Stage 5 Data Analysis - NFDC history Stage 5 Data Analysis Barton-on-Sea Cliff Instability Preliminary Study New Forest District Council This document has …

Stage 5 Data Analysis

Barton-on-Sea Cliff Instability Preliminary Study

New Forest District Council

28 March 2014




28 March 2014

Halcrow Group Limited

Lyndon House, 62 Hagley Road, Edgbaston, Birmingham

B16 8PE

Tel 01214562345 fax 0121 4561569

halcrow.com

Halcrow Group Limited has prepared this report in accordance with

the instructions of client New Forest District Council for the client’s sole and specific use.

Any other persons who use any information contained herein do so at their own risk.

© Halcrow Group Limited 2014

Document history




This document has been issued and amended as follows:

Version Date Description Created by Verified by Approved by

1.0 28/03/2014 Draft for comment R Brooks P Fish R Moore

Contents

1 Introduction 7

1.1 Background 7

1.2 Scope of work 7

1.3 Terms of reference 9

2 Site description 10

2.1 Site location 10

2.2 Coastal management 10

2.3 Previous studies 13

2.4 The 2013 ground investigation 13

3 Ground conditions 20

3.1 General 20

3.2 Palaeogene strata 20

3.2.1 Becton Sand Formation (Zones I & J) 20

3.2.2 Chama Sand Formation (Zones H and G) 20

3.2.3 Barton Clay Formation (Zone F2) 26

3.2.4 Barton Clay Formation (‘F2 Shear’ horizon) 30


3.2.6 Barton Clay Formation (Zone E) 33

3.2.7 Barton Clay Formation (Zone D) 33

3.2.8 Barton Clay Formation (Zone C) 38

3.2.9 Barton Clay Formation (Zones B and A) 39

3.2.10 Superficial deposits Brickearth, Plateau Gravel and Colluvium 41

3.3 Surface drainage and hydrogeology 41

3.3.1 Surface drainage 41

3.3.2 Hydrogeology 41

3.4 Material properties 45

3.4.1 Specialist testing of the ‘D Shear’ horizon 46

4 Geological and geomorphological model 49

4.1 Model development 49

4.1.1 Purpose of model 49

4.1.2 Methodology 49

4.1.3 Modelling results 50

4.2 Distribution and thickness of strata 52

4.2.1 Becton Sand Formation (Zones I & J) 52

4.2.2 Chama Sand Formation (Zones H & G) 53



4.2.5 Barton Clay Formation (Zone E) 55

4.2.6 Barton Clay Formation (Zone D) 55

4.2.7 Barton Clay Formation (Zones C to A) 57

4.2.8 Brickearth 58

4.2.9 Plateau Gravel 58

4.2.10 Colluvium and Made Ground 59

4.2.11 Summary 61

4.3 Relationship between geology and geomorphology 61

5 Cliff behaviour review 63

5.1 Cliff behaviour units 63

5.2 Landslide failure mechanisms 64

5.2.1 General 64

5.2.2 Cliff House Hotel landslide 64

5.2.3 Controls on shear zone development in the Palaeogene strata 68

6 Conclusions 71

6.1 Strata distribution and thickness 71

6.2 Hydrogeology 71

6.3 Controls on principal shear horizons 72

6.4 The Cliff House Hotel landslide ground model 72

7 References 73

Appendix

A Borehole logs from current and past ground investigations

B Results of specialist testing by Drs Barton and West

C Guide to the GIS

List of tables

Table 2.3: Chronology of previous engineering studies and ground investigation.

Table 2.4: 2013 Ground Investigation boreholes.

Table 3.1: Significant features of Zone F2 of the Barton Clay Formation

Table 3.2: Summary of lithostratigraphic units encountered at the site during the

2013 Ground Investigation

Table 3.3: Groundwater monitoring instrumentation installed during the 2013

Ground Investigation.

Table 3.4: Summary of preliminary analyses of aggregated geotechnical testing data

Table 4.1: Summary of typical unit thicknesses at the site.

Table 5.1: Cliff behaviour units identified at Barton-on-Sea (after Moore et al., 2003).

List of figures

Figure 1: Site location plan showing key landslides and engineered coastal defences

elements.

Figure 2: Geological long section of cliffs between Highcliffe and Milford-on-Sea

Figure 3: Previous ground investigation locations.

Figure 4: 2013 Ground Investigation locations.

Figure 5: Clay and sand content (%) v. depth below top of stratum for Zone H of the

Chama Sand Formation.

Figure 6: Schematic section along the cliff line at Barton-on-Sea showing average

groundwater levels and their relationships to strata dip and Zone H.

Figure 7: Geological strata looking onshore (5x vertical exaggeration).

Figure 8: Extract from the 3D geological model showing contoured base of Zone I.

Figure 9: Extract from the 3D geological model showing contoured base of Zone G.

Figure 10: Extract from the 3D geological model showing contoured base of Zone F2

and thickness isopachytes.

Figure 11: Extract from the 3D geological model showing contoured base of Zone F1

and thickness isopachytes.

Figure 12: Extract from the 3D geological model showing contoured base of Zone E.

Figure 13: Extract from the 3D geological model showing contoured base of Zone D.

Figure 14: Extract from the 3D geological model showing contoured base of Zone C.

Figure 15: Extract from TerraDat geophysical report showing approximate thickness

and distribution of the superficial deposits.

Figure 16: Extract from TerraDat geophysical report showing approximate thickness

and distribution of the superficial deposits.

Figure 17: 3D view of Cliff House Hotel.

Figure 18: 3D view of undercliff between the Sea Road Access and Hoskin’s Gap.

Figure 19: Results of seismic refraction and resistivity geophysical surveys along

Line 1.

Figure 20: Results of seismic refraction and resistivity geophysical surveys along

Line 2.

List of photographs

Photo 1: The Chama Sand Formation (Zone H) outcropping adjacent to the Sea

Road Access (visible at top right).

Photo 2: ‘G Shear’ at c. 4mBGL in BH14/2012

Photo 3: ‘G Shear’ in BH14/2012

Photo 4: Developing shear at the ‘G Shear’ horizon at c. 25mBGL in BH19/2012.

Photo 5: Developing ‘G Shear’ in BH19/2012.

Photo 6: Composite photograph showing fossiliferous mudstone bands in the cores

recovered from Zone F2

Photo 7: Detail of two of the extremely weak, pale green-grey, variably fossiliferous

mudstone bands encountered in the upper part of Zone F2 in borehole

BH12/2012 (left) and BH01/2012

Photo 8: Detail of one of the hard, pale brown-cream, highly calcareous silty clay

bands that frequently occur near the centre of Zone F2

Photo 9: Detail of the red-brown weathering band located above the basal

concretionary limestone (below 0.5m scale) at outcrop below the Sea Road.

Photo 10: Detail of an exposure below the central part of the Naish Farm Holiday

Park where a possible second level of limestone concretions is seen lying

around 1.0m above the persistent horizon marking the base of Zone F2

Photo 11: Geological hammer resting on the active ‘F2 Shear’ surface, at the top of

the undercliff.

Photo 12: Detail of ‘F2 Shear’ at 10.75mBGL in borehole BH13/2012.

Photo 13: The developing ‘F2 Shear’ at 24.55mBGL in borehole BH07/2012

Photo 14: The developing ‘F2 Shear’ at 19.7mBGL in borehole BH19/2012

Photo 15: Large burrows infilled with green glauconitic sand, from the central part

of Zone D in borehole BH01/2012.

Photo 16: Sample from immediately below the ‘D Shear’ surface exposed on the D

bench below Naish Farm.

Photo 17: Colour enhanced, composite photograph showing the sections of core

from boreholes BH01/2012, BH02/2012, BH04/2012 and BH19/2012

containing the ‘D Shear’ horizon and the immediately overlying

mudstone.

Photo 18: Detail showing the ‘D Shear’ horizon (30mm ‘chocolate brown’ band at

right) and immediately overlying pale, green-beige mudstone in borehole

BH19/2012

Photo 19: Detail showing the ‘D Shear’ horizon (brown band at extreme left) and

immediately underlying material in borehole BH19/2012

Photo 20: Coarse sand to fine gravel of sub-rounded quartz and possible jasper in a

matrix of glauconitic sandy, silty clay from near the base of Zone C at c.

36.5mBGL in BH02/2012.

Photo 21: Burton’s ‘pale-grey marly clay’ seen in situ below the ‘D Shear’ at the foot

of the undercliff complex below Naish Farm (left), and in BH02/2012 at

36mBGL.

Doc no: Version: 1.0 Date: 28 March 2014 Project code: GNFDCB Filename: Stage 5 data analysis report (final).docx

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1 Introduction

1.1 Background

The coast at Barton-on-Sea, Hampshire, exposes a sequence of sedimentary strata

comprising clays, sands and gravels of Palaeogene and Quaternary geological ages.

These strata are relatively weak and the cliffs fronting the town have historically been

subject to landslides and erosion.

The basic geology and broad mechanisms of landsliding have been previously

established, based upon a series of previous site investigations carried out on behalf

of New Forest District Council (NFDC) and its predecessors between 1960 and 1994.

Understanding of engineering geology and landslide mechanisms has also been

improved by extensive academic studies and published literature. A combination of

toe erosion resulting from wave action, and cliff failure from toe erosion and excess

groundwater is postulated to have triggered a series of translational landslides with

superimposed mudslides. Multiple shear-prone horizons are suspected within the

exposed strata, which together with the general eastwards dip, allow distinct cliff

behaviour units to be recognised along the frontage.

Knowledge existing prior to this study supported localised stabilisation of the cliffs

using sheet piling, anchor walls, engineered drainage systems and rock armour toe

protection. However, these measures have only been partially successful and the

cliffs have continued to experience instability, particularly in response to high

antecedent rainfall. In the future, continuing cliff instability will threaten commercial

interests and residential properties across the frontage, some locations sooner than

others.

In order to develop a forward plan for cliff instability management that is in line with

the adopted shoreline management plan, NFDC has commissioned a programme of

work to better understand cliff instability at Barton on Sea. The planned work aims

to:

identify gaps in our understanding of the engineering geology and

geomorphology of the cliffs that causes uncertainty in landslide ground models

identify and implement additional ground investigation works and monitoring

that will reduce this uncertainty and underpin improved landslide ground

models

develop appropriate and viable engineering and management options that will

reduce or control groundwater and cliff instability.

1.2 Scope of work

The work commissioned focuses on the 1.85km stretch of coastal frontage between

Naish Farm Holiday (caravan) Park in the west and Barton-on-Sea Golf Course in the

east. This site corresponds with the maximum extent of the residential built areas of

Barton-on-Sea developed above the unstable coastline (Figure 1). The Preliminary

Study comprises the following stages:


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Stage 1: Desk study review of existing information and preparation of a

summary report, which will inform the design and scope of the ground

investigations and monitoring works (Halcrow 2011)

Stage 2: Design of ground investigation works and monitoring instrumentation

including preparation of site programmes and procedures for analysis of data

(Halcrow 2012). This phase of work also included geomorphological mapping

the site frontage that was used to develop landslide ground models and

support ground investigation design. Note that parts of the frontage

reactivated over the winter of 2012/13 meaning the geomorphological mapping

requires updating

Stage 3: Produce tender brief to enable an NFDC to prepare tender documents

for the ground investigation and monitoring. The GI contract was awarded to

Geotechnical Engineering Limited (GEL) in November 2012.

Stage 4: Site supervision of the ground investigation. The works were

undertaken during May and June 2013 having been deferred from their

planned start date due to the wet winter of 2012/2013 that led to localised

reactivation of the landslide.

Stage 5: Data analysis of the information obtained from the ground

investigation. This report. Includes borehole logs from past and the present

ground investigation, results of specialist testing and key project information in

a Geographical Information System (GIS) in appendices.

Following installation of monitoring equipment, data readings, checks and

preliminary analysis are to be taken for a period of two years (i.e. June 2013 to

June 2015).

Stage 6: Feasibility report detailing the range of alternative engineering and

management options for groundwater control and cliff stabilisation measures,

including outline designs, cost estimates and a separate non-technical

summary. This work will be undertaken during 2015.

This report therefore presents the findings of Stage 5 data analysis and acts a

companion volume to GEL’s factual report (Geotechnical Engineering, 2014). It is

underpinned by three data sets:

GEL’s factual report, which includes terrestrial geophysics survey data from

TerraDat and borehole geophysical logging by European Geophysical Services.

Observations made on site by Halcrow during supervision of the ground

investigation.

Additional laboratory analyses of sediment samples by Drs Max Barton and

Ian West of Southampton University.

The report provides an update to the previous Stage 1 desk study review report

(Halcrow, 2011) and provides definitive statements on the stratigraphy and materials

in Section 2, the groundwater regime and hydrogeology in Section 3 and the

landslide ground models in Section 4. A GIS database is also provided that includes

the location of past and present boreholes and 3D surfaces of the main strata

constructed from the new ground investigation data (described in Appendix C).


9

1.3 Terms of reference

Following successful pre-qualification, Halcrow Group Limited (Halcrow) responded

to an invitation to tender for a preliminary feasibility study at Barton-on-Sea in June

2011. Halcrow subsequently submitted its tender for this work on 14 July 2011, the

tender document having been prepared in accordance with the instructions for

tendering and the brief provided by NFDC.

Halcrow was subsequently commissioned by NFDC to undertake the project in

September 2011. The scope of this commission is summarised in Section 1.3.


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2 Site description

2.1 Site location

The site comprises the 1.85 km of coastline at Barton-on-Sea, Hampshire, between

Naish Farm in the West and Barton Golf course in the East (Figure 1). The coastal

frontage is formed of weak degraded cliffs in Barton Group Strata, which are exposed

within the western limb of the Hampshire Basin syncline (Figure 2).

The site frontage is characterised by a narrow beach above which are a series of

higher benches backed by the cliff top and cliff top plateau. The crest of the upper

cliff is between 30m and 35mAOD and is characterised by a near vertical cliff face,

varying from between 5m to 10m high, fronted by a steep talus slopes. The body of

the landslide comprises an undercliff that consists of a series of steep scarps

separated by benches. The lowest bench is protected by rock armour. Five rock

armour strong points extend into the sea at regular intervals across the frontage to

promote formation of a beach.

2.2 Coastal management

The Poole and Christchurch Bays Shoreline Management Plan Review (SMP2, Royal

Haskoning, 2011) was adopted in 2011. The overall vision for the Barton-on-Sea area

is to continue to manage the rate of shoreline recession through a long-term policy of

Managed Realignment (MR). This policy acts as a transitional section of coast from

Highcliffe to Friars in the west that will continue to be protected by Hold the Line

(HTL) policies, and Hordle Cliff to Barton in the east that will be allowed to evolve

naturally under a No Active Intervention (NAI) policy.

SMP2 states that the intent of the policies for Barton-on-Sea is to develop a long term

readjustment of the defence approach for the town. This envisages protecting the

eastern seafront of the town (Marine Drive East) from erosion whilst maintaining the

important open space of the cliff and coastal slope. Works would be undertaken to

improve the stability of the slope but accepting further loss due to the cliff top

recession that would still occur, particularly along the coastal sections comprising

steep cliffs.

Gradual failure of the defences at the western end of the town (Marine Drive West)

would not be prevented. This means that losses of property and the Naish Farm

Holiday Park are likely consequences and so adaptation measures will need to be

developed and implemented to manage this process.


11

Figure 1: Site location plan showing key landslides and engineered coastal defence elements


12

Figure 2: Geological long section of cliffs between Highcliffe and Milford-on-Sea showing easterly dip of strata and stratigraphical relationships (after Melville & Freshney (1982) and West (2014)).


13

2.3 Previous studies

The classic academic work on the stratigraphy of the Barton Group strata was that of

Ernest St. John Burton (1929), who also published further significant research in 1925

and 1933. Burton established the zonal division of the strata at Barton-on-Sea that

remains largely unchanged to this day and is used here.

In more recent times, work directed by Max Barton has provided the most detailed

and frequently referenced body of work on the engineering geology of the Barton

Group, including the probable causes and mechanisms of coastal instability at Barton-

on-Sea (Barton 1973, Barton & Coles 1984, Barton et al. 2006, Garvey 2007, Barton &

Garvey 2011).

A number of engineering studies and associated schemes have been completed at the

site on behalf of NFDC or its predecessor. The distribution of these previous

investigations within and surrounding the site is presented on Figure 3.

A summary of the scope of these various investigations summarised in Table 2.3. Full

references for the cited studies are provided in Section 7. Detailed information on all

work previously undertaken at Barton on Sea is provided in Halcrow 2011. All

available borehole logs from past ground investigations are presented in Appendix

A. The locations and details of past boreholes are shown in the project GIS files

appended to this report.

Despite this history of extensive research and investigation at the site, a number of

important uncertainties remain. The uncertainties principally relate to the details of

the stratigraphy, in particularly variability in thickness of geological beds and their

engineering geological properties; the hydrogeology; and the landslide ground

models. These have been addressed by the ground investigation undertaken in the

current study.

2.4 The 2013 ground investigation

The 2013 Ground Investigation was undertaken between 20 May and 3 July 2013,

with the technical scope being based on recommendations made by Halcrow in the

Stage 1 desk study review report (Halcrow, 2011). The contract tender process was

managed by New Forest District Council and awarded to GEL, with the specialist

geophysical components of the contract being sub-contracted to TerraDat (terrestrial

geophysics) and European Geophysical Services (downhole geophysics). The entire

site works programme was supervised by Halcrow. The principal elements of the

2013 Ground Investigation were:

16 rotary cored and rotary coreless (core recovery at specific horizons only)

boreholes (BH01/2012 to BH07/2012 and BH11/20112 to BH19/2012) and two

cable-percussion boreholes (BH09/2012 & BH10/2012), to depths of between

20m and 43m below ground level, the samples from which were geologically

logged and sub-sampled for subsequent testing

logging of majority of rotary open holed boreholes (BH01/2012, BH04/2012,

BH06/2012, BH12/2012, BH13/2012, BH15/2012, BH16/2012 and BH18/2012)

using down-hole geophysical techniques (natural gamma, gamma-density,

dual neutron and/or induction) to identify and correlate marker horizons


14

Table 2.3: Chronology of previous engineering studies and ground investigation (see also Section 7).

Date Description Consultant GI Contractor

1959-1969 Barton-on-Sea Cliff

Stabilisation (Stages 1-3)

Sir William Halcrow

& Partners

George Wimpey &

Cementation Ground

Engineering Ltd 1969-1970 Barton-on-Sea Cliff

Stabilisation

1970-1973 Highcliffe Cliff Drainage

Scheme

1987-1988 Landslide between

Groynes 15 and 17

Sir William Halcrow

& Partners

Soil Mechanics Ltd

1989-1991 Cliff Instability at

Hoskin’s Gap

Robert West &

Partners

Structural Soils Ltd

1990 Landslide between

Groynes 15 and 17

Rendel Geotechnics Soil Mechanics Ltd

1991 Barton-on-Sea Cliff

Stabilisation (Phase I)

Rendel Geotechnics Fugro McClelland

Ltd


Stabilisation (Phase II)


Stabilisation (Phase III)


Stabilisation (Phase IV)

2002-2006 Christchurch Bay

Strategy Study

Halcrow Group Ltd &

High-Point Rendel

2003 Barton-on-Sea Outfall Southern Water Costain Limited

six in situ, variable head permeability tests (Five in standpipes, one in-

borehole)

laboratory geotechnical soil and chemical testing on sub-samples collected

from the boreholes

surface geophysical surveys comprising:

o three intersecting electrical resistivity tomography and seismic refraction

(P & S wave) on the undercliff below the Cliff House Hotel

o an electrical resistivity tomography survey line located just behind the

crest of the sea cliffs along the full length of the site

o an electromagnetic ground conductivity survey covering the open green

spaces on the cliff top, and forming the bulk of the site


15

a conventional buried services survey in the cliff top areas of the site and

including the seaward margin of Marine Drive, Marine Drive East and Marine

Drive West.

specialist laboratory analysis of sediment samples undertaken by Dr Max

Barton and Dr Ian West of Southampton University, comprising:

o X-ray diffraction (XRD) analysis of samples from shear surfaces to

determine changes in clay mineralogy

o preparation and analysis of microscope slides from key sampled materials

to determine micro-structures.

The locations of the points of investigation are shown in Figure 4, with borehole

positions also being summarised in Table 2.4. Full details of the scope of work and

the resulting data are provided in GEL’s factual report (Geotechnical Engineering,

2014). Borehole logs and GIS data associated with the 2013 ground investigation are

provided in Appendix A and on the DVD supplied with this report.


16

Figure 3: Previous ground investigation locations.


17

Table 2.4: 2013 Ground Investigation boreholes.

Position I.D.

NGR Elevation (mAOD)

Depth (m)

Drilling technique

Sampling In situ testing Installations (ref also Table 3.3)

mE mN Geophysics Permeability SPTs

BH01/2012 422903.80 93150.80 34.75 36.50 Rotary coreless Isolated coring √ Inclinometer

BH02/2012 423016.30 93131.40 34.70 37.00 Rotary coreless Isolated coring 3 VWPs

BH03/2012 423296.30 93063.30 33.90 35.40 Rotary cored Continuous core 2 VWPs

BH04/2012 423300.90 93064.10 33.80 40.10 Rotary coreless Isolated coring √ √ Inclinometer

BH05/2012 423412.80 93027.60 33.85 36.10 Rotary cored Continuous core √ (SP) √ 2 VWPs, 1 SP

BH06/2012 423519.20 93045.70 34.25 43.00 Rotary coreless Isolated coring √ √ (SP) 3 VWPs, 1 SP

BH07/2012 423624.30 93001.60 33.75 35.50 Rotary cored Continuous core √ (SP) 2 VWPs, 1 SP

BH09/2012 423451.00 92982.80 24.65 20.00 Percussion Tubes/bags/pots Inclinometer

BH10/2012 423494.10 92980.10 24.50 20.00 Percussion Tubes/bags/pots 1 SIT, 1VWP


BH12/2012 423728.50 92985.30 33.45 36.00 Rotary coreless Isolated coring √ √ (borehole) 2 VWPs

BH13/2012 423786.50 92927.10 18.45 22.80 Rotary coreless Isolated coring √ Inclinometer


BH15/2012 423867.80 92902.50 12.65 20.50 Rotary coreless Isolated coring √ 1 VWP

BH16/2012 423933.50 92950.60 32.60 35.50 Rotary coreless Isolated coring √ √ (SP) 2 VWPs, 1 SP

BH17/2012 424217.10 92917.50 31.50 34.00 Rotary coreless Isolated coring √ 1 VWP

BH18/2012 424192.60 92859.50 11.25 20.10 Rotary coreless Isolated coring √ 2 VWPs

BH19/2012 423304.00 93067.40 33.80 39.80 Rotary coreless Isolated coring √ (SP) 2 VWPs, 1 SP

Notes:

AOD = Above Ordnance Datum

SP = Standpipe piezometer

SPTs = Standard penetration tests

VWP = Vibrating wire piezometer

SIT = Slip indicator tubing


18

Figure 4: 2013 Ground Investigation locations.


19


20

3 Ground conditions

3.1 General

The 2013 Ground Investigation has provided some of the highest quality,

undisturbed samples yet recovered from the major shearing horizons within the

Palaeogene strata. These samples have allowed the shearing horizons and the

associated lithostratigraphic units within the Barton Group to be studied in

unprecedented detail.

The following sections provide summary descriptions of the character of the

lithostratigraphic units of the Becton, Chama and Barton Clay Formations, with key

details summarised in Table 3.1. Details are also provided regarding the character of

the overlying superficial deposit, and the hydrological and hydrogeological regimes

at the site. Detailed reporting on the hydrogeology will be conducted in 2015,

following 24 months of groundwater monitoring. The geotechnical properties of the

key strata, based upon the findings of preliminary analyses of in situ and laboratory

geotechnical testing data, are presented and briefly discussed in Section 3.4.

The distribution, continuity and attitude of the various strata have been determined

via 3-dimensional modelling, and the results of this work are described in Section 4.

3.2 Palaeogene strata

3.2.1 Becton Sand Formation (Zones I & J)

The Becton Sand Formation comprises three subdivisions, these correlating with

Burton’s Zones I, J (‘Becton Bunny Member’), and K (‘Long Mead End Beds’). As was

anticipated, only the lowest division (i.e. Zone I) of the formation was encountered

during the 2013 Ground Investigation, although Zone J was observed to probably

sub-crop below the easternmost 150m of the site (Section 4).

Zone I was found to comprise c. 9m (Section 4) of dense to very dense, medium

fractured, thinly laminated, grey to dark grey, clayey, fine to medium sand, this being

compatible with boreholes previously completed on the Barton-on-Sea Golf Course,

beyond the eastern end of the site (Costain, 2003). At depths above 10mBGL,

oxidation and bleaching of the weakly pyritiferous sands was seen to have affected

the unit and to have led to the overprinting of the grey colour of the unweathered

sands with colours ranging from vivid orange to pale yellow-white. Such alteration is

conspicuous in the exposed cliffs below the Central Amenity Area and between that

location and the Sea Road Access, with cross-stratification also being clearly visible,

picked out by weathering, in these cliff exposures (Photo 1).

No fossils were noted in this unit, this being compatible with previous investigations.

3.2.2 Chama Sand Formation (Zones H and G)

3.2.2.1 Zone H

The Chama Sand Formation has for many years been poorly exposed and obscured

by talus. However, a reasonable exposure of this key lithostratigraphic unit was

available for study at the top of the cliff next to the Sea Road Access during the 2013

Ground Investigation (Photo 1). The Zone H sub-cropped below the intensely


21

bleached, cross-stratified sands of the feather edge of Zone I and was oxidised to a

vivid orange colour in the upper c. 0.3m, below which there was seen to be a rapid

transition to its characteristic blue-grey colour (Photo 1).

Photo 1: The Chama Sand Formation (Zone H) outcropping adjacent to the Sea Road Access (visible at top right). The presumed upper contact of Zone H with the overlying bleached sands of Zone I is marked by the red line at the top of the 0.5m scale, at the base of marked cross stratification. Inset shows an extremely weak spheroidal concretion around 0.6m in diameter which was observed within slumped Zone H material below borehole BH01/2012.

Zone H was encountered in all of the boreholes completed behind the cliff line at the

site during the 2013 Ground Investigation, and was found to be around 8m thick

in the central part of the site (Section 4). The upper 5m (approx.) was found to

comprise dense to very dense, widely fractured, thinly to thickly laminated, blue-grey

mottled orange, clayey to very clayey, silty, fine to medium sand. The lower 3m

comprised stiff to very stiff, fissured, thinly laminated, grey-blue, slightly sandy to

very sandy, silty clay. Index testing data (Figure 5), however, indicate clay content to

increase gradationally with depth, the above defined boundary between ‘sand’ and

‘clay’ being approximate only.

The scatter in the data plotted on Figure 5 highlights the probable existence within

Zone H of distinct beds and lamina of sandier material in more clayey material (and

vice-versa), particularly in the lower part. The inspection of cores retrieved during the

2013 Ground Investigation, however, did not reveal the presence of significant

traceable beds of clay and sand which might localise the postulated ‘H1-H2 Shear’ at

their contact.


22

Figure 5: Clay and sand content (%) v. depth below top of stratum for Zone H of the Chama Sand Formation.

Zone H was found to be intensely bioturbated throughout, although fossil debris was

confined to the base of the unit, as has been previously noted (Bristow et al. 1991 and

West 2014). Here the diagnostic bivalve fossil Chama squamosa (Solander) and the

gastropod Turitella edita (Solander) and Turitella imbricataria (Lamarck) were found in

relative abundance. Rare, pebble-sized segregations of carbonaceous matter and

pyritiferous material were also noted in the cliff exposures and in the borehole core.

Spheroidal masses of cemented, fossiliferous sand (i.e. concretions), up to 0.6m in

diameter, were also observed within this unit at exposure on the undercliff west of

the Cliff House Hotel landslide (Photo 1), the occurrence of such masses having been

previously reported by Burton (1929). No evidence of such concretions was noted in

the boreholes.

3.2.2.2 Zone G

The base of the Chama Sand Formation is marked in coastal exposure by a frequently

cemented, grey mottled red-brown, shelly and ferruginous (sideritic) band up to 0.3m

thick – the Zone G ‘stone band’ (Burton, 1929).


23

Relatively fresh, in situ exposures of Zone G were located during the 2013 Ground

Investigation at the top of the undercliff complex below the site of borehole

BH02/2012 (423040mE, 93074mN, 21.6mAOD approx.) and below the Sea Road

Access (423348mE, 92960mN, 17.0mAOD approx.). These locations were found to tie

in relatively well with the projected borehole intersections of this marker horizon

(Section 4). At exposure the unit was seen to comprise loose, very clayey gravel to

weak, green weathering red-brown, very gravelly pyritiferous mudstone. The unit

was <70mm in thickness, with the gravel component comprising largely of both intact

and broken Turitella gastropods. Multiple similar, but thinner seams of fossil debris

were noted to occur in the clays immediately above Zone G, and particularly in the

upper part of the underlying Zone F2 of the Barton Clay Formation (Sub-section

3.2.3.1). Within the boreholes completed during the 2013 Ground Investigation, Zone

G was seen to be of similarly restricted development, to generally comprise very

clayey gravel, and to be also overlain and underlain by multiple but generally thinner

seams of fossil debris.

The in situ and borehole evidence collected during the 2013 Ground Investigation was

compatible with Burton’s postulation (Burton 1929), that Zone G may be typically

uncemented in the sub-surface, only becoming indurated due to the oxidation and

leaching of the enclosing and bounding pyritiferous clay in the shallow sub-surface

or at exposure. Zone G also appears to exist as a number of discontinuous

fossiliferous lenses occurring within a horizon 1-2m thick; hence, Zone G is not

considered to be a reliable marker at the small scale.

3.2.2.3 ‘H1-H2’ and ‘G Shear’ horizons

As stated in Sub-section 3.2.2.1, no evidence was observed in the cores for the

existence of significant, traceable beds of clay and sand in Zone H that might

potentially aid in localising the postulated ‘H1-H2 Shear’ (Barton & Garvey, 2011). The

probable presence of thick sand and clay laminations in the lower part of the unit,

however, would seem to offer the potential for such strain localisation.

Shear surfaces at the ‘G Shear’ horizon were observed at field exposure and in the

boreholes completed during the 2013 Ground Investigation, this shearing horizon

having been previously located at approximately 0.15m above the base of Zone H

(Barton, 1973). The relatively fresh exposures studied below BH02/2012 (Sub-section

3.2.2.2) showed sandy colluvium sat directly onto in situ seams of fossil debris located

<0.5m above a candidate Zone G fossil debris band. Within borehole BH14/2012 of the

2013 Ground Investigation, which was located on the undercliff below the Central

Amenity Area, disturbed Zone H sand was found in contact with a sub-horizontal,

polished and slickensided shear surface developed on basal Zone H clay

approximately 0.15m above Zone G (Photos 2 & 3). The ‘G Shear’ was possibly also

encountered in borehole BH09/2012, located on the undercliff near the Sea Road

Access (at and disturbing a candidate Zone G band), and in borehole BH10/2012,

around 40m further to the east (at 0.23m above a candidate Zone G band).

The presumed developing ‘G Shear’ surface was encountered 25m behind the cliff

line in BH19/2012 near the Cliff House Hotel, possibly at the base of a thick sand

lamination, and 0.15m above the presumed Zone G band (Photos 4 & 5).


24

Photo 2: ‘G Shear’ at c. 4mBGL in BH14/2012 of the 2013 Ground Investigation. Red arrow shows shear surface separating disturbed and probably remoulded upper Zone H sand (right) from intact lower Zone H clay (left), with probable Zone G ‘stone band’ (largely obscured by clay smear) indicated by black arrow, 0.15m below ‘G Shear’. Note presence of thin seams of fossil debris between ‘G Shear’ and Zone G.

Photo 3: ‘G Shear’ in BH14/2012 of the 2013 Ground Investigation. Polished and slickensided ‘G Shear’ surface formed on the basal Zone H clays. Note existence of multiple sub-parallel polished surfaces, and of an inclined fissure surface (in shadow at bottom of specimen at left).


25

Photo 4: Developing shear at the ‘G Shear’ horizon at c. 25mBGL behind the current cliff line in BH19/2012 of the 2013 Ground Investigation. Red arrow shows developing ‘G Shear’ within basal Zone H sandy clays, 0.15m above the presumed Zone G (black arrow). Note presence of multiple thin seams of fossil debris between ‘G Shear’ and Zone G.


26

Photo 5: Developing ‘G Shear’ in BH19/2012 of the 2013 Ground Investigation. Polished ‘G Shear’ surface formed in the basal Zone H clays. Note formation of shear surface immediately above a thin seam of fossil debris (sample is inverted) and/or below an apparently more sandy horizon.

3.2.3 Barton Clay Formation (Zone F2)

3.2.3.1 General

Zone F2 of the Barton Clay Formation is potentially of key importance to the

engineering geological understanding of cliff instability at Barton-on-Sea, with

multiple authors (Rendel Geotechnics 1993a, Garvey 2007, Barton & Garvey 2011 and

Hosseyni et al. 2012) postulating that variability in the recorded thickness of this unit

within the site, and/or of the relative positioning of ‘iron-stained’ clays above the

basal concretionary mudstone layer may help explain the distribution of instability in

the western part of the site, and in particular below Cliff House Hotel.

Inspection of in situ exposures below the site of borehole BH02/2012 (423040mE,

93074mN) and below the Sea Road Access (423341mE, 92969mN approx.), and of

borehole cores retrieved during the 2013 Ground Investigation, indicate the strata of

Zone F2 to range from very stiff to hard, fissured, thinly laminated, grey-blue mottled

brown, slightly sandy, silty clay to extremely weak mudstone, with frequent seams of

fossil debris. This finding is wholly compatible with previous studies.

The cores retrieved from the boreholes located behind the cliff line during the 2013

Ground Investigation have, however, provided unprecedented detail on the

distribution of the fossil and ‘iron-stained’ bands within this unit, and of the its

thickness where not truncated by landsliding. Multiple boreholes indicated Zone F2

to be of relatively uniform thickness behind the cliff line (c. 4m –Section 4), with a

series of laterally persistent, red-brown weathering, fossiliferous and pyritiferous

mudstone bands with Zone G affinities occurring in the upper half, and with an

argillaceous limestone layer/concretions marking its base. A summary of these

findings is presented in Table 3.1 below, and in Photos 6, 7 and 8.

Table 3.1: Significant features of Zone F2 of the Barton Clay Formation (where encountered immediately behind the cliff line).

Height above base of unit

(m)

Feature Typical thickness

(m)

Comments

3.5-4.5 Zone G ‘stone band’ (base) <0.3 Taken as the thickest and/or best cemented of a series of

fossiliferous lenses at this approximate horizon

3.0-3.5 Extremely weak, pale green-grey,

variably fossiliferous mudstones

<0.2 Variably pyritiferous and locally brecciated. Up to four

bands in fossiliferous clay (Photos 6 & 7)

2.5 Hard, pale brown-cream, highly

calcareous silty clay

<0.3 Frequently very subtle and diffuse, generally devoid of

fossils and showing signs of brecciation (Photos 6 & 8)


27

Height above base of unit

(m)

Feature Typical thickness

(m)

Comments

0.5-1.5 Hard, pale brown-cream, highly

calcareous silty clay

<0.3 Very subtle and diffuse, occasionally pyritiferous and

generally devoid of fossils (Photo 9). Only observed in

BH19/2012, BH05/2012, BH09/2012and at outcrop below

the Sea Road Access. Possibly analogous to a rarely

exposed 2nd level of concretions at Naish (Photo 10)

0.2-0.3 Basal concretionary layer:

extremely weak to medium strong,

dark grey mottled brown, calcite

veined concretionary argillaceous

limestone band/concretions

0.2-0.3 Locally absent or present only as a subtle and diffuse,

pale brown mottling and hardening of locally highly

calcareous clays (e.g. Photo 8). Alternatively, the mottled

clay horizon may be analogous with the red-brown

weathering clays occurring between isolated limestone

concretions in the exposures below Naish Farm


28

Photo 6: Composite photograph showing fossiliferous mudstone bands in the cores recovered from Zone F2 during the 2013 Ground Investigation, and their very approximate cross correlation. The cores represent the uppermost part of Zone F2, with the Zone G ‘stone band’ lying just above the top of the core sections illustrated.

Photo 7: Detail of two of the extremely weak, pale green-grey, variably fossiliferous mudstone bands encountered in the upper part of Zone F2 in borehole BH12/2012 (left) and BH01/2012 (right) of the 2013 Ground Investigation (see also Photo 6). Note ‘ringing’ on exterior curve of core from BH12/2012 illustrating the increased hardness of the band relative to the bounding clays, and also the evidence of brecciation. The band on the right has very strong affinities with the Zone G ‘stone band’.


29

Photo 8: Detail of one of the hard, pale brown-cream, highly calcareous silty clay bands that frequently occur near the centre of Zone F2 (sample from 26.8mBGL in BH12/2012 for the 2013 Ground Investigation), with similar layers occurring just above the basal concretionary limestone layer east of the Sea Road Access (Photo 9), and in place of/between the basal limestone layer where this is absent/present as isolated concretions (Photo 10).

Photo 9: Detail of the red-brown weathering band located above the basal concretionary limestone (below 0.5m scale) at outcrop below the Sea Road Access and previously noted by Garvey (2007). A band at this horizon was proven in boreholes BH19/2012, BH05/2012and BH09/2012 of the 2013 Ground Investigation, these being located completed inland of this location.


30

Photo 10: Detail of an exposure below the central part of the Naish Farm Holiday Park where a possible second level of limestone concretions is seen lying around 1.0m above the persistent horizon marking the base of Zone F2 (Table 3.1 – 0.3m long hammer for scale). Note also presence of red-brown weathering nodular mudstone between the concretions at both horizons (at extreme right).

3.2.4 Barton Clay Formation (‘F2 Shear’ horizon)

The’ F2 Shear’ horizon has been previously placed at 0.1m above the top of the

concretionary limestone layer marking the base of Zone F2 by Barton (1973). The

occurrence of physical shearing at this depth has been confirmed by the 2013 Ground

Investigation, and more significantly, so has the progressive development of this

shear behind the cliff line at this horizon (Section 5).

At the relatively fresh, in situ exposures of Zone F2 located during the 2013 Ground

Investigation at the top of the undercliff complex below the site of borehole

BH02/2012 (423052mE, 93059mN), the polished ‘F2 Shear’ was located 0.1m above the

basal concretionary limestone layer, i.e. at approximately 18.4mAOD, and

immediately above a thin seam of fossils (Photo 11). A zone of shearing was also

encountered at this horizon at the top of the undercliff complex below the Central

Amenity Area in borehole BH13/2012 of the 2013 Ground Investigation (Photo 12). In

the latter case the shear zone comprised multiple sub-parallel polished, undulating

and slickensided partings with entrained clay intraclasts within a c. 20mm band of

clay, located around 0.2m above the basal concretionary limestone layer.

The development of the ‘F2 Shear’ behind the cliff line was proven in boreholes

BH07/2012 (Photo 13) and BH19/2012 (Photo 14) of the 2013 Ground Investigation.

Once again, the shear surface was seen to have developed immediately above a thin

seam of fossils, but in this case no more than two polished and planar shear surfaces

where observed.


31

Photo 11: Geological hammer on the active ‘F2 Shear’ surface, at the top of the undercliff below the location of borehole BH02/2012 of the 2013 Ground Investigation. The shear surface was associated with a thin fossil seam and was situated around 0.1m above the concretionary limestone layer marking the base of Zone F2.

Photo 12: Detail of ‘F2 Shear’ at 10.75mBGL in borehole BH13/2012 of the 2013 Ground Investigation, which was located at the top of the undercliff below the Central Amenity Area and to the immediate east of Hoskin’s Gap. The shear zone (dark band at right), which comprised multiple sub-parallel polished, undulating and slickensided partings with ‘intraclasts’ within a c. 20mm band of clay, was located around 0.2m above the basal concretionary limestone layer (pale broken core at left).


32

Photo 13: The developing ‘F2 Shear’ as encountered behind the cliff line at 24.55mBGL in borehole BH07/2012 of the 2013 Ground Investigation. The shear comprised of two polished and planar surfaces separated by around 10mm of intact clay and was closely associated with thin seams of fossil debris.

Photo 14: The developing ‘F2 Shear’ as encountered behind the cliff line at 19.7mBGL in borehole BH19/2012 of the 2013 Ground Investigation. The shear comprised a polished surface located immediately above a thin seam of crushed fossil shells. Significantly, this shear surface was not located just above the basal concretionary limestone as is normal, but had been pushed higher (0.5m above limestone, approx.) due to the local development of a hard, pale brown-cream, highly calcareous silty clay just above the basal limestone (i.e. as in the exposure featured in Photo 9).


33


Where encountered in boreholes during the 2013 Ground Investigation, Zone F1 was

found to comprise a relatively monotonous sequence of very stiff to hard,

prominently fissured, thinly laminated, grey-brown, slightly sandy, silty, clay to

extremely weak mudstone, around 9m thick (local to the cliff line - Section 5). Lenses

and more persistent seams of fossil debris were recorded in the borehole core, and at

outcrop below the Sea Road Access, these being of a character consistent with the

‘shelly drifts’ recorded by Burton (1929).

No shear surfaces were observed in the Zone F1 clays, this being consistent with the

findings of previous studies. Evidence of the localisation of failures in the exposed ‘F

Bench’ along inclined and arcuate (in plan and section), polished and slickensided

fissures occurring in this unit was, however, observed below the Sea Road Access

during the 2013 Ground Investigation.

3.2.6 Barton Clay Formation (Zone E)

The Zone E unit was described by Burton (1929) as comprising a lower, highly

fossiliferous ‘Earthy Bed’ some three feet in thickness overlain by a ‘hard clay’ around

two feet in thickness and capped by a horizon of large light-coloured septarian

(limestone concretions).

The limestone concretions marking the top of Zone E where encountered in boreholes

BH01/2012, BH03/2012, BH05/2012, BH07/2012, BH14/2012 and BH18/2012 of the 2013

Ground Investigation, with the material lying below this being found to comprise

very stiff to hard, fissured, thinly laminated, brown-grey, slightly sandy, silty, clay to

extremely weak mudstone. The base of this unit, however, proved more difficult to

locate, the transition to the comparatively sand-rich clays of Zone D being sufficiently

gradational to not allow definitive placement either in the field or based on

laboratory grading data (probably due to the occurrence of the glauconitic sand in

discrete and, in the vicinity of the Zone E to D boundary, relatively widely separated

lamina and segregations). The thickness of 1.5m to 2.0m recorded for Zone E on the

borehole logs associated with the 2013 Ground Investigation is, therefore, whilst

compatible with previous studies (Section 5), approximate only.

3.2.7 Barton Clay Formation (Zone D)

Zone D is poorly exposed in the field, largely due to the fact that the occurrence of

shearing towards the base of the unit results in the fragmentation and reworking of

all material lying above this shear surface. The rotary cores obtained from

considerable depths both below the undercliff complex and from behind the cliff line

during the 2013 Ground Investigation have, therefore, provided a rare opportunity to

examine the full thickness of Zone D, and in particular the character of the material

immediately overlying the ‘D Shear’.

As has been alluded to in Sub-section 3.2.3.3, the upper boundary of Zone D was

seen, in the boreholes completed during the 2013 Ground Investigation, to be

gradational, with the transition from very stiff to hard, fissured, thinly to thickly

laminated, grey-brown to a distinctly dark grey-green, slightly sandy, silty, clay to

extremely weak mudstone occurring over a vertical height of approximately 1m.

Below this transition, Zone D was seen to be very heavily bioturbated, with the


34

presence of burrow features up to several centimetres in diameter being highlighted

by a bulk infill or internal coating of dark green, glauconitic sand (Photo 15). Some of

these glauconitic burrow fillings were seen to be themselves cut by later features

infilled with sand-poor, grey-brown clays. Besides occasional pieces of fossilised

wood, coarse gravel-sized pieces of pyritic material and coarse sand to fine gravel-

sized, sub-rounded clasts of detrital quartz where also recorded, particularly below

the ‘D Shear’ horizon near the boundary with Zone C.

Note that although the Zone D clays were seen to contain both segregations and thin

lamina of glauconitic sand, the clays were only very rarely found to be ‘sandy’, with a

bulk sand content of typically <30%, based on laboratory grading tests.

3.2.7.1 ‘D Shear’ horizon

Previous work by Barton et al. (2006) has focused on the clay mineralogy and

engineering significance, in terms of the localisation of shearing in Zone D, of a layer

of ‘chocolate brown’ clay that can be seen in situ, at the toe of the undercliff complex

below Naish Farm, to immediately underlie the ‘D Shear’ surface (Photo 16). This

subtle colour marker, along with the recorded occurrence around 0.5m below the ‘D

Shear’ horizon of isolated limestone concretions at the top of the underlying Zone C

unit, allowed the ‘D Shear’ horizon to be readily identified in the boreholes

completed during the 2013 Ground Investigation.


35

Photo 15: Large burrows infilled with green glauconitic sand, from the central part of Zone D in borehole BH01/2012 of the 2013 Ground Investigation. Note that the burrow infill in the lower core sample is itself stratified, and that it connects directly with relatively more glauconitic clay at extreme right (top of core section is to the right of the image).

Photo 16: Sample from immediately below the ‘D Shear’ surface (top of sample is the shear surface), where exposed on the D bench below Naish Farm. The uppermost 25mm of the sample comprises the softened ‘chocolate brown’ clay of Barton et al. (2006), the underlying pale green-beige silty clay/mudstone breaking with a conchoidal fracture.

The ‘chocolate brown’ band, which was confirmed to actually comprise the ‘D Shear’

horizon, was seen in the borehole cores to have a total thickness of around 30mm to

(max.) 40mm and to comprise stiff, brown, silty clay containing multiple, sub-parallel,

undulating and slickensided shear surfaces (Photos 17 to 19), some locally arching

around entrained, gravel-sized ‘intraclasts’ of clay/mudstone. Geotechnical index

testing indicates the ‘D Shear’ to have a higher clay content than the material

bounding it (Section 3.4).


36

Also of potentially great significance, was the observed occurrence directly above the

‘D shear’ of an extremely weak, pale green-beige and apparently only lightly

bioturbated mudstone around 0.3m thick. This mudstone had a comparatively low

sand content and was characteristically closely to very closely fractured, these

fractures being inclined at around 45 degrees and terminating against the top of the

‘D Shear’. Locally, the surfaces of these fractures showed possible evidence of

alteration, with cream halos/infillings being recorded (ref core from BH01/2012 at top

of Photo 16). Complex fabrics suggestive of syn-sedimentary and/or tectonic

disturbance, including angular clasts cemented into paler matrices (ref core from

BH19/2012 at bottom of Photo 17 and Photo 18) and inclined and or disrupted

lamination, were also common.

Below the ‘D Shear’, the green-beige mudstone was replaced by the very stiff to hard,

fissured, thinly laminated and intensely bioturbated, green-grey, slightly sandy, silty

clay to extremely weak mudstone typical of Zone D. A notable exception to this was

in borehole BH19/2012, where the mudstone (or a similar mudstone) was present to

around 100mm below the ‘D Shear’ (Photo 19).


37

Photo 17: Colour enhanced, composite photograph showing the sections of core from boreholes BH01/2012, BH02/2012, BH04/2012 and BH19/2012 containing the ‘D Shear’ horizon and the immediately overlying mudstone. The ‘D Shear’ horizon is marked at lower right and can be seen in the core from all of the boreholes as a subtle band of ‘chocolate brown’ clay around 30mm thick. Immediately overlying this (i.e. to left in the photo) is a comparatively pale, green-beige mudstone around 300mm thick with steeply inclined, sub-parallel fractures and complex sedimentary/tectonic textures. It will be noted that the inclined fractures terminate against the ‘D Shear’ horizon in every case.

Photo 18: Detail showing the ‘D Shear’ horizon (30mm ‘chocolate brown’ band at right) and immediately overlying pale, green-beige mudstone in borehole BH19/2012 (top of sample is to left). Note the inclined, glauconitic sand infill/clast to left of centre and the overlying pale green-cream band, which is possibly developed in association with an inclined fracture (just visible at left).


38

Photo 19: Detail showing the ‘D Shear’ horizon (brown band at extreme left) and immediately underlying material in borehole BH19/2012 (top of sample is to left). Note that the fractured, pale green-beige mudstone that overlies the ‘D Shear’ also continues for around 100mm below it in this borehole, with the relatively sharp transition back to the slightly sandy and darker thinly laminated clays that are typical of the bulk of Zone D being located towards the centre of the sample.

3.2.8 Barton Clay Formation (Zone C)

Zone ‘C’ (the Voluta suspensa Zone of Burton 1929), forms a scarp below the ‘D Shear’

at the toe of the slumped cliffs below Naish Farm, and is a useful marker bed in the

field due to the occurrence of horizons of limestone concretions at its upper and

lower boundaries.

Boreholes BH01/2012, BH02/2012, BH04/2012 and BH19/2012 of the 2013 Ground

Investigation produced reasonable quality core from Zone C, with the fortuitous

coring of a limestone concretion in BH04/2012 indicating the upper horizon of

concretions to lie approximately 0.5m below the ‘D Shear’, this being wholly

compatible with previous studies (Barton, 1973). In all boreholes, Zone C was seen to

comprise very stiff, fissured, thinly to thickly laminated, dark green-grey, slightly

sandy, silty clay to extremely weak mudstone, with pockets and bands up to 0.3m

thick of sandy/gravelly clay, particularly in the lower half. In the latter pockets and

bands, the coarse sand to fine gravel-sized clasts comprised sub-rounded detrital

quartz (Photo 20), with possible jasper and bone fragments having also been

tentatively identified by Ian West (pers. Comm., 2013). Large, as yet unidentified

bone fragments were also found at around 36mBGL in BH01/2012, along with

frequent clasts of fossilised wood.

Zone C was found to be intensely bioturbated throughout, but at a vertical depth of

around 0.7m below the upper horizon of limestone concretions, the boreholes

encountered a pale green-cream extremely weak mudstone around 0.15m thick. This

marker band was heavily bioturbated, the burrow traces standing out clearly due to

their being infilled with the same green-grey sandy clay that was found overlying the

comparatively much paler grey mudstone. The same marker band was also identified

in the scarp below the ‘D Shear’ at the foot of the undercliff complex below Naish

Farm (Photo 21), and its character was wholly compatible with ‘pale-grey marly clay’

identified by Burton (1929) as forming a useful marker in the cliffs 2.5 feet (0.76m

approx.) below the upper horizon of limestone concretions.


39

Photo 20: Coarse sand to fine gravel of sub-rounded quartz and possible jasper in a matrix of glauconitic sandy, silty clay from near the base of Zone C at 36.5mBGL (approx.) in BH02/2012 of the 2013 Ground Investigation.

Photo 21: Burton’s ‘pale-grey marly clay’ seen in situ below the ‘D Shear’ at the foot of the undercliff complex below Naish Farm (left), and in BH02/2012 of the 2013 Ground Investigation at 36mBGL.

3.2.9 Barton Clay Formation (Zones B and A)

The boreholes completed during the 2013 Ground Investigation were not deep

enough to penetrate Zones B and A of the Barton Clay Formation, with the exception

of borehole BH01/2012, which penetrated 0.3m into the very stiff, fissured, thinly to

thickly laminated, sandy, slightly gravelly, silty clays to extremely weak mudstones

of Zone B.


40

Table 3.2: Summary of lithostratigraphical units encountered at the site during the 2013 Ground Investigation (for unit thicknesses Section 4).

Formation Zone Description

Plateau Gravels Dense to very dense, orange-brown-yellow, sandy GRAVEL with

subordinate gravelly SAND, sometimes as distinct bands.

Becton

Sand

Fm

‘Up

per

Ba

rto

n B

eds’

I

Dense to very dense, medium fractured, thinly laminated, grey to

dark grey mottled orange-yellow, clayey, fine to medium SAND

Ch

ama

San

d F

orm

atio

n

H

Dense to very dense, widely fractured, thinly to thickly laminated,

blue-grey mottled orange, clayey to very clayey, silty fine to medium

SAND with a transition to stiff to very stiff, fissured, thinly

laminated, grey-blue, slightly sandy to very sandy, silty CLAY

towards the centre of the unit. ‘G shear’ 0.1-0.2m above base (or

possibly in the upper part of the underlying Zone F2)

G

Loose very clayey GRAVEL of fossil debris to weak, green,

weathering red-brown, very gravelly (shell debris) pyritiferous

MUDSTONE. Taken as the thickest and/or better cemented of a

series of discontinuous lenses of shell debris occurring at this

horizon.

Bar

ton

Cla

y F

orm

atio

n

‘Mid

dle

Bar

ton

Bed

s’

F2

Very stiff to hard, fissured, thinly laminated, grey-blue mottled

brown, slightly sandy, silty CLAY to extremely weak MUDSTONE,

with frequent seams of fossil debris, mudstone bands and a basal

concretionary limestone layer/horizon of concretions up to 0.3m

thick (possible two layers below Naish Farm). ‘F2 Shear’ 0.1-0.5m

above basal limestone band/concretions

F1

Very stiff to hard, fissured, thinly laminated, grey-brown, slightly

sandy, silty, CLAY to extremely weak MUDSTONE with a number

of persistent horizons with lenses of shell debris

E

Very stiff to hard, fissured, thinly laminated, brown-grey, slightly

sandy, silty, CLAY to extremely weak MUDSTONE. Top of unit is

marked by a horizon of limestone concretions up to 0.3m thick

D

Very stiff to hard, fissured, thinly laminated and intensely

bioturbated, green-grey, slightly sandy to rarely sandy, silty CLAY

to extremely weak MUDSTONE. Clay-rich ‘D Shear’ 0.5m above base

of unit and below a characteristically closely to very closely fractured

pale green mudstone with complex sedimentary/tectonic fabrics

C

Very stiff, fissured, thinly to thickly laminated, dark green-grey,

slightly sandy, silty CLAY to extremely weak MUDSTONE, with

pockets and bands up to 0.3m thick of sandy/gravelly clay. Horizons

of limestone concretions up to 0.3m thick at top and bottom

‘Lo

we r

Bar

ton

Be

ds’

B Very stiff, fissured, thinly to thickly laminated, sandy, slightly

gravelly, silty CLAY to extremely weak MUDSTONE


41

Formation Zone Description

A

(A0-

A3)

(not encountered)

3.2.10 Superficial deposits Brickearth, Plateau Gravel and Colluvium

The superficial deposits mantling the bedrock within the site comprise Quaternary

(Pleistocene) Brickearth and Plateau Gravel, with local occurrences of engineered and

non-engineered fill or ‘Made Ground’. Landslide debris or ‘colluvium’ is also present

on the landslide benches in the undercliff complex.

Where sampled during the 2013 Ground Investigation, the Brickearth was found to

comprise a soft to firm, light brown mottled orange, slightly sandy, slightly gravelly

silt, everywhere overlain by an organic topsoil layer, their combined thickness being

around 1.2m (Section 4). This finding is wholly compatible with those of previous

investigations at the site.

Plateau Gravel rests uncomfortably upon the gently dipping strata of the Palaeogene

Barton Group and was deposited by Pleistocene ancestors of the Rivers Stour and

Avon.

Where encountered in the boreholes completed during the 2013 Ground Investigation

the Plateau Gravel was recorded to comprise up to 6.3m (Section 4) of generally

dense to very dense, orange-brown-yellow, sandy gravel with subordinate gravelly

sand, sometimes as distinct bands. The gravel clasts were angular to sub-rounded,

fine to coarse, and of black flint. The sand component was typically medium to

coarse. This finding is in agreement with those of previous investigations.

Coastal landsliding, erosion and cliff remediation at Barton-on-Sea have resulted in

thick deposits of colluvium mantling the undercliff.

The colluvium deposits encountered in the boreholes completed on the undercliff

during the 2013 Ground Investigation comprised a assortment of reworked sands

principally from the upper portion of the Chama Sand Formation, mixed locally with

sand and gravel from the overlying Pleistocene deposits and Made Ground material

that has fallen over the edge of the cliff line onto the surface of the undercliff

complex. Made Ground in the form of a layer of flint gravel was also found to

underlie the trackways maintained along the undercliff.

3.3 Surface drainage and hydrogeology

3.3.1 Surface drainage

There are no natural surface water courses within the site; however, water frequently

ponds at the base of landslide backscarps and on the benches of the undercliff during

the winter months. These ponds are fed both by groundwater springs issuing at the

top of and behind the undercliff complex (Sub-section 3.3.2 for further details) and by

direct precipitation.

3.3.2 Hydrogeology


42

A number of standpipe piezometers and vibrating wire piezometers were installed

across the site at a range of depths during the 2013 Ground Investigation (Table 3.3).

These installations will be monitored for the next 24 months to capture the

groundwater response to seasonal fluctuations in precipitation and landsliding. The

current understanding of the hydrogeology of the site, based upon analyses of the

first six months of data collected from the installations constructed in 2013 (i.e. July

2013 to January 2014), is summarised below.

The superficial deposits are relatively permeable (Sub-section 3.4) and allow

precipitation to percolate rapidly down to the underlying Palaeogene strata of the

Barton Group. At the far-eastern end of the site the sub-cropping sands of the Becton

Sand Formation (i.e. Zone I) permit these infiltrating waters to continue to descend

within the underlying Palaeogene strata. Moving westward (i.e. up strata dip),

however, the significantly more clayey and less permeable strata of the Chama Sand

Formation (i.e. Zone H) rise towards the base of the superficial deposits and force a

similar westward rise in the groundwater level (Figure 6). West of the Cliff House

Hotel, groundwater mounding due to the close proximity of the top of Zone H forces

the groundwater table to the top of the Palaeogene strata and during prolonged

rainfall, into the base of the overlying superficial deposits.

Barton (1973) has postulated that during prolonged periods of wet weather channels,

cut into the buried upper surface of the Chama Sand and Barton Clay Formations

during the deposition of the Plateau Gravel, may funnel groundwater towards the

crest of the coastal cliffs to form localised springs that are closely associated with

development of mudslides. Surface geophysical surveys and boreholes completed

during the 2013 Ground Investigation do support the existence of such channels

(Section 4). However, inspection of the cliff exposures indicates they are relatively

shallow (<2m). These geophysical surveys (Section 2.4 and Geotechnical Engineering

2014) have also highlighted the existence of a network of historical field drains,

boundary ditch drains and Made Ground-infilled hollows on the cliff top at the site.

Although not maintained, based upon observations made in 2013 many of these

features still have the capacity to conduct run-off to the cliff edge and onto the

undercliff during times of prolonged or intense precipitation. Such have also

previously acted to localise tension cracks, where they existed as linear features of

cliff line-sub-parallel orientation.

Outside of periods of extreme weather, groundwater flows toward the cliff line and

exits into the undercliff complex via spring lines variously located either within the

central part of or near the base of Zone H, the exact position depending on both the

elevation of the base of Zone H and the damming effect of the undercliff complex.

The temporary loss of flush water at the G horizon, in borehole BH02/2012 of the 2013

Ground Investigation, suggests that the locally porous and permeable lenses of fossil

debris that occur at this horizon may also be important local controls on spring

location, particularly west of Cliff House Hotel. The spring lines occurring above the

Barton Clay Formation effectively separate the groundwater regime in the

unconfined Pleistocene and Palaeogene aquifers from the confined and potentially

layered aquifers within the underlying, low permeability clays.

Groundwater flow within the relatively impermeable Barton Clay Formation is

generally insignificant, with piezometer data indicating pore water pressures to be

approximately 4m to 5m below hydrostatic at the base of Zone F2, and >15m below


43

hydrostatic at the ‘D Shear’ horizon near the base of Zone D (Figure 6). It has been

suggested by previous authors (e.g. Barton, 1973), that this inverted piezometric

profile is a legacy of historically rapid rates of coastal erosion and lateral unloading,

with pressures possibly now slowly rebounding in response to the reduction in the

erosion rate effected by the cliff remediation works completed in the 1960’s and

1970’s (see also Vaughan & Walbancke, 1973). Insufficient data exist currently to

confirm this hypothesis; however, it is likely that the ubiquitous presence of fissuring

within the clays of the Barton Clay Formation, of possible drainage along shear

horizons and of sand bands in depth, will also be important controlling factors.

Within the undercliff, the intermingling of granular and cohesive strata within the

highly anisotropic colluvium; the local presence of groundwater springs, deep cracks

and daylighting shear horizons; and the existence of a locally failed engineered

drainage system, has led to the development of complex hydrological and

hydrogeological regimes, with ephemeral ponds and perched groundwater bodies

having been recorded at multiple levels. The rapid response of perched groundwater

bodies and surface ponds to precipitation events has previously been cited as a key

control on instability within the undercliff (Barton (1973) and Barton & Garvey

(2011)), and this has been tentatively confirmed by groundwater monitoring over the

exceptionally wet winter of 2013-2014. Borehole groundwater strikes and piezometer

monitoring data also suggest that the confinement of more permeable horizons in

hydraulic connection with strata behind the cliff line, by particularly clayey

colluvium or by a thin capping of in situ clay strata, may lead to the local

development of sub-artesian or artesian groundwater pressures. At borehole

BH15/2012 located below the Central Amenity Area (Figure 4), artesian pore

pressures were encountered at the Zone G horizon even though an engineered deep

drain was present <1m away.

In summary, the Plateau Gravels, Becton Sand and Chama Sand Formations form the

primary aquifer units at the site, and it is within these strata that groundwater flows

towards the coastal cliffs are concentrated. The underlying and largely separate

groundwater regime in the relatively impermeable clays of the Barton Clay

Formation is characterised by sub-hydrostatic piezometric pressures, this probably

reflecting historically high rates of erosion and unloading. The hydrological and

hydrogeological regimes operating in the undercliff are complex and are subject to

multiple temporal and spatially controls.


44

Table 3.3: Groundwater monitoring instrumentation installed during the 2013 Ground Investigation.

Borehole No.

Elevation (mAOD)

Piezometer response zones & [tip depths] (mBGL)

Strata/Zones monitored

SP VWP VWP VWP (s = Shear)

BH02/2012 34.70 - 12.0-14.0

[13.0]

16.5-18.0

[17.25]

33.5-35.5

[34.5] H, F2/F2s, D/Ds

BH03/2012 33.90 - 8.0-12.0

[10.0]

18.0-20.0

[19.0] - H, F2/F2s

BH05/2012 33.85 13.0-15.0 13.0-15.0

[14.0]

20.0-22.0

[21.0] - H, H, F2/F2s

BH06/2012 34.25 13.5-15.5 13.5-15.5

[14.5]

21.5-23.5

[22.5]

37.0-40.0

[39.0] H, H, F2/F2s, D

BH07/2012 33.75 13.5-15.5 13.5-15.5

[15.0]

23.5-25.5

[24.6] - H, H, F2/F2s

BH10/2012 24.50 - 5.0-6.5

[5.75] - - Col

BH11/2012 18.75 - 3.0-5.0

[4.7]

7.0-9.0

[8.5] - Col/Gs, F/F2s

BH12/2012 33.45 - 15.0-17.0

[16.0]

25.0-27.0

[26.0] - H, F/F2s

BH14/2012 12.50 - 2.5-4.0

[3.4]

6.0-7.0

[6.4] - Col/Gs, F/F2s

BH15/2012 12.65 - 3.0-5.0

[4.0] - - Col/Gs

BH16/2012 32.60 18.0-20.0 18.0-20.0

[19.0]

27.5-29.0

[28.3] - H, F/F2s

BH17/2012 31.50 - 23.0-25.0

[24.0] - - H

BH18/2012 11.25 - 5.0-7.0

[6.0]

10.0-11.0

[10.5] - H, F/F2s

BH19/2012 33.80 15.5-16.5 19.5-20.5

[20.0]

37.0-38.0

[37.5] - F/F2s, D/Ds


45

Borehole No.

Elevation (mAOD)

Piezometer response zones & [tip depths] (mBGL)

Strata/Zones monitored

SP VWP VWP VWP (s = Shear)

Notes:

mBGL = Metres below ground level

mAOD = Metres above Ordnance Datum

SP = Standpipe piezometer

VWP = Vibrating wire piezometer

Col = landslide colluvium

Figure 6: Schematic section along the cliff line at Barton-on-Sea showing the average groundwater levels measured within the various strata occurring at the site (July 2013 to January 2014), and their relationships to strata dip and Zone H

3.4 Material properties

The results of preliminary analyses indicated that the in situ and laboratory

geotechnical testing data associated with the 2013 Ground Investigation data were

sufficiently compatible with those associated with previous investigations at the site

to allow aggregation of these data populations. Re-evaluation of historical borehole

logs and more detailed sub-division of associated data by lithostratigraphic unit also

allowed data for the individual zones within the Barton Clay Formation to be

separated out, as summarised in Table 3.4.

The index properties for each of the lithostratigraphic units presented in Table 3.4

highlight the effects of grain-size variations within the Barton Clay Formation, with

Zone F1 being seen to have the highest clay content. The ranges of these index

parameters are compatible with previously published data for the Barton Clay

Formation, and indicate the clays of that formation to be generally of intermediate to

very high plasticity.

No significant variations in index or peak effective shear strength properties were

detected at the ‘G Shear’ or ‘F2 Shear’ horizons. This finding is compatible with

previous studies (Barton et al., 2006), and with a cliff erosion-effected rebound

provenance for these shears (i.e. as opposed to shearing along a pre-existing,

intrinsically more weak layer).

5

10

15

20

25

30

35

423000 423200 423400 423600 423800 424000 424200

Ele

vati

on

(m

AO

D)

National Grid Easting (m)

EastWest


46

It is noted that the residual effective shear strengths associated with the 2013 Ground

Investigation are generally significantly reduced compared to those previously

assessed. Although this finding may be partly due to the geotechnical tests employed

during the 2013 Ground Investigation (small ring shear apparatus), a detailed

analysis of this finding has yet to be completed (detailed analyses of the available in

situ and laboratory geotechnical testing data will form part of future studies).

3.4.1 Specialist testing of the ‘D Shear’ horizon

While no significant compositional or geomechanical anisotropy was detected at the

‘G Shear’ or ‘F2 Shear’ horizons during the data review (Section 3.4.1), subtly

increased clay contents and reductions in residual effective shear strength were

detected at the ‘D Shear’ horizon (Table 3.4). Additionally, a preliminary review of X-

ray diffraction (XRD) and scanning electron microscope (SEM) analyses, completed at

the University of Southampton in early 2014 on samples of the ‘D Shear’ and

immediately surrounding lithological units collected from behind the cliff line during

the 2013 Ground Investigation (Appendix B), indicate the ‘D Shear’ horizon to have a

comparatively higher smectite clay content and lower quartz content than the

bounding clays, and also a sheared micro-fabric.

The above findings are compatible with the previous work of Barton et al. (2006) and

suggest that primary sedimentalogical anisotropy in Zone D may at least partly

explain the tendency for shearing at the ‘D Shear’ horizon.


47

Table 3.4: Summary of preliminary analyses of aggregated geotechnical testing data (range of reported results, with median values in rounded parentheses and suggested low characteristic parameter values in square parentheses, as appropriate. Effective strength parameters have been assessed based on the raw stress and strain data and not on interpretations by others).

Material Property Plateau Gravel Becton Sand Fm

Chama Sand Fm Barton Clay Formation

Zone I Zone H Zone F2 Zone F1 Zone E Zone D ‘D Shear’

SPT N60 5-232 (42) 52-300 (100) 15-181 33-600

(155)

31-200

(174) - - -

Natural moisture content (%) - 16.0-40.0 (24.0) 19.0-40.0 17.6-49.2

(26.0)

19.6-29.1

(25.3)

16.9-23.4

(21.0)

13.6-28.0

(21.1) -

Clay content (%) - 4.0-23.0 (8.5) 8.0-52.0 26.0-61.0

(52.5)

30.0-61.0

(54.0)

30.0-46.0

(40.0)

27.0-53.0

(46.0)

49.0-51.0

(50.0)

Liquid limit (%) - 34.0-51.0 (44.0) 30.0-78.0 58.0-86.0

(71.0)

50.0-79.0

(69.0)

37.0-62.0

(50.0)

37.0-71.0

(58.0) -

Plastic limit (%) - 15.0-18.0 (16.0) 15.0-28.0 18.0-27.0

(23.0)

18.0-30.0

(25.0)

17.0-22.0

(19.0)

17.0-27.0

(22.0) -

Plasticity index (%) - 19.0-33.0 (28.0) 6.0-51.0 38.0-62.0

(47.0)

29.0-54.0

(44.0)

20.0-41.0

(30.0)

20.0-49.0

(35.0) -

Bulk mass density (gb Mg/m3) - 1.68-2.19 (1.89) 1.84-2.17 1.92-2.11

(2.03)

1.92-2.11

(2.01)

2.08-2.13

(2.11)

1.85-2.25

(2.05) 2.14

Peak undrained shear strength (cu

kN/m2) - - 123-206

52-273

(157) [150]

115-248

(175) [150] 287

221-436

(388) -

Peak effective angle of friction (ᶲ’p

degrees)

29.0-44.0 (40.0)

[35.0]

35.0-44.0 (41)

[35.0] 25.0-35.0 [22.6] [21.5] - [25.0] -

Peak effective cohesion (c’ kN/m2) 0.0

0.0 0.0-5.0 [2.0] [5.0] - [5.0] -

Residual effective angle of friction (ᶲ’r

degrees)

-

- 8.1 [7.2] [7.2] - [7.2] [5.7]

Residual effective cohesion (c’r kN/m2) -

- 0.0 [0.0] [0.0] - [0.0] [0.0]

Coefficient of volume compressibility

(mv m2/MN)

-

- - 0.007-0.02 0.01-0.02 0.01 0.01 0.01

Modulus of elasticity (MN/m2) -

- - 50-143 50-100 100 75 75


48

Material Property Plateau Gravel Becton Sand Fm

Chama Sand Fm Barton Clay Formation

Zone I Zone H Zone F2 Zone F1 Zone E Zone D ‘D Shear’

Coefficient of permeability (k m/s) >1.0E-04 3.5E-06 to 6.7E-03 1.9E-08 to 4.9E-03 - 1.0E-07 to

4.8E-07 - - -


49

4 Ground model

4.1 Model development

4.1.1 Purpose of model

To better understand the relationships between the geological units described in

Section 3 and surface morphology, borehole data from the 2013 Ground Investigation

and select, re-evaluated historical borehole logs, were used to create a 3D geological

model of the site. These data are described in Appendix C. This model presents 3D

surfaces representing the key stratigraphical horizons, the LiDAR ground surface

data and geomorphological data from site mapping and interpretation of laser

scanning data captured before and after the recent landslide reactivation (late 2012 to

early 2013). The 3D model has been used to/can be used in the future to:

understand the thickness and attitude (i.e. dip) of the geological strata

encountered in the boreholes

understand the relationship between the strata and the geomorphology of the

cliff line

support development of landslide ground models

support understanding of the hydrological and hydrogeological regimes.

Data on the surface morphology (a LiDAR survey from 2011) and geomorphological

mapping undertaken in 2011 used in the development of the ground model both pre-

date the landslide reactivation of winter 2012/13 and require revision and update.

4.1.2 Methodology

Data from the 18 boreholes and from the down-hole geophysical logging completed

during the 2013 Ground Investigation, and from selected historical deep boreholes

with associated high quality engineering geological logging records available for

review (Halcrow, 2011), were analysed in order to identify the elevations of the bases

of the following key stratigraphical units at the various points of investigation:

Plateau Gravel

Becton Sand Formation (Zone I)

Chama Sand Formation (Zone H)

Chama Sand Formation (Zone G)

Barton Clay Formation (Zone F2)

Barton Clay Formation (Zone F1)

Barton Clay Formation (Zone E)

Barton Clay Formation (Zone D)

Barton Clay Formation (Zone C).


50

The results of this detailed review were used to interpolate the base of each unit

across the site using the triangular irregular network (TIN) method. The TIN method

was selected as it interpolates a straight line between points of known position and

does not seek to create curved surfaces.

As the TIN method does not seek to interpolate beyond known points, the surfaces

created did not intersect with the cliff face. This made correlation of the sub-surface

geology with the geomorphological mapping and LiDAR data difficult. A series of

‘pseudo points’ were therefore created to allow each surface to be extrapolated to the

cliff face. This was achieved by:

visualising the TIN surfaces created from the ‘known’ data and checking that

the dip was consistent and predictable

creating transects perpendicular to the coast across the TINs, extracting

elevation data and plotting in spreadsheet format

fitting linear trendlines along these data and extrapolating them to the cliff face

to determine the expected outcrop for each stratum – i.e. the ‘pseudo point’

adding the ‘pseudo point’ to the input dataset and recreating the TIN such that

it now extended to the cliff face.

4.1.3 Modelling results

Example outputs from the 3D model developed, which has been visualised using

ESRI’s ArcScene software, are shown in Figure 7. The 3D model has confirmed:

the generally gentle (1 to 2 degrees) and consistently east-northeast dipping,

laterally persistent nature of the Barton Group strata (blue to red surfaces)

the expected close relationship between strata dip and distribution and the dip

and distribution of the topographic scarps and benches exposed on the

undercliff.

In addition to the above, the 3D model has highlighted:

the gradual thinning of Zone H of the Chama Sand Formation toward its sub-

crop at the western-end of the site

the relatively rapid thinning of Zone F2 of the Barton Clay Formation towards

the southwest (i.e. seaward). This is thought to be due to the basal

concretionary limestone horizon rising in the same direction

the relatively rapid thickening of Zone F1 of the Barton Clay Formation

towards the southwest (i.e. seaward). This is thought to be due to the

topography on the base of Zone F2 (ref above)

the consistent thickness of Zone F (i.e. combined thickness of Zones F1 and F2)

the undulating character of the erosive contact (dark green surface in Figure 7)

between the superficial deposits (i.e. Plateau Gravel) and the underlying

Palaeogene strata, reflecting erosion of a series of channels by an ancient river

system.


51

Figure 7: Geological strata looking onshore (5x vertical exaggeration). Vertical black lines indicate the positions of the boreholes. Upper diagram shows surfaces formed at the base of each unit; lower diagram shows predicated outcrop/sub-crop below Colluvium of the base of each unit on undercliff.

Cliff House

Hotel

Central Amenity Area

Cliff House

Hotel

Central Amenity Area Strata from base: Bed C (red), Bed D (purple), Bed E (orange), Bed F1 (green), Bed F2 (red), Beds H & G Chama Sand (blue), Beds I and J Becton Sand (indigo), Plateau gravel (dark green).


52

It is proposed that the results of the current phase of inclinometer and piezometer

monitoring be fed into the 3D model in late 2015, with these additional data possibly

allowing further significant insights to be achieved. A more detailed interpretation of

the results of 3D modelling using the currently available datasets, informed where

appropriate by the results of the geophysical surveys completed during the 2013

Ground Investigation (Section 2.4, is presented in the following sections.

4.2 Distribution and thickness of strata

4.2.1 Becton Sand Formation (Zones I & J)

Zone I was encountered in boreholes drilled as far west as the Cliff House Hotel

during the 2013 Ground Investigation, with the broad sub-crop of this unit at the base

of the superficial deposits, which covers the majority of the site, terminating local to

borehole BH02/2012 (based upon borehole and surface geophysical evidence).

The 3D surface modelling the interpreted base of Zone I is presented in Figure 8. This

surface indicates that, like all of the Palaeogene strata, Zone I dips relatively

uniformly towards the east-northeast at around one degree, although a number of

local anomalies are apparent. These are interpreted to be the result of errors in the

logging of and/or in the subsequent interpretation of the drillers’ logs for isolated

boreholes, most probably due to the erroneous inclusion of a portion of the oxide-

stained top of the underlying Zone H. They are therefore not thought to be

representative of the true geology.

Figure 8: Extract from the 3D geological model showing contoured base of Zone I.

The thickness of Zone I is interpreted to range from 0.7m to 11.29, with much of this

variation being due to the sub-cropping nature of the unit (i.e. it is, to a greater or

lesser degree, truncated by erosion). Where concealed below a capping of the

overlying Becton Bunny Bed (Zone J), on the Barton-on-Sea Golf Course, it is

interpreted to be around 11m thick. Burton (1929) states the thickness of Zone I as 26

feet (7.9m); however, it is uncertain where this thickness was measured. The

thickness of 14.6m recorded in BH17/2012 is based solely on drillers’ logs and likely


53

includes the locally sandy base of the overlying Plateau Gravel, the same being true

of the 13.4m recorded in B3/1960 (Section 4.2.2.2).

Zone J was not encountered in the boreholes completed as part of the 2013 Ground

Investigation. Geophysical surveys and inspections of the cliff exposures completed

during the site works period, however, suggest that the sub-crop of this unit probably

underlies the eastern-most 150m of the site (Section 4.2.2.1).

4.2.2 Chama Sand Formation (Zones H & G)

Zones H and G were encountered in all of the boreholes completed during the 2013

Ground Investigation and in the majority of historical boreholes. The sub-crop of

these units at the base of the superficial deposits extends between the old field

boundary hedgeline near borehole BH02/2012 to well beyond the western boundary

of the site.

The modelled base of the Zone G marker is presented in Figure 9. Zones G and H

appear to dip relatively uniformly towards the east, although the twisting of contours

in the areas of densest investigation suggests that the fact that Zone G exists as a

series of discontinuous lenses bounded by multiple similar fossil seams has

introduced a degree of randomised variability. This presumption is supported by the

observation that boreholes reporting anomalously decreased thicknesses in Zone H

frequently also report anomalously increased thicknesses of Zone F2, and vice-versa.

Figure 9: Extract from the 3D geological model showing contoured base of Zone G.

The total combined thickness of Zones H and G was recorded during the 2013

Ground Investigation to range from approximately 7m to 10m, with the thickness of

this unit being recorded to gradually increase from west to east. Zone G generally

represented <0.2m of this total. This range in thickness is compatible with new

interpretations of the historical borehole data, although Burton (1929) states the

thickness of Zone H as 18 feet (5.5m) and of Zone G as generally one to two feet


54

(≤0.6m). The reasons for the apparent significant deviation of the interpreted

thicknesses of Zones H and G from Burton’s historical measurements is probably due

to Burton’s having measured his sections significantly further to the west of the site,

or if they were recorded local to the site, due to the measured sections being subtly

truncated due to shearing at the postulated ‘H1-H2 Shear’.


Zone F2 was encountered in all boreholes completed during the 2013 Ground

Investigation, except for borehole BH17/2012 at the eastern end of the site. Some of

the historical boreholes located at and beyond the eastern end of the site were also

not sufficiently deep to penetrate this unit. The sub-crop of this unit is located well

beyond the western boundary of the site at Highcliffe.

The modelled base of Zone F2 is presented in Figure 10. Significantly, the base of this

unit dips with a more northerly component, i.e. to the northeast rather than east/east-

northeast. The contours on the basal surface are also closer together, with the dip

angle being estimated at up to two degrees.

Figure 10: Extract from the 3D geological model showing contoured base of Zone F2 (upper image) and thickness isopachytes (lower image).

Previous authors have suggested that the thickness of Zone F, and of Zone F2 in

particular, is locally variable and that this might be an important local control on

instability at the site. The findings of the 2013 Ground Investigation, and of the 3D

modelling completed subsequently, support this hypothesis, with the aforementioned

steepened dip of the basal concretionary limestone layer resulting in the modelled


55

thickness of Zone F2 varying from around 4m just behind the cliff line to <2m at

exposure, near the toe of the undercliff complex (Figure 11). This finding is

significant, and allows the findings of the 2013 Ground Investigation and previous

investigations to be squared with Burton’s (1929) previous measurement of Zone F2

as being only five feet (1.5m) thick, this presumably having been recorded

significantly seaward of the present cliff line.


Zone F1 of the Barton Clay Formation was proven in all of those boreholes completed

to the east of Fisherman’s Walk during the 2013 Ground Investigation, and in a

number of historical boreholes, boreholes BH10/2012, BH11/2012, BH16/2012 and

BH17/2012 of the 2013 Ground Investigation being too shallow to prove the base of

this unit. Zone F1 does not sub-crop at the site.

The contoured base of Zone F1 (upper image in Figure 11) shows a similar dip and

dip direction to that of the units located above Zone F2, suggesting sedimentation

conditions during the deposition of Zone F2 were probably anomalous and, based on

the occurrence of disturbed fabrics/brecciation in the latter unit (Section 3.2.3.1),

possibly partly erosive into Zone F1. This tentative conclusion is supported by an

isopachyte plot (lower image in Figure 11) that shows Zone F1 to thicken seaward as

the overlying Zone F2 thins.

The interpreted total thickness of around 13m for Zone F is broadly compatible with a

previous estimate of 11.6m (Barton et al., 2006), but is significantly at variance with

Burton’s (1929) measurement of 25 feet (7.6m). This is most likely explained by

Burton’s having measured the thickness of Zone F1 further to the east-northeast

below Naish, where the northerly directed thinning would have significantly reduced

the thickness of this unit (lower image in Figure 11).

4.2.5 Barton Clay Formation (Zone E)

As stated in Sub-section 3.2.3.3, the relatively widely separated limestone concretions

marking the top of Zone E where encountered in a number of the boreholes

completed during the 2013 Ground Investigation, but never those marking the base of

this unit. The topography of the basal surface is presented in Figure 12 and, whilst the

estimated thickness of around 2.0m for this unit is compatible with previous

estimates, it is approximate only.

Note that the apparently more easterly (as opposed to east-northeast) dip of the basal

surface in Figure 12 is likely an artefact of the small number of data points rather than

a true rotation in dip direction.

4.2.6 Barton Clay Formation (Zone D)

Zone D has only been encountered in boreholes completed to the west of the Central

Amenity Area located near to the centre of the site (Figure 13). Based upon the

limited data available, this unit follows the general trend at the site and dips at

around one degree to the east-northeast.

The thickness of Zone D has previously been estimated at between 6.0m (Burton,

1929) and 7.5m (Barton et al., 2006), this range being estimate being compatible with

the findings of the 2013 Ground Investigation.


56

Figure 11: Extract from the 3D geological model showing contoured base of Zone F1 (upper image) and thickness isopachytes (lower image).

Figure 12: Extract from the 3D geological model showing contoured base of Zone E.


57

Figure 13: Extract from the 3D geological model showing contoured base of Zone D.

4.2.7 Barton Clay Formation (Zones C to A)

Due to its relatively great depth of occurrence at the site, the base of Zone C was only

penetrated in borehole BH01/2012 during the 2013 Ground Investigation. Based on

this penetration and on a limited number of historical borehole penetrations, the

modelled base of this unit is presented in Figure 14. Once again, the modelled surface

indicates a consistent east-northeast dip of around one degree. A unit thickness of

around 3.5m is indicated. Due to the lack of borehole penetrations of the base of the

units of the Barton Clay Formation occurring below Zone C, they have not been

modelled and will, therefore, not be discussed further herein.

Figure 14: Extract from the 3D geological model showing contoured base of Zone C.


58

4.2.8 Brickearth

The distribution and thickness of the Brickearth deposits (inclusive of topsoils) have

not been modelled in 3D; however, the surface geophysical surveys completed

during the 2013 Ground Investigation provide a useful indication of the likely

variability within this unit (Figures 15 and 16).

Borehole data from the 2013 Ground Investigation indicate a typical thickness of 1.0m

to 2.0m of Brickearth on the cliff top plateau at the site; however, the rotary drilling

technique employed did not generally allow the lower boundary of this unit to be

accurately placed. The geophysics data presented in Figures 15 and 16, however,

provide a wealth of information, although interpretation of the anomalies is

complicated.

Based upon detailed observations of the backscarp at the top of the undercliff

complex during the 2013 Ground Investigation, and on the interpretations of

TerraDat presented on Figures 15 and 16, the cool-coloured areas (i.e. high

conductivity) on Figure 16 do generally tally with the observed presence of

thickening within the Brickearth deposits. Most notably, the large area of high

conductivity occurring at the far eastern-end of the site (Figure 15), and separated

from lower conductivity areas to the west by a west-southwest – east-northeast

trending lineament, correlates well with the observed eastern boundary of a broad

Brickearth-filled channel in the top of the Plateau Gravel. Note that the sub-crop of

the more clayey strata of Zone J of the Becton Sand Formation (aka the ‘Becton Bunny

Beds’) also likely contributes to the generally higher conductivity of the shallow sub-

surface at the far eastern-end of the site.

4.2.9 Plateau Gravel

The base of the Plateau Gravel is an unconformity formed by fluvial erosion of the

underlying Palaeogene strata by the former ‘Solent River’ during the Pleistocene

Period. The erosive contact is sub-horizontal; however, geophysical anomalies and

the variable thickness proven within the boreholes suggest this contact has a subtle

topography, this probably being manifested as a series of shallow river channels.

Based on the aggregated borehole data population within the 3D model (i.e. 2013 and

selected previous), the Plateau Gravel is around 5m thick on average, with increased

thicknesses detected at the Cliff House Hotel (up to 6.3m), local to Hoskin’s Gap and

the Central Amenity Area (up to 8.1) and in the central part of Marine Drive East (up

to 8.9m). These local anomalies correlate very well with the geophysical

(electromagnetic) survey results presented in plan on Figure 15, which show

decreased surface conductivities (i.e. brown colours) at all of these locations. There is

also tentative evidence in cross-section for these local thickenings (Figure 16);

however, the very low conductivity zones shown as occurring east of chainage 450 on

resistivity section Line 5 on Figure 16 are interpreted to primarily reflect the sub-crop

of Zone I of the Becton Sand Formation.

Inspection of the cliff exposures indicates that the above picture is complicated by the

presence of gravel units within a broader depth of gravelly sand. Such subtle

variability was not generally detected in the boreholes and, consequently, the plan

positions of the in-filled channels in the Plateau Gravel remain relatively poorly

constrained.


59

4.2.10 Colluvium and Made Ground

The character, distribution and comparative thicknesses of the masses of colluvium

existing upon the lithological benches within the undercliff complex at Barton-on-Sea

have previously been described in some detail by Barton and Coles (1984). For this

reason, and because of the temporal and spatial variability of such deposits and their

insignificance with regard deep-seated landsliding, further discussion of the

character and distribution of these deposits will not be provided here.

Discussion of the Made Ground present at the site will also not be discussed here, for

similar reasons to those outlined above.

Figure 15: Extract from TerraDat’s geophysical report showing approximate thickness and distribution of the superficial deposits (TerraDat Figure 8 in Appendix D of Geotechnical Engineering, 2014).


60

Figure 16: Extract from TerraDat’s geophysical report showing approximate thickness and distribution of the superficial deposits (TerraDat Figure 6 in Appendix D of Geotechnical Engineering [2014]).


61

4.2.11 Summary

The assessed thicknesses of the various geological units are summarised in Table 4.1.

Table 4.1: Summary of typical unit thicknesses at the site.

Formation Unit/Zone Thickness (m) Comments

Site wide (inc. undercliff)

Central Amenity Area to the Cliff

House Hotel

(Su

per

fici

al

dep

osi

ts) Colluvium 1-10 n/a Based on Barton & Coles (1984)

Brickearth 1-2 Local thickening within subtle, open channels

Plateau Gravel 5-9 Local thickening within subtle, open channels

Becton

Sand

Zone J ≤3 0 Sub-crops below far eastern-end of site

Zone I ≤11 Sub-crops below majority of site

Chama

Sand

Zone H 6-10 8 Thins toward, and sub-crops at, west-end of site

Zone G <0.2 As a series of pinching and swelling lenses

Bar

ton

Cla

y

Zone F2 1-6 4 Thins rapidly to the southwest as base rises

Zone F1 5-11 9 Thickens rapidly to the southwest as top rises

Zone E 2 Approximate only

Zone D 7 Approximate only

Zone C 3.5 Approximate only

4.3 Relationship between geology and geomorphology

A visual comparison of the modelled 3D geological surfaces with the digital elevation

model (Figures 17 and 18) have allowed the following previously identified spatial

relationships to be confirmed:

the base of Zone D barely rises above beach level within the site and underlies

a mantle of colluvium on the D bench in the Naish Farm area, this bench being

formed at the ‘D Shear’ horizon

Zone F1 forms the scarp face below the lowest bench to the immediate east of

Sea Road Access, with the shear overlying the base of Zone F2 (i.e. the ‘F2

Shear’) forming the top of this bench

the base of Zone G is closely associated with the upper bench at Naish Farm,

this reflecting movement at the ‘G Shear’ horizon.

The ability to use the 3D modelling to explore such relationships highlights the

benefit of its development as part of the present study, and following the receipt of

new monitoring data in late 2015, further significant insights into the causes and

mechanisms of landsliding at Barton-on-Sea can be achieved (Section 4.1.3).


62

Figure 17: 3D view of Cliff House Hotel (5x vertical exaggeration) showing projected outcrop of bases of key strata on the surface of the underclifff and highlighting the relationships between key strata and undercliff topography (Note: Plateau Gravel excluded for clarity).

Figure 18: 3D view of underclifff between the Sea Road Access and Hoskin’s Gap (5x vertical exaggeration) showing projected outcrop of bases of key strata on the surface of the underclifff and highlighting the relationships between key strata and undercliff topography (Note: Plateau Gravel excluded for clarity).


63

5 Cliff behaviour review

5.1 Cliff behaviour units

Based upon field reconnaissance and analysis of aerial photographs using digital

photogrammetry, cliff behaviour units (CBUs) were previously defined across the

Barton-on-Sea frontage by Moore et al., (2003). A CBU reflects the interrelationship

between the processes, mechanisms and associated landforms identified at specific

locations, each CBU being somewhat independent of its neighbours. In this context a

CBU defines a discrete coastal cliff unit of similar physical form, composition and

behaviour.

A preliminary review of the findings of the 2013 Ground Investigation indicates that

no changes to these previously defined CBUs are necessary, although on-going

monitoring of inclinometers and piezometers, and analysis of landslide behaviour

following reactivation over the wet winter of 2013-2014 will provide additional data

and opportunity to review the CBU extents and understanding. The previously

identified CBUs are summarised in Figure 1 and in Table 5.1.

Table 5.1: Cliff behaviour units identified at Barton-on-Sea (after Moore et al., 2003).

CBU Name Description Ground movement potential Historical mean annual cliff retreat

rate (m/yr)

1

Naish

Farm

Active unprotected cliff slopes

subject to seasonal

mudsliding and shallow

translational bench sliding

Seasonal mudslide reactivation and retrogression

following periods of intense or prolonged rainfall.

Episodic movement and retrogression failures occur

during periods of exceptional three month antecedent

effective rainfall (e.g. 2000-2001 winter rainfall)

1.66 ± 0.08

2

Cliff

House

Hotel

Active cliff slopes subject to

deep-seated episodic

translation landslide

movement

Deep seated translational ground movement

associated with high three month antecedent effective

rainfall.

Landslide reactivation and headscarp retrogression

associated with exceptional three month antecedent

effective rainfall

0.63 ± 0.08m

3

Marine

Drive

West


landsliding including seasonal

mudsliding of the lower

slopes and episodic

translational movement of the

Upper cliff slopes



Reactivation and headscarp retrogression associated

with exceptional three month antecedent effective

rainfall

0.40 ± 0.08

4

Barton

Court


landsliding including seasonal

mudsliding of the lower

slopes and episodic

translational movement of the

Upper cliff slopes



Reactivation and headscarp retrogression associated

with exceptional three month antecedent effective

rainfall

0.53 ± 0.08

5

Marine

Drive

East

Unprotected cliff slopes

subject to localised shallow

failures

Localised shallow mudslide reactivation following

periods of intense or prolonged rainfall

0.65 ± 0.08


64

5.2 Landslide failure mechanisms

5.2.1 General

A number of failure mechanisms have previously been identified by various authors

as controlling the location of landsliding across the frontage at Barton-on-Sea. In

summary these include:

Translational sliding at the interface between the Chama Sand Formation and

the underlying Barton Clay Formation (i.e. on the ‘G Shear’). In addition, shear

surfaces have been identified near the base of Zone F2 (the ‘F2 Shear’) in the

Barton Clay Formation (e.g. Hoskin’s Gap), and in the lower part of Zone D

(the ‘D Shear’ - e.g. between Cliff House Hotel to Naish Farm).

Shallow mudslides, associated principally with the Barton Clay Formation and

previously displaced colluvium, where over-steep scarp slopes have been

subject to weathering and localised spalling (Barton & Garvey, 2011). The

mudslides are generally considered to be relatively slow moving masses of

saturated or partially saturated clay rich debris which displace along

translational shear surfaces, often associated with lithological boundaries.

Landslide reactivations of previously failed materials that have been stabilised

in the past through replacement of slip debris with coarse aggregate rock fill.

Landslide ground model sections developed by others for the major named

landslides have previously been presented in the Stage 1 desk study review report

(Halcrow, 2011). A review of the data associated with the 2013 Ground Investigation

indicates that these models remain valid, although future monitoring datasets may

change this.

The exception to this is the Cliff House Hotel landslide, with geophysical surveys

from the 2013 Ground Investigation having provided evidence in support of shearing

both at the ‘F2’ and ‘D Shear’ horizons. Discussion of the findings of geophysical

surveys of the Cliff House Hotel landslide, completed during the 2013 Ground

Investigation, are therefore, presented below.

Discussions are also presented on the possible origin of the major shear surfaces

controlling landsliding at the site, based upon the evidence of shearing encountered

in the borehole core retrieved during the 2013 Ground Investigation.

5.2.2 Cliff House Hotel landslide

5.2.2.1 Geometry and mechanism of landsliding

Barton and Garvey (2011) have previously postulated that the morphology of the

Cliff House Hotel landslide, taken together with data from short-lived

instrumentation previously installed in the undercliff hereabouts, indicates

landsliding at this location to have occurred only at the ‘D Shear’ horizon, and

significantly (Section 5.2.2.2) not at the ‘F2 Shear’ horizon. Conversely, and in direct

response to Barton and Garvey (2011), Hosseyni et al. (2012) have proposed, based

upon morphology alone, that the Cliff House Hotel landslide is a two-tier failure that

involves both a small upper compound landslide perched on the ‘F2’ or ‘H Shear’


65

horizon, combined with a lower small compound slide with its base at the ‘D Shear’

horizon.

Surface seismic refraction (‘P’ and ‘S wave’), resistivity tomography and electro-

magnetic ground conductivity surveys were completed by TerraDat, around the Cliff

House Hotel landslide during the 2013 Ground Investigation (Figure 4). The results of

these geophysical surveys present a complex picture, with interpretation of the

resistivity data being complicated by the presence of standing water and perched

groundwater within the colluvium. However, a number of features of interest have

been identified within the seismic refraction data, which are relatively insensitive to

groundwater saturation.

Based upon Figures 19 and 20, which are reproduced from TerraDat’s survey report

in Appendix D of the 2013 Ground Investigation factual report (Geotechnical

Engineering, 2014), the following tentative conclusions are drawn:

Seismic refraction Lines 1 and 2 suggest that the boundary between intact clay

and slipped material is located at or just below 0mAOD towards the bottom of

the landslide.

The same data suggest this basal boundary rises from around 0mAOD to

around 15mAOD below the central part of the landslide, with Line 1

suggesting a distinct scarp between these two elevations (Survey Line 3 also

supports the presence of a boundary between disturbed and relatively intact

strata at 15mAOD below the upper half of the landslide).

Pronounced steps in the shallow sub-surface suggest the presence of multiple

rotated and/or translated intact blocks within the upper section of the

landslide.

Mudsliding subsequent to the main deep-seated landslide has significantly

reworked the uppermost, saturated layers of the landslide.

Subject to the results of further detailed analyses of these data and the findings of the

on-going monitoring at the site during 2014 and 2015, therefore, it is tentatively

concluded that the seismic data indicate the Cliff House Hotel landslide to be a two-

tiered system similar to that proposed by Hosseyni et al., (2012). The tentative

identification of a developing shear zone at the ‘F2 Shear’ horizon behind the Cliff

House Hotel landslide would seem to further support the existence of a two-tiered

failure at this location (Sections 3.2.3.1 and 5.2.3).

Note that a re-analysis of the data associated with monitoring instrumentation (now

lost) that previously existed within this landslide has not been completed as part of

the present report.


66

Figure 19: Results of seismic refraction and resistivity geophysical surveys along Line 1 (bottom right – extract from Geotechnical Engineering [2014]).


67

Figure 20: Results of seismic refraction and resistivity geophysical surveys along Line 2 (bottom right – extract from Geotechnical Engineering [2014]).


68

5.2.2.2 Presence of local anomalies within and/or absence of ‘F2 Shear’ in Zone F2

Barton and Garvey (2011) have also previously noted that, though prominent within

the undercliff west of Naish Farm, exposures of the ‘F2 Shear’ are very rare between

there and the areas east of the Sea Road Access, possibly due to the absence of

shearing at this horizon within this interval, including at the Cliff house Hotel

landslide. Conversely, and once again in direct response to Barton and Garvey’s

paper, Hosseyni et al., (2012) dismiss Barton and Garvey’s implicit suggestion (based

on their landsliding models for the Hoskin’s Gap, Hoskin’s Gap West and Central

Amenity Area landslides), that the ‘F2 Shear’ is present at different (although closely

spaced) stratigraphic positions east of the Cliff House Hotel landslide, stating that

they had located and traced this slip surface in the undercliff west of the Cliff House

landslide and were confident that it only followed one stratigraphic horizon.

As discussed in Section 3.2.3.1 (see also Table 3.1 and Photos 6 to 10) one of the key

findings of the 2013 Ground Investigation was the existence of significant lithological

complexity in Zone F2 of the Barton Clay Formation. Of potentially greatest

significance was the observation that the local development of a red-brown

weathering band of hardened nodules above the base F2 concretionary limestone

horizon within borehole BH19/2012 had apparently forced the developing ‘F2 Shear’

encountered in this borehole to rise from around 0.15m above the concretionary

limestone horizon to around 0.5m above this horizon (this layer is also present at

outcrop below the Sea Road Access, and has been previously noted by Garvey 2007).

This finding is critical as it may provide a simple explanation for the postulated

‘absence’ of this shear at and immediately west of the Cliff House Hotel – i.e. it might

not lie where it was looked for. The recorded presence of second horizon of fully

developed limestone concretions located even further above the basal horizon of

concretionary limestone at exposure below Naish Farm (Photo 10), may indicate that

the ‘F2 Shear’ can be forced even further from its normal position locally. In short, the

local development of stiff layers just above the basal concretionary limestone horizon

in Zone F2 may locally have forced the ‘F2 Shear’ to develop at a higher elevation than

is normal further to the east.

The above hypothesis for the reported ‘absence’ of the ‘F2 Shear’ west of Cliff House

Hotel is both broadly compatible with Barton and Garvey’s (2011) observation that

the F2 bench has a complex morphology below Naish Farm, and with their implicit

suggestion that it may be present at different but closely spaced stratigraphic

positions east of the Cliff House Hotel landslide. The above explanation is also

compatible with the landsliding model proposed by Hosseyni et al., (2012) for Cliff

House Hotel landslide, and with the observed occurrence during the 2013 Ground

Investigation of a propagating shear zone behind the Cliff House Hotel Landslide.

Note that the detailed analysis of further movements below the Cliff house Hotel, of

data from the inclinometer installed in borehole BH04/2012 behind this landslide in

2013, and of any new in situ exposures of Zone F2 revealed below Naish Farm, may

allow further confidence in the above hypothesis to be achieved.

5.2.3 Controls on shear zone development in the Palaeogene strata

The factors possibly controlling development of shearing at preferred horizons at

Barton-on-Sea have long been debated. Barton et al., (2006), developing the work of

Barton (1973), speculated that the development of shear horizons at Barton-on-Sea


69

might be related to elastic rebound in response to the lateral stress relief brought

about by rapid coastal recession or, flexural slip along intrinsically weak horizons

during tectonic deformation. With regard to the latter mechanism, Edward

Bromhead, formerly professor of geotechnical engineering at Kingston University,

has speculated (Bromhead, 2013) that such an intrinsically weak horizon could be

afforded by the presence of an altered volcanic ash layer.

Burland et al. (1977) determined that shear zones could propagate behind an

excavated face in over-consolidated, high-plasticity stratified Oxford Clay, to a

horizontal distance of between 1.0 and 1.5 times the height of the excavated face. The

mechanism of shear zone formation was determined to be progressive failure in

response to the elastic rebound of the clay strata following relatively rapid lateral

unloading due to continued clay extraction. Significantly, the authors also

determined that shear zone development at that site was focused a small distance

above a comparatively stiff sand layer. Later instrumented field trials by Cooper et al.,

(1998) in Gault Clay at Selborne in Hampshire broadly confirmed the findings of

Burland et al.

Based upon the relationships determined in these papers, and presuming that coastal

erosion at Barton-on-Sea has been rapid enough to allow the current depth of the

shear horizons to be taken as the height of the ‘excavated face’, the theoretical

possible inland distances that shear surfaces could have developed at Cliff House

Hotel (existing ground level 33.8mAOD approx., 1.5 plan distance to face height ratio

assumed) are estimated as:

‘G Shear’ (16.1mBGL in BH19/2012) = 24m

‘F2 Shear’ (19.6mBGL in BH19/2012) = 30m

‘D Shear’ (37.5mBGL in BH19/2012, approx.) = 56m

The postulated shears encountered just above each of the two shallower horizons in

borehole BH19/2012 of the 2013 Ground Investigation (Section 3.2) could, therefore,

feasibly have been the margins of the ‘G’ and ‘F2 Shears’, propagating inland in

response to continued cliff instability stress relief, as could have the polished parting

found just above the base F2 concretions further to the east at 24.6mBGL in borehole

BH07/2012. The physical character of these shears surfaces, i.e. single or up to two

highly polished, planar partings, is also compatible with a progressive failure origin

(Cooper et al., 1998).

The presence of a potential shear behind the cliff line at the ‘D Shear’ horizon in

borehole BH19/2012 is less easily attributed to stress relief alone. Feasibly the local

presence at the ‘D Shear’ horizon of intrinsically weaker clays (Section 3.4) might

allow the developing ‘end region’ of this shear to propagate further inland behind the

cliff line than is predicted based on the work of Burland et al. (2006). Alternatively (or

feasibly in combination), the Cliff House Hotel landslide could be a single-tier system

with a very steep rear failure surface that has allowed stress relief to within 30m or so

(in plan) of the cliff crest (such a failure surface geometry has previously been

proposed by Barton and Garvey [2011]). In the latter case, however, such a failure

geometry is not well supported by geophysical surveys completed during the 2013

Ground Investigation (Section 5.2.2).


70

The physical character of the potential ‘D Shear’ recorded behind the cliff line in

borehole BH19/2012 (i.e. multiple polished, undulating planes with rolled

‘intraclasts’, within some 30mm of anomalously coloured, clay-enriched material), is

also notably different to the two shallower shears. This distribution of shear strains

through a comparatively thicker zone could be simply due to the presence of a

similar thickness of intrinsically weak clay material; however, the ‘D Shear’ also has

strong affinities with the active shears encountered in the undercliff complex – i.e. the

‘D Shear’ existing behind the cliff line at the site may have been subject to

significantly higher strains then than the shallower shear horizons. The intimate

association of the ‘D Shear’ with the base of a characteristically very closely fractured

mudstone exhibiting complex, syn-sedimentary and/or tectonic fabrics (Section

3.2.3.4) is also unique at the site.

Based upon the above described evidence, and upon a preliminary review of the

results of laboratory testing on material from the ‘D Shear’ horizon (Sub-section 3.4)

that suggests the presence of compositional anisotropy at the that horizon, it is

tentatively concluded that an alternative mechanism to stress relief-effected rebound

(i.e. a tectonic mechanism) may have been at least partly involved in the propagation

of the ‘D Shear’.


71

6 Conclusions

Following a review of the data associated with the 2013 Ground Investigation, of

detailed observations made of the available Palaeogene exposures during the course

of these works, and of a reinterpretation of the borehole logs and data associated with

previous investigations completed at the site, an innovative 3D ground model has

been developed for the site in a GIS format.

Further review of the observational data collected during 2013 in combination with

3D visualisation of the strata in the sub-surface using the 3D model developed, has

subsequently allowed the following significant conclusions to be drawn.

6.1 Strata distribution and thickness

The Palaeogene strata are generally inclined at around one degree to the east-

northeast, although the base of Zone F2 dips anomalously at around two

degrees to the northeast (below).

Zone H of the Chama Sand Formation gradually thins toward its sub-crop at

the western-end of the site. The Zone G ‘stone band’ is generally uncemented

in the sub-surface, and is likely present as a series of laterally discontinuous

but closely associated lenses spread through a metre of more of clay.

Zone F2 of the Barton Clay Formation thins, and the underlying Zone F1

correspondingly thickens, relatively rapidly towards the southwest as the base

of the former unit rises steeply in the same direction, this explaining the

significant variability in the thicknesses of these units previously recorded,

most notably on the undercliff. The total thickness of Zone F, however, is

relatively consistent across the site.

The erosive contact between the superficial deposits (i.e. Plateau Gravel) and

the underlying Palaeogene strata is of an undulating character, although the

plan locations of the generally shallow and broad ‘channels’ cut into the top of

the Palaeogene strata remain relatively poorly constrained.

6.2 Hydrogeology

The Plateau Gravels, Becton Sand and Chama Sand Formations form the

primary aquifer units at the site, and it is within these aquiferous units that

groundwater flows towards the coastal cliffs are concentrated.

The Plateau Gravel-infilled channels in the top of the Palaeogene strata may

funnel groundwater onto the top of the undercliff in the western third of the

site, whilst the presence of sand and clay lamina in the lower half of Zone H

and of locally relatively porous and permeable lenses of fossil debris at the

Zone G horizons of the Chama Sand Formation are likely significant factors

focusing sub-surface groundwater migration into the undercliff complex.

The largely separate groundwater regime in the low permeability clays of the

Barton Clay Formation is characterised by sub-hydrostatic piezometric

pressures in the form of an inverted piezometric profile, this probably largely

reflecting historically high rates of erosion and unloading.


72

6.3 Controls on principal shear horizons

The highly polished and planar character of the partings encountered in Zone

H and in Zone F2, in boreholes located a short distance behind the cliff line

during the 2013 Ground Investigation, is wholly compatible with these having

been the ‘end regions’ of the ‘G’ and ‘F2 Shears’ proven to exist within the

undercliff complex, propagating inland in response to continued landsliding-

effected stress relief and consequent elastic rebound of the Palaeogene strata.

This is supported by the proven absence of significant variations in the

material properties of the clays located at these two horizons that might have

previously resulted in the localisation of strains thereabouts during tectonic

movements.

The lengths of shear zone that have propagated at the ‘G’ and ‘F2 Shear’

horizons behind the cliff line can be expected to increase to the east, due to the

dip of the strata and the consequential increase in effective cliff height. The

significant northerly dip component on the base of Zone F2 will presumably

also result in an increase in the length of shear forming behind the cliff line at

any point as the cliff line regresses north-northeast.

The presence of Zone G of the Chama Sand Formation as a series of laterally

discontinuous but closely associated lenses may have led to the propagation of

the ‘G Shear’ at subtly different horizons across the site, as has been speculated

by previous authors.

The presence, at the ‘D Shear’ horizon encountered behind the cliff line during

the 2013 ground investigation, of multiple, undulating partings within up to

40mm of material (in stark contrast to the simple partings observed at the

overlying shear horizons); of consistent yet distinct fabrics and material

associations; and of varied physical properties, supports previous speculation

by others that the location and propagation of this shear may have been at least

partly due to the prior presence of an intrinsically weak layer at this horizon.

6.4 The Cliff House Hotel landslide ground model

Surface geophysical surveys completed during the 2013 Ground Investigation

suggest the Cliff House Hotel landslide is a probably a two-tiered system. The

tentative identification of a developing shear zone at the ‘F2 Shear’ horizon

behind this landslide would seem to further confirm the ‘F2 Shear’ to be a

permissible failure horizon at this location, this having previously been the

subject of much debate. Updating the geomorphological map of this region will

help support revision of the ground model for this landslide.

The existence of significant lithological complexity in Zone F2 of the Barton

Clay Formation, including the local development of hardened nodules and/or a

second level of limestone concretions above the base F2 concretionary

limestone, may force the ‘F2 Shear’ to rise above its expected position between

the Sea Road Access and Naish Farm, this potentially providing a relatively

simple explanation for the postulated ‘absence’ of this shear at its ‘normal’

position, and for the locally complex morphology of the F2 bench on the

undercliff below Naish Farm. .


73

7 References

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Burton, E. St.J. 1933. Faunal horizons of the Barton Beds of Hampshire. Proceedings of

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Costain. 2003. Barton on Sea Outfall: Ground Investigation Factual Report (Contract

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Fort, D.S., Clark, A.R. & Cliffe, D.G. 2000. The Investigation and Monitoring of

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McClelland Limited to New Forest District Council (Project No. 93/2019).

Fugro-McClelland Limited. 1991. Final Report, Soils Investigation: Static Cone

Penetration Tests at Barton-on-Sea Hampshire (Phase I). Report by Fugro-McClelland

Limited to New Forest District Council (Project No. 91/2390).

Gardner, J.S., Keeping, H. & Monkton, H.W. 1888. The Upper Eocene, comprising the

Barton and Upper Bagshot Formations. Quarterly Journal of the Geological Society of

London, 6, 252-281.

Garvey, P.M. 2007. A study of the reactivation of landsliding at Barton-on-Sea,

Hampshire, following stabilisation works in the 1960s. Unpublished MSc dissertation,

University of Southampton.

George Wimpey. 1960. Report on Site Investigation for Proposed Cliff Stabilisation at

Barton-on-Sea, Hampshire. Report by George Wimpey & Co. Ltd (Ref: S/2189) to

Borough of Lymington, March 1960.

Geotechnical Engineering. 2014. Barton-on-Sea Cliff Instability – Factual Report on

Ground Investigation (Report ref 28004). Report for New Forest District Council,

February 2014.

Halcrow. 2011. Barton on Sea Cliff Instability Preliminary Study: Stage 1 Desk Study

Review. Report by Halcrow for New Forest District Council, November 2011.

Halcrow. 2006. Christchurch Bay Coastal Defence Strategy Study. Report by Halcrow

Group Limited for New Forest District Council.

Halcrow. 1987. Barton-on-Sea Cliff Stabilisation: Report on Permanent Remedial

Measures for the Landslide between Groynes 15 and 17 (Hoskin’s Gap). Report by Sir

William Halcrow & Partners for New Forest District Council, December 1987.

Halcrow. 1971. Highcliffe: Report on the instability of the Cliffs and Recommended

Remedial Works. Report by Sir William Halcrow & Partners for Borough of

Christchurch.

Halcrow. 1969. Barton-on-Sea Cliff Stabilisation (Stage 3): Report on Proposed

Protection Works to and the Stabilisation of Barton Cliffs from Limit of Stage 2 (Cliffe

Road) Westward to Chewton Bunny. Report by Sir William Halcrow & Partners to

the Borough of Lymington, January 1969.

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works of undercliff drainage carried out between May and October 1964. Report by

Sir William Halcrow & Partners to the Borough of Lymington, August 1965.

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William Halcrow & partners to the Borough of Lymington, April 1960.

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review. Report to New Forest District Council, November 2011.

Halcrow. 2012. Barton on Sea cliff instability preliminary study: Stage 2 ground

investigation options. Report to New Forest District Council, March 2012.


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Hosseyni, S., Torii, N. & Bromhead, E.N. In Press. Discussion of Barton & Garvey

2011. Reactivation of landsliding following partial cliff stabilisation at Barton-on-Sea,

Hampshire, UK Quarterly Journal of Engineering Geology & Hydrogeology.

Melville, R.V. & Freshney, E.C. 1982. British Regional Geology: The Hampshire Basin and

Adjoining Areas. British Geological Survey, London. Her Majesty's Stationery Office.

146 pp.

Moore, R., Rogers, J., Woodget, A. & Baptiste, A. 2010. Climate change impact on cliff

instability and erosion. EA Flood and Coastal Risk Management Conference 2010, Telford.

Moore, R., Fish, P., Glennerster, M. & Bradbury, A. 2003. Cliff Behaviour Assessment:

A Quantitative Approach using Digital Photogrammetry and GIS. In: 38th DEFRA

Flood & Coastal Management Conference.

Rendel. 2002. The Coastal Landslides at Barton-on-Sea, Hampshire, UK: Christchurch

Bay Strategy Study Input. Report by High-Point Rendel (Ref: 1589/R/01/02) for New

Forest District Council and Halcrow Group Ltd, October 2002.

Rendel. 1994. Barton-on-Sea Cliff Stabilisation: Recommended Stabilisation Measures

(Ref: R/H240/03). Report by Rendel Geotechnics for New Forest District Council,

October 1994.

Rendel. 1993a. Cliff Instability at Barton-on-Sea and Options for Stabilisation:

Discussion Document. Report by Rendel Geotechnics Ltd for new Forest District

Council (Ref: R/H240/02), October 1993.

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Rendel Geotechnics Ltd for new Forest District Council (Ref: R/H240/01), September

1993.

Robert West. 1991. Report on Analysis of Cliff Stability at Hoskin’s Gap, Barton-on-

Sea. Report by Robert West & Partners for New Forest District Council (Ref:

4314/01/MAT/bs/WP13), April 1991.

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Review. Report by Royal Haskoning to Bournemouth Borough Council.

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‘Boreholes Data’ and ‘Inclinometer Data’). Report by Soil Mechanics Ltd for New

Forest District Council, May 1991.

Soil Mechanics. 1988. Baron-on-Sea Cliff Stabilisation: Site Investigation for the

Landslide between Groynes 15 and 17 (Hoskin’s Gap). Report by Soil Mechanics Ltd

for New Forest District Council (Ref: 7500/17), June 1988.

Structural Soils. 1989. Barton on Sea: Engineering logs for boreholes BH1 to BH6.

Extract of ground investigation report by Structural Soils Ltd for Robert West &

Partners, 1989.

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cutting slopes in overconsolidated clay. Geotechnique, 23, 531-539.

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southern England. Internet site: www.soton.ac.uk/~imw/barton.htm. By Ian West, Romsey


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and School of Ocean and Earth Science, National Oceanography Centre, Southampton,

Southampton University. Version: January 2014.

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scale geological sheet 329 (England and Wales). His Majesty's Stationery Office. 79pp.


77

A Borehole logs from current and past ground investigations


78

B Results of specialist testing by Drs Barton and West

The following tables, figures and photographs provide detailed information on the

specialist analysis undertaken at University of Southampton on the area of the

insipient shear at the ‘D’ horizon in BH4 from Cliff House Hotel. Samples were taken

from between 37.8 and 37.3m BGL (equivalent to -3.5 to -4.0m OD).

Information presented comprises:

A sketch log showing the location of samples

Results of X-ray diffraction (XRD) analysis showing bulk mineralogy of

samples

Results of X-ray diffraction analysis showing clay mineralogy of samples

Scanning electron microscope (SEM) imagery of polished thin section

samples taken parallel and at right angles to the bedding.


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Figure 1. Borehole log, showing location of samples relative to the lithological units

Lithological Units

In following tables

A

B

C

D


80

Table 1. Bulk mineralogy determined by XRD, Barton-on-Sea Core 4-2012, core depths 37.20 - 38.30m. (Abbreviations: K-Feld = potassium feldspar, Plag Feld= plagioclase feldspar). A = Very dark green clay, B= Pale greenish grey clay, C = Chocolate brown clay, D = Very dark green clay.

A. Absolute data

Lithology

Sample

No.

Core Depth Mineralogy

BULK

TOTAL Lower Upper K-Feld Plag Feld Calcite Pyrite Quartz

Total

Clay

A 2 37.28 37.29 1.5 1 1.1 29.1 53.1 85.8

7 37.33 37.34 1.4 1.4 32.9 57.9 93.6

8 37.34 37.35 1.2 1 1 33.3 56.3 92.8

11 37.37 37.38 1.1 0.8 1.1 33.1 57.9 94

B 18 37.44 37.45 1.5 0.6 0.8 32.8 54.1 89.9

22 37.48 37.49 1 0.8 0.7 33.8 54.8 91.2

28 37.54 37.555 1.1 0.9 1.6 34.2 56.4 94.3

C 30 37.56 37.57 1.6 1.5 28.7 56 87.8

D 32 37.575 37.585 1.4 0.8 0.7 40 48 90.9

35 37.6 37.61 1.3 0.7 1 31.5 55.1 89.6

37 37.62 37.63 1.1 0.8 0.9 34 57.1 93.9

40 37.83 37.84 1.2 0.6 1.2 31.8 54.7 89.5


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B. Normalized data.

Lithology

Sample

no.

Core Depth Mineralogy

BULK

TOTAL

Clay/Quartz

Ratio Lower Upper K-Feld

Plag

Feld Calcite Pyrite Quartz

Total

Clay

A 2 37.28 37.29 1.7 1.2 1.3 33.9 61.9 100.0 1.82

7 37.33 37.34 1.5 1.5 35.1 61.9 100.0 1.76

8 37.34 37.35 1.3 1.1 1.1 35.9 60.7 100.0 1.69

11 37.37 37.38 1.2 0.9 1.2 35.2 61.6 100.0 1.75

B 18 37.44 37.45 1.7 0.7 0.9 36.5 60.2 100.0 1.65

22 37.48 37.49 1.1 0.9 0.8 37.1 60.1 100.0 1.62

28 37.54 37.555 1.2 1.0 1.7 36.3 59.8 100.0 1.65

C 30 37.56 37.57 1.8 1.7 32.7 63.8 100.0 1.95

D 32 37.575 37.585 1.5 0.9 0.8 44.0 52.8 100.0 1.20

35 37.6 37.61 1.5 0.8 1.1 35.2 61.5 100.0 1.75

37 37.62 37.63 1.2 0.9 1.0 36.2 60.8 100.0 1.68

40 37.83 37.84 1.3 0.7 1.3 35.5 61.1 100.0 1.72


82

Table 2. Clay mineralogy (<2 micron fraction) determined by XRD, Barton-on-Sea Core 4-2012, core depths 37.20 - 38.30m. A. Sample data, B. Average lithology values. Note: Peak widths are the half height widths in degrees two theta (Abbreviations: Avg = average, SD = standard deviation). A = Very dark green clay, B= Pale greenish grey clay, C = Chocolate brown clay, D = Very dark green clay.

A Sample data

Lithology

Sample

No.

Core Depth Clay % Total Clay Clay

Ratios

(K+C)/I

S/I

Lower Upper

Smectite(S

)

Illite (I) Kaolinite(

K)

Chlorite(C

)

K+C

A 2 37.28 37.29 48.3 35.3 10.3 6.1 16.4 100 0.46 1.37

7 37.33 37.34 51.9 32 10.3 5.8 16.1 100 0.50 1.62

8 37.34 37.35 51.6 31.8 10.3 6.3 16.6 100 0.52 1.62

11 37.37 37.38 50.5 32.2 11 6.3 17.3 100 0.54 1.57

B 18 37.44 37.45 44.3 34.9 12.7 8.1 20.8 100 0.60 1.27

22 37.48 37.49 47.8 34 11.8 6.4 18.2 100 0.54 1.41

28 37.54 37.555 49.4 33.5 10.6 6.6 17.1 100.1 0.51 1.47

C 30 37.56 37.57 52.3 31.8 10.1 5.8 15.9 100 0.50 1.64

D 32 37.575 37.585 46.8 34.2 12.3 6.7 19 100 0.56 1.37

35 37.6 37.61 46.8 34 11.4 7.8 19.2 100 0.56 1.38

37 37.6 37.63 48.4 32.7 11.9 7 18.9 100 0.56 1.48

40 37.83 37.84 48 34.5 11 6.5 17.5 100 0.56 1.39


83

B. Average lithology values

Lithology S I

K+C Ka C

A Avg 50.6 32.8 16.6 10.5 6.1

SD 1.63 1.66 0.51 0.35 0.24

B Avg 47.2 34.1 18.7 11.7 7.0

SD 2.61 0.71 1.90 1.05 0.93

D Avg 47.5 33.9 18.7 11.7 7.0

SD 0.82 0.79 0.78 0.57 0.57


84

Figure 1. Results of XRD analysis of clay minerals


85

SEM imagery of lithological units taken normal to bedding. A (very dark green), B (pale green), C (chocolate

brown) and D (very dark green)

Note the chocolate brown zone material shows an incipient shear surface. This borehole was drilled in an area

that has not yet failed, indicating microfabric developments are proceeding inland of the main headscarp. At

the time of drilling, the borehole was 22.5m inland of the headscarp, but cliff recession over the winter of

2013/14 means this distance is now reduced to c. 15m.

Sample 1 Dark green unit, depth 37.320 – 37.355 m BGL, magnification x 250


86

Sample 2. Pale green unit. Depth 37.455 – 37.490 m BGL, magnification x 250


87

Sample 3. Chocolate brown unit, depth 37.555 – 37.565 m BGL, magnification x 250


88

Sample 3. Chocolate brown unit, depth 37.555 – 37.565 m BGL, magnification x 700


89

Sample 4. Dark green unit, depth 37.600 – 37.630 m BGL, magnification x 250


90

SEM imagery of lithological units taken parallel to bedding .A (very dark green), B (pale green), C (chocolate

brown)

Sample ST1. Dark green unit, depth 37.320 – 37.355 m BGL, magnification x 400


91

Sample ST2. Pale grey unit, depth 37.455 – 37.490m BGL, magnification x 400


92

Sample ST3. Chocolate brown unit, depth 37.555 – 37.565m BGL, magnification x 400


93

C Guide to the GIS

The GIS provided with this report comprises two components: an ArcView 10.1 GIS

database that includes all data assembled and analysed to date and an ArcScene 3D

model that can be used to visualise some of the data. Both applications point to the

same source data.

In order to keep file sizes to a manageable level, we have excluded from the database

all aerial imagery that have been provided by the Council. The Council is free to add

these, and other data, to their own version of the GIS.

The ArcView GIS layer ‘Barton Ground Model.mxd’ links to all data provided and is

the recommended method of viewing the data provided. It is designed to open in

ArcGIs version 10.1 and comprises the following layers:

GI data

- 2013 boreholes and monitoring

- Historical ground investigations (used to develop the 3D model)

- TerraDat geophysics lines / polygons – as surveyed locations of the 2013

resistivity and seismic surveys. See Geotechnical Engineering Ltd’s factual

report for results of these surveys

Geomorphological mapping (results of field mapping in 2011)

- Cliff behaviour units

- Slope morphology recorded during 2011 field mapping

- Interpreted geomorphology following 2011 mapping

Geological data (processed results of 2013 and historical GI data)

- Modelled surfaces for principal geological horizons (the surface represents

the base of the named geological unit and was derived from interpolation

between known elevations interpreted from new and past boreholes)

- Contours on these surfaces (highlights the dip direction and angle of the

strata)

- Strata outcrops (where these surfaces crop out on the cliff face)

Laser scans and difference models

- Three surveys undertaken in Sept 2012, November 2012 and March 2013

- Calculated differences between these three surfaces – Sept 12 to Nov 2012,

Nov 2012 to March 2013 and total change between Sept 2012 and March

2013. Red colours indicate reduction in elevation (through erosion or

movement of material), green colours indicate increase in elevation

(through accumulation of material)

- Mapped change of sheet piling and the headscarp between Sept 2012 and

March 2013


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LiDAR. Only the 2009 LiDAR data are provided here. Other LiDAR and aerial

photography can be added is wished.

The ArcScene 3D Model comprises the following layers. The scene has 10x vertical

exaggeration. This can be adjusted in the Layer properties:

Boreholes (location of 2013 and historical boreholes used to create the modelled

surfaces)

Modelled surfaces for principal geological horizons (the surface represents the

base of the named geological unit and was derived from interpolation between

known elevations interpreted from new and past boreholes)

Strata outcrops (where these surfaces crop out on the cliff face)

2009 LiDAR data

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