MERSEY GATEWAY BRIDGE PROJECT...MERSEY GATEWAY BRIDGE MER-DJV-REP-ENV-02-333014 I 8 MARCH 2017 Document Control Sheet Rev. Status Date By Check Approved 0 1st Draft 02/10/2014 DT DJC

MERSEY GATEWAY

BRIDGE PROJECT

MERSEY GATEWAY EMBANKMENT

PORE PRESSURE & GROUNDWATER MIGRATION ANALYSIS (GUSSION TRANSPORT SITE) MARCH 2017

MER-DJV-REP-ENV-02-333014

MERSEY GATEWAY BRIDGE

MER-DJV-REP-ENV-02-333014 I 8 MARCH 2017

Document Control Sheet

Rev. Status Date By Check Approved

0 1st Draft 02/10/2014 DT DJC DH

1 2nd

Draft 19/12/2014 DT DJC LB

2 For Approval 08/03/2017 RE/DT LB DJC

URS Infrastructure & Environment UK Limited Bridgewater House Whitworth Street Manchester M1 6LT

Telephone: +44 (0) 161 907 3500 Fax: +44 (0) 161 907 3501 Web: www.urs.com

This submission is prepared on behalf of and submitted by the Merseylink Consortium in accordance with the provisions of the Project Agreement for the Mersey Gateway Bridge

Project executed between Halton Borough Council and Merseylink CJV which comprises FCC Construccion, Samsung C&T Corporation and Kier Infrastructure and Overseas

Limited.

This report may not be relied upon by any other party (save for Halton Borough Council) without the prior written agreement of one or all of FCC Construccion, Samsung C&T

Corporation and Kier Infrastructure and Overseas Limited, save to the extent that (i) disclosure and/or reliance of this report is permitted in accordance with any purpose and

intention of the Project Agreement and (ii) disclosure is permitted by Halton Borough Council to the Board, to its advisors and otherwise in accordance or as contemplated by the

terms of the Project Agreement.


MER-DJV-REP-ENV-02-333014 II 8 MARCH 2017

CONTENTS

1 INTRODUCTION ...................................................................................................... 1

2 ASSESSMENT ......................................................................................................... 2

2.1 BASIS OF ASSESSMENT ........................................................................................ 2

2.2 CONCEPTUAL SITE MODEL – MADE GROUND OVERLYING ALLUVIUM AND GLACIAL TILL .......................................................................................................... 2

2.2.1 MODEL DETAILS AND LIMITATIONS ........................................................................ 2

2.2.2 RESULTS ................................................................................................................... 3

3 CONCLUSION .......................................................................................................... 5

Appendices

Appendix A: ConSim Risk Assessment Model Input Parameters

Appendix B: Pore water Pressure Calculations


MER-DJV-REP-ENV-02-333014 1 8 MARCH 2017

1 Introduction

URS was commissioned to assess the possibility of lateral groundwater migration due to

embankment construction activities for the Gussion Transport Site within the Mersey

Gateway project and potential risks of resultant contaminant migration. A similar

assessment has been undertaken for the adjacent Sammy Evans Scrap Yard site.

This assessment updates a previous Embankment Pore Pressure & Groundwater

Migration Analysis for Gussion Transport Site dated 12th February 2014 by taking into

account further detailed assessment of potential excess pore water pressures that may

be generated by the proposed embankment construction, including maximum potential

pore water pressure, timescales for pore water pressure dissipation and the likely radius

of influence of the embankment loading assumed to cause the excess pore water

pressure.

As a worst case the assessment does not take into account of the influence of soil mixed

columns (see MER-DJV-REP-GEO-01-440153 – Ground Treatment Section 1:

Geotechnical Design Report) which would reduce the potential impact on pore water

pressure from the embankment construction. The assessment of potential excess pore

water pressures also takes into account a conservative time period for embankment

construction (see MER-DJV-REP-GEO-01-440153).



2 Assessment

2.1 Basis of Assessment

One ground model scenario has been assessed. The assumed sequence of strata is as

follows:

- Made Ground (0.3m to 3.5) overlying Alluvial Silts and Clays (~0.5m) overlying Glacial Till

It was considered appropriate to assess a scenario that included for the flow of

groundwater within the Made Ground and Alluvium taking into account the nature of the

ground conditions at the Gussion Transport site. The potential for pore water pressure

increase in the granular Glacial Till is considered to be negligible and has therefore not

been included in the assessment. The detail of the scenario is given below.

The aim of the assessment was to determine the travel time for contaminant migration

from a groundwater contaminant source of arsenic, ammonia, and TPH (Aromatic EC12-

EC16) from the Gussion Transport site to a hypothetical down-gradient receptor located

250m from the site boundary. It should be noted that the distance to the receptor is

conservative as the River Mersey is located at a distance of over 700m down-gradient of

the site. This assessment is not intended to be a detailed quantitative controlled waters

risk assessment for the site and as such does not include all potential contaminants of

concern within the site (see MER-DJV-REP-ENV-00-333001 - Ditton Junction / Mersey

Vale Area: Conceptual Site Model Report). The determinands included in the

assessment were selected to provide an indication of the potential impacts of the

embankment construction on contaminants which exhibit differing physico-chemical

properties. The ConSim risk assessment model was used for the analysis. Details of the

input parameters for the ConSim risk assessment model are included in Appendix A.

Three ConSim simulations were made for the scenario; one for the existing case i.e.

current groundwater gradient, a second for when the embankment load induces an

increase in pore water pressure resulting in a temporary artificially increased hydraulic

gradient in the radius of influence of embankment loading and a final simulation for

contaminant travel outside the radius in influence of embankment loading.



2.2 Conceptual Site Model – Made Ground Overlying Alluvium and Glacial Till

The graphical representation of the scenario is detailed below.

2.2.1 Model Details and Limitations

— The ground investigation reports show that the groundwater within the Glacial Till is primarily encountered within the granular components within the Till. As such the predominantly cohesive Glacial Till is effectively a barrier to vertical migration of groundwater within the Made Ground and Alluvium.

— The ground model and groundwater body included in the assessment is based on the details included in the report MER-DJV-REP-ENV-00-333001.

— Groundwater gradient of 0.003 in pre-embankment case (from Gifford, 2011 report).

— Excess pore water pressures were calculated from assessment of the degree of consolidation of the Made Ground during and following embankment construction, assuming the embankment is constructed in 2m lifts. A conservative value for the coefficient of consolidation (cv) of 4 m2/year (taken from review of geotechnical test results of the Made Ground) was used in the analysis.

— Worst case pore water pressures due to embankment construction were calculated to be 156 kPa (maximum), 80 kPa (average) and 20 kPa (minimum) (see Appendix B). This maximum pore water pressure is based on a five-week construction period.

— Pressure Head (ψ ) in metres was calculated as detailed below:

w

p

γψ =

where,

p = pressure in kPa (i.e. 156, 80 and 20 kPa)

10m Receptor assumed to be 250m from embankment

Glacial Till

Made Ground

Pressure Head = 16 – 2m

Pressure Head = 0.0m

25m from embankment centre section

Embankment

Groundwater Flow

Alluvium (Sand and Clay)



wγ = unit weight of water (9.81 kN/m3)

— The calculated pressure heads were 15.8m, 8.2m and 2.0m for the maximum, most likely and minimum pore water pressures respectively.

— Calculated localised groundwater gradients of 0.64, 0.32 and 0.08 during/following embankment construction, based on a pressure head differences of 15.8m, 8.2m and 2.0m over a lateral distance of 25m.

— 25m distance from embankment is the distance at which the excess pore water pressure is assumed to return to zero. This is based on a conservative approach where a theoretical line of zero excess pore water pressure extends at an angle of 45° from the toe of the embankment slope. This assumption is validated by additional detailed pore water pressure calculations undertaken using the finite element analysis software package Plaxis for the Catalyst Pore Water Pressure report (MER-DJV-REP-ENV-02-0333022).

2.2.2 Results

The results of the analysis are given in the table below.

Condition

Retarded Travel Time1 (years)

Time Period for EQS / DWS Exceedance

2 at Receptor

(years)

Time for excess pore water

pressure full dissipation Arsenic Ammonia

TPH Aromatic C12-C16

Arsenic Ammonia TPH

Aromatic C12-C16

Existing Condition at 250m Receptor

2846 4.4 166 1000 1.0 54 -

Following Embankment Construction at 25m Receptor

3

3.7 0.006 0.2 1 1.0 1.0 1.5 years

Following Embankment Construction at 250m Receptor

3

2494 3.9 145 824 1.0 51 1.5 years

1 at the 5

th percentile

2 exceedance at the 95

th percentile

3 assuming maintained excess pore water pressures

The results show that although there is a slight decrease in retarded travel time due to

the artificial localised increase in hydraulic gradient the predicted time period for

excess pore water dissipation is insignificant when compared to the predicted travel

time to the receptors and time period for EQS exceedance. The model assumes

steady state hydraulic conditions. The predicted travel times could only be plausible if

the increased hydraulic gradient is maintained. In practice the maximum hydraulic

gradient created by the initial excess pore water pressure is very short lived. Therefore

as the excess pore water pressure is predicted to dissipate within 1.5 years the

reduction in travel times at the 5th Percentile for Arsenic (from 2,846 to 2,498 years),



Ammonia (from 4.4 to 3.9 years) and TPH Aromatic C12-C16 (from 166 to 145 years)

is rendered insignificant. The embankment construction could theoretically result in the

lateral movement of mobile contaminants. However, it should be noted that the risks of

lateral contaminant migration from the Gussion Transport site currently exist

irrespective of the proposed embankment construction activities.



3 Conclusion

Analysis has shown that the embankment loading would temporarily increase pore

water pressures within a limited lateral distance of the embankment. This increase in

pore water pressure will temporarily increase the local hydraulic gradient within the

underlying groundwater should groundwater be encountered within the loading bulb of

influence. This could result in a decrease in travel time of groundwater (and any

mobile contaminants) from below the embankment to a distance at which no excess

pore water pressures are predicted. However, the predicted time scales for dissipation

of excess pore water pressure are generally very short when compared to the reduced

travel times for groundwater migration.

These reduced travel times correspond to the assumption of a constant excess pore

water pressure driving the groundwater flow. However, the actual travel time will be

very similar to that of the existing condition (i.e. the excess pore water pressure will

dissipate generally well in advance of the predicted reduced travel time).

A further element of conservatism inherent in the ConSim model is that it does not

take into account the progressive dissipation of pore pressures, as such the predicted

travel times are conservative worst case time scales.

During the course of the works pore pressure measurements will be taken in

conjunction with groundwater monitoring to assess the potential for off-site migration

of contaminants. Monitoring will also be used to check the rate of pore pressure

dissipation which in turn may be used as the basis for controlling the rate of

embankment construction.

In summary the risk of causing significant lateral migration of contaminants is

considered negligible.


MER-DJV-REP-ENV-02-333014 8 MARCH 2017

APPENDIX A – ConSim Risk Assessment Model Input Parameters

CONSIM - Hydrogeological Risk Assessment

LEVEL 3 (GROUNDWATER) ASSESSMENT - INPUTS & JUSTIFICATION

Project Number

Project Title

Date

Simulation Details

min most likely max

Length of contaminant source

(in direction of groundwater flow)m - 100 - Measurement of source zone (BH58, WS17, WS22)

Width of contaminant source

(perpedicular to groundwater flow)m - 125 - Measurement of source zone (BH58, WS17, WS22)

Rainfall mm/year - 837 - Based on average annual rainfall in Widnes 1981-2010 from The Met Office

Infiltration Factor % - 0.25 - Conservative assumption of unsurface ground.

Infiltration mm/year - 209 - Based on unsurfaced ground.

Saturated aquifer thickness m 0.3 (0.15) 0.9 (0.75) 3.5 (0.1)Range of values (and Probabilities) dervied from groundwater monitoring and strike data in Made Ground / Glacial Sands and

Gravels

Bulk density of aquifer materials g/cm3 - 1.9 - Assumed value

Effective porosity of aquifer fraction 0.01 - 0.3 Minimum and maximum values for Clay and Sand from Domenico & Schwartz, 1990

Hydraulic gradient (exisiting case) fraction - 0.003 - From Gifford report for Made Ground

Hydraulic gradient fraction 0.08 0.32 0.64 From assessment of excess pore water pressure due to embankment construction

Hydraulic conductivity of aquifer m/s 1.00E-08 1.00E-03 Conservative values for Clay/Silt and Sandy Gravel.

Fraction of Organic Carbon fraction 0.0058 Based on a conservative 1% SOM

Distance to compliance point m - 250 - Conservative figure. No surface watercourses within 500m of site.

Longitudinal dispersivity m - 25 - 10% of pathway length

Transverse dispersivity m - 2.5 - 1% of pathway length

Parameter Units Input Value(s)

47067604

Mersey Gateway

Analysis of potential migration of elevated concentration of Arsenic, Ammonia and Aromatic EC12-EC16 at Gussion site within the groundwater in the Made Ground /

Alluvium to off-site surface water receptors.

04-Feb-14

Source / Justification


MER-DJV-REP-ENV-02-333014 8 MARCH 2017

APPENDIX B – Pore Water Pressure Calculations

Days Surcharge (kPa) Excess PWP (kPa) cv (m2/yr) 4 t (years) Tv U% 100-U%

0 0 0 H (m) 2.5

7 40 40

14 40 35 0.019 0.012 12.5 87.5

21 80 75

28 80 66 0.019 0.012 12.5 87.5

35 120 106

42 120 92 0.019 0.012 12.5 87.5

49 160 132

56 160 116 0.019 0.012 12.5 87.5

63 200 156

70 200 136 0.019 0.012 12.5 87.5

100 200 111 0.101 0.065 28.7 71.3

125 200 98 0.170 0.109 37.2 62.8

150 200 87 0.238 0.153 44.1 55.9

175 200 78 Key 0.307 0.196 50.0 50.0

200 200 70 0.375 0.240 55.3 44.7

225 200 62 0.444 0.284 60.1 39.9

250 200 56 0.512 0.328 63.9 36.1

275 200 50 0.581 0.372 67.6 32.4

300 200 45 0.649 0.416 71.0 29.0

325 200 41 0.718 0.459 73.9 26.1

350 200 36 0.786 0.503 76.6 23.4

375 200 33 0.855 0.547 79.0 21.0

400 200 29 0.923 0.591 81.2 18.8

425 200 26 0.992 0.635 83.1 16.9

Design Input

During Construction

Post Construction

Excess Pore Water Pressure Dissipation Profile During and After Embankment Construction at Sammy Evans and Gussion Transport Sites

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300 350 400 450

Exce

ss P

WP

(k

Pa

)

Time (Days)

Excess Pore Water Pressure Profile

MERSEY GATEWAY BRIDGE PROJECT...MERSEY GATEWAY BRIDGE MER-DJV-REP-ENV-02-333014 I 8 MARCH 2017 Document Control Sheet Rev. Status Date By Check Approved 0 1st Draft 02/10/2014 DT DJC

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