20110209 GroundAnchors Presentation

GROUND ANCHORSGROUND ANCHORS thetheimportance of maintenance andimportance of maintenance and

inspections and some recentinspections and some recentdevelopmentsdevelopments

presentation by

Dr Devon Mothersille

SBMA Ltd

FEBRUARY 2011

Some special applications

Changes in European Standards

The importance of maintenance andinspections

Final Remarks

Tonights presentation

Recent developments

INTRODUCTIONAND

BACKGROUND

TERMINOLOGYFigure 1 from EN 1537:2000 - Sketch of a ground anchor(details of anchor head and head protection omitted)

SECTOR 1

GROUND ANCHOR MARKET SECTORS

Rock bolts and soilnails

Lengths up to 10mLoads up to 100kN

SECTOR 1 Low capacity ground

anchors Lengths up to 12m Loads up to 500kN

GROUND ANCHOR MARKET SECTORS

Rock bolts and soilnails

Lengths up to 6mLoads up to 100kN

SECTOR 1 Low capacity ground

anchors Lengths up to 12m Loads up to 500kN

Medium to high capacity ground anchors

Lengths: 12m to 130m

Test loads up to 20000kN

SECTOR 2

GROUND ANCHOR MARKET SECTORS

SOME SPECIALAPPLICATIONS

Burnley Tunnel,Melbourne,Australia

Tunnel subjected to amaximum hydrostatic headof 60m(courtesy PSM)

Typical cross section through the central section of the tunnel showingfanned arrays of 46mm diameter monobar anchors with overall lengthsvarying between 7 - 10m.

In total some 5200 anchors with working load of 1000kN wereinstalled over a distance of 2km to resist uplift pressures

Seven Mile Dam, British Columbia, Canada(Courtesy Con-Tech Systems Ltd)

Fabrication of 92 strandtendons up to 126m long(Courtesy Con-Tech Systems Ltd)

57 tendonstransported to damsite via road(Courtesy Con-Tech Systems Ltd)

Homing of tendon in 400mmdiameter boreholes(Courtesy Con-Tech Systems Ltd)

Anchors proof loaded to19177kN(Courtesy Con-Tech Systems Ltd)

The Aviva Stadium Dublin, Ireland

Foundation structure incorporating eight anchors withworking loads up to 1250kN

Coupling of 63.5mm diameter bars to form 20m long tendons

Placement of bearingplate and nut withaccess manhole

Placement of bitumencoated, steelprotective cap filledwith corrosioninhibiting compound

Substantial slope stabilisation project utilising high capacitysingle bore multiple anchors (SBMAs) at Degendamm, Austria

Large reinforced concrete stressing blocks used with 3600kNwork load anchors in highly weathered rock

Fabrication of tendonscomprising 18No. 18mmdiameter strands with 20moverall fixed length and totallength of 85m.

Use of double protected 18mm diameter Dyform strandfor anchors at Degendamm, Austria

On completion 200No. anchors of working load 3600kNeffectively replace 400No. anchors of 1500kN working load

specified in the tender.

Al-Quds Endowment Tower,Doha, Qatar with multi-levelbasement and 100 floors.

Installation of 1537 No. removable SBMAs with workingload of 750kN to support excavation for the Al-Quds

Tower Project, Doha, Qatar

Excavation support by propping and shoring

Reinforced concrete props supporting temporaryworks for deep excavation in China

Combining propping and anchoring during the constructionof Central Station, Hong Kong

FOUNDATIONCONSTRUCTIONFOR THE WORLDTRADE CENTRE,NEW YORK, USA

MAINTENANCE ANDINSPECTIONS

Closing Remarks

Benefits

Guidelines

Consequences

Section Overview

THE BENEFITS

Routine programmes of inspection andmonitoring, where satisfactory condition and

service performance are confirmed,can extend the service life of anchored

structures

Where investigations highlightunacceptable tendon exposure to corrosion

or tendon over-stressing,the results provide early warning of the need

for precautionary or remedial measures,in order to safeguard the integrity andperformance of the anchored structure

In spite of these benefits, insufficient attentionis paid to routine maintenance inspections

and service behaviour monitoringin current practice

The potential consequences should not beignored as anchored structures represent key

elements of a countrys infrastructure

Anchors installed over 30 years agomay have corrosion protection

considered inferior or inadequateby todays standards!

THE NATURE OF CORROSION

SCHEMATIC REPRESENTATION OF TYPES OF CORROSION

Stress corrosioncracking

Generalised attack

Localised attack

Unprotected steel tendon in Victorian soilretention system

THE CONSEQUENCES OFNOT CARRYING OUTMAINTENANCE AND

INSPECTIONSPROGRAMMES

FAILURES

Anchored quay wall failure, River Thames

Anchored quay wall failure, River Thames

Soil nailed slope, South Korea

Failed slope, South Korea

Failed slope, South Korea

Failure of large anchored slope in Asia

Failure of large anchored slope in Asia

Some features encounteredduring anchor inspections

BARCELONA, SPAIN

Anchored wall support during excavation

Removal of grout plug to expose anchor headcomponents

Expose strands and anchor head block

Severely corroded strand exhibiting slippagethrough wedges after 18 months in service

HARBOUR IN SOUTHERNENGLAND

1300kN working load anchors in alluvial deposits.Anchors in tidal range restrain quay wall

Removal of protective cap reveals severely corrodedbarrel and wedges after 11 years in service

Inspection within protective caps reveals inadequatefilling with corrosion inhibiting compound

Corrosion induced fractured barrel and wedges andstrand slippage after 11 years in service

Degradation of rubber gaskets after 11 years in service

Emulsification of corrosion inhibiting compoundafter 11 years in service

RIVER THAMES, ENGLAND

Severe corrosion up to 10mmdeep recorded on protectivesteel cap after 28 years in amarine environment

Severe corrosion and deterioration of bitumenpainted steel cap after 30 years in service

Anchor head after removal of cap showingcorroded barrels and remnant grease after

30 years in service

Severely corroded anchor head showing strandslippage and protective cap loss after 30 years in

service

RIVER CLYDE, SCOTLAND

Anchor heads subjectedto severe exposurewithin the tidal rangeafter 21 years in service

Exhumed anchor head in concrete deck protected by greaseimpregnated tape after 21 years in service

NoteGrease has dried out leaving the tape material ineffective in protecting the anchorhead

Strand loss and slippage at exposed anchor headafter service of 33 years

Sample of strand with barrel and wedges attachedshowing damage to sheathing at a location beneath the

anchor head

Severe corrosion on failed strand showing section loss

Macrograph showing section loss of up to 16% inperipheral wires after 30 years in service

MAINTENANCE AND REPAIR

Sequence of remedialmeasures for corroded

anchor headafter loss of protective

cover in service

Glass fibre reinforced protective cap

FAILURES OF GROUND ANCHORSIN SERVICE

BYTENDON CORROSION

1934 TO 1980

FIP (1986)

Corrosion is localised and independent oftendon type i.e. bar, strand or wire

Short term failures due to stress corrosioncracking or hydrogen embrittlement

Period of service ranges from a few weeks tomany years

FINDINGS FROM 35 CASE HISTORIES

19 incidents at or within 1m of anchor head

2 incidents in fixed length

21 incidents in the free length

FAILURE LOCATIONS

Typical anchor head detail (BS 8081:1989)

Absence of protective sheathing immediately below stressing head

GUIDELINES

RESPONSIBILITIES OF DESIGNER

highlight value and necessity ofinspection/monitoring

produce a performance specification andmaintenance manual

provide access for investigations

stipulate record keeping

ensure appropriate reporting

MAINTENANCE TESTING

Has anchor sufferedcorrosion or mechanical damage?

ESSENTIAL QUESTIONS

+where appropriate, testing to determinethe nature and severity of the condition

involves inspection of the condition of anchormaterials and components

Are the conditions recorded within acceptable limits?

SERVICE BEHAVIOUR MONITORING

Have individual anchors maintained their design loadin compliance with acceptance criteria?

Are the trends in movement/deformationacceptable?

ESSENTIAL QUESTIONS

+individual anchors e.g. residual load

and anchor head displacement

focuses on performance ofanchored structure e.g. overall movement

and local deformation

TESTING AND MONITORING OF IMPORTANTANCHORS

EXTENT AND FREQUENCY

annual intrusive inspections of 5-10% of anchorsto assess anchor head condition and monitoringby load lift-off checks

annual visual inspection of outer headprotection of all anchors, or at least arepresentative sample

Frequency ofvisual survey

Monitoring of individual anchorsNo. of anchors1 Frequency of anchor inspection

and residual load measurementExtent and frequency ofspecial grease checks2

High risk category meaning risk to life where failures affect occupied buildings and economic risk relating tofailures affecting urban trunk roads, essential services or excessive structural damage to buildings

Weekly (up to endof maintenance

period) and every 6months thereafter

15% of first 50anchors

12% of second 50anchors

10% of additionalanchors

2 weeks, 1 month,3 months, 6 months,

9 months, 1 year,18 months, 2 years, and every

year thereafter

Three anchors2 years, 5 years and every

5 years thereafter

Low risk category meaning risk to life where failures affect densely used open spaces and recreationalfacilities, roads with high traffic density and public waiting areas, and economic risk relating to failures affectingrural or primary distributor roads that are not sole accesses and temporary loss of essential services

Fortnightly (up toend of maintenance

period) andannually thereafter

10% of first 50anchors

7% of second 50anchors

5% of additionalanchors

2 weeks, 1 month,3 months, 6 months,

9 months, 1 year,18 months, 2 years, and every

2 years thereafter

Two anchors2 years, 5 years and every

5 years thereafter

Negligible risk category meaning risk to life where failures affect country parks, lightly used open recreationalareas, roads with low traffic density and storage compounds for non-dangerous goods, and economic riskrelating to failures affecting country parks, open air car parks, rural feeder and local distributor roads that are notsole accesses

Monthly (up to endof maintenance

period) andannually thereafter

7% of first 50anchors

3% of additionalanchors

2 weeks, 1 month,3 months, 6 months,

9 months, 1 year,18 months, 2 years,

5 years and every 5 yearsthereafter

One anchor2 years, 5 years and every

5 years thereafter

1.The same anchors are to be monitored each time. The number may be increased if necessary to provide arepresentative sample.2.Different anchors are to be selected each time for special grease checks so that the original undisturbedgrease can be sampled.

GUIDE TOEXTENT ANDFREQUENCYOFMONITORINGOF ANCHORS(AFTERGEOSPEC 1,1989)

USA

Depending on the number of anchors and the importance ofthe measurements, typically 3% to 10% of the anchors, ormore if desired, are monitored for service behaviour on anygiven project (PTI ,2004).

In general, monitoring commences at short intervals of 1-3months and later at intervals not greater the 2 years,depending on the results. When an anchor load gain ismeasured, monitoring should continue until the loadstabilises. If the load in the anchorage approaches theoriginal proof load, the anchorage is destressed to the designworking load, additional anchors are installed and the overallanchored structure is monitored until the overall systemstabilises.

UK (BS8081:1889 clause 11.5.3):

Duration and frequency of monitoring. Where thepurpose of monitoring is the detection of failuredue to corrosion e.g. unprotected anchorages,testing should be carried out at not greater than 6month intervals for a period of 3 years andthereafter at long regular intervals of not greaterthan 5 years throughout the entire life of thestructure.

Closing RemarksConcern about the condition of existing anchoredstructures constructed circa 30 years ago

Remedial measures employed at the anchor headwill serve to prolong the effective service life ofexisting anchorages

The condition of existing ground anchorssupporting waterside structures can only beassessed by implementing programmes ofinspections, monitoring and testing

RECENT CHANGESTO EUROPEANSTANDARDS

British Standard Code of Practice forGround Anchorages

Design

Construction

Testing

Essentially covering three main aspects:

BS8081:1989

EC7 (EN 1997-1, Ch. 8): Design of Anchors

EN 1537: Execution of GroundAnchors

EN ISO 22477-5: Testing ofAnchors

Harmonisation

BS8081:1989 will be replaced by three documents:

Countries regularly represented on committeeCEN/TC288/WG14

Austria Belgium Denmark France Germany Netherlands Norway Portugal Sweden Spain Switzerland United Kingdom

DESIGN IN ACCORDANCEWITH EC7

Basis of the EC7 anchor design approach

The fundamental basis for the limit state designapproach adopted in EC7 is that actions (theanchor force) are resisted by internalresistances (the steel tendon strength) andexternal resistances (the ground/grout interfacialbond).

Partial factors are assigned in the designprocess

EXECUTION INACCORDANCE WITH EN1537

CLAUSE 8.4 Stressing

Cl.8.4.5: Stressing of anchors with staggered free lengths

Cl.8.4.5.1: For anchors in these cases, special consideration shall begiven to the stressing operation to avoid overstressing of eachindividual tendon unit.

CLAUSE 9.10 Monitoring

Cl.9.10.4: If monitoring is to be carried out a minimum of 5% of theanchors should be monitored on a regular basis during their designlife.

[BS8081 recommends 10% or three anchorages whichever is greater forprojects with < 100 anchorages and at least 5% of the excess over 100.]

Cl.9.10.6: The monitoring should include the inspection of corrosionprotection of the accessible parts of the anchor head.

TESTING IN ACCORDANCEWITH EN ISO 22477-5

EN ISO 22477-5

Proposals have been made to reduce the proofload factor from 1.5 to 1.25 for both temporaryand permanent anchors

The national foreword to EN1537:2000 states that it supersedesthose parts of BS8081:1989 that deal with the construction ofground anchors.

Closing remarks

Until the publication of EC7 all aspects ofBS8081:1989 dealing with design still apply.

Until the publication of EN ISO 22477-5 all aspects ofBS8081:1989 that deal with testing also still apply.

It anticipated that full publication of the harmonized documentswill not emerge until 2011/12.

RECENTDEVELOPMENTS

RESEARCH INTO THEUSE OF CARBON

FIBRE TENDONS FORGROUND ANCHORS

Carbon fibre tendons

THE USE OF REAL TIMEMONITORING

TECHNOLOGY ON ANCHORS

Anchored slope in SouthKorea

Fibre optic sensors usedin tendon as part of a realtime monitoring system

INTERNATIONALCONFERENCE ON

GROUND ANCHORS

LONDON 2017

Millions of anchors installed over the past 70 yearswith relatively few recorded failures

Final Remarks

No room for complacency

Ongoing need to maintain high standards in anchordesign and construction

Need for rigorous maintenance inspections andservice behaviour monitoring to ensure satisfactoryperformance in the future

THANK YOU FORYOUR

ATTENTION