GROUND ANCHORS – theimportance of maintenance and inspections and some recent developments presentation by Dr Devon Mothersille SBMA Ltd FEBRUARY 2011 Tonight’s presentation Some special applications The importance of maintenance and inspections Changes in European Standards Recent developments Final Remarks INTRODUCTION AND BACKGROUND TERMINOLOGY Figure 1 from EN 1537:2000 .Sketch of a ground anchor (details of anchor head and head protection omitted) . GROUND ANCHOR MARKET SECTORS GROUND ANCHOR MARKET SECTORS SECTOR 1 Rock bolts and soil nails Lengths up to 10m Loads up to 100kN Low capacity ground anchors Lengths up to 12m Loads up to 500kN . GROUND ANCHOR MARKET SECTORS SECTOR 1 Rock bolts and soil nails Lengths up to 6m Loads up to 100kN Low capacity ground anchors Lengths up to 12m Loads up to 500kN SECTOR 2 Medium to high capacity ground anchors Lengths: 12m to 130m Test loads up to 20000kN . SOME SPECIAL APPLICATIONS . Tunnel subjected to a maximum hydrostatic head of 60m (courtesy PSM) Burnley Tunnel. Melbourne. Australia . Typical cross section through the central section of the tunnel showing fanned arrays of 46mm diameter monobar anchors with overall lengths varying between 7 . .10m. In total some 5200 anchors with working load of 1000kN were installed over a distance of 2km to resist uplift pressures . British Columbia. Canada (Courtesy Con-Tech Systems Ltd) .Seven Mile Dam. Fabrication of 92 strand tendons up to 126m long (Courtesy Con-Tech Systems Ltd) 57 tendons transported to dam site via road (Courtesy Con-Tech Systems Ltd) . Homing of tendon in 400mm diameter boreholes (Courtesy Con-Tech Systems Ltd) Anchors proof loaded to 19177kN (Courtesy Con-Tech Systems Ltd) . Ireland .The Aviva Stadium Dublin. Foundation structure incorporating eight anchors with working loads up to 1250kN . Coupling of 63.5mm diameter bars to form 20m long tendons . Placement of bearing plate and nut with access manhole Placement of bitumen coated. steel protective cap filled with corrosion inhibiting compound . Austria .Substantial slope stabilisation project utilising high capacity single bore multiple anchors (SBMAs) at Degendamm. Large reinforced concrete stressing blocks used with 3600kN work load anchors in highly weathered rock . .Fabrication of tendons comprising 18No. 18mm diameter strands with 20m overall fixed length and total length of 85m. Use of double protected 18mm diameter Dyform strand for anchors at Degendamm. Austria . anchors of 1500kN working load specified in the tender. .On completion 200No. anchors of working load 3600kN effectively replace 400No. Qatar with multi-level basement and 100 floors.Al-Quds Endowment Tower. . Doha. Doha. removable SBMAs with working load of 750kN to support excavation for the Al-Quds Tower Project. Qatar .Installation of 1537 No. Excavation support by propping and shoring . Reinforced concrete props supporting temporary works for deep excavation in China . Combining propping and anchoring during the construction of Central Station, Hong Kong FOUNDATION CONSTRUCTION FOR THE WORLD TRADE CENTRE, NEW YORK, USA MAINTENANCE AND INSPECTIONS Section Overview Benefits Consequences Guidelines Closing Remarks . THE BENEFITS . can extend the service life of anchored structures . where satisfactory condition and service performance are confirmed.Routine programmes of inspection and monitoring. in order to safeguard the integrity and performance of the anchored structure .Where investigations highlight unacceptable tendon exposure to corrosion or tendon over-stressing. the results provide early warning of the need for precautionary or remedial measures. insufficient attention is paid to routine maintenance inspections and service behaviour monitoring in current practice The potential consequences should not be ignored as anchored structures represent key elements of a country’s infrastructure .In spite of these benefits. Anchors installed over 30 years ago may have corrosion protection considered inferior or inadequate by today’s standards! . THE NATURE OF CORROSION . SCHEMATIC REPRESENTATION OF TYPES OF CORROSION Generalised attack Localised attack Stress corrosion cracking . Unprotected steel tendon in Victorian soil retention system . THE CONSEQUENCES OF NOT CARRYING OUT MAINTENANCE AND INSPECTIONS PROGRAMMES . FAILURES . River Thames .Anchored quay wall failure. Anchored quay wall failure. River Thames . Soil nailed slope. South Korea . South Korea .Failed slope. Failed slope. South Korea . Failure of large anchored slope in Asia . Failure of large anchored slope in Asia . Some features encountered during anchor inspections . BARCELONA. SPAIN . Anchored wall support during excavation . Removal of grout plug to expose anchor head components . Expose strands and anchor head block . Severely corroded strand exhibiting slippage through wedges after 18 months in service . HARBOUR IN SOUTHERN ENGLAND . Anchors in tidal range restrain quay wall .1300kN working load anchors in alluvial deposits. Removal of protective cap reveals severely corroded barrel and wedges after 11 years in service . Inspection within protective caps reveals inadequate filling with corrosion inhibiting compound . Corrosion induced fractured barrel and wedges and strand slippage after 11 years in service . Degradation of rubber gaskets after 11 years in service . Emulsification of corrosion inhibiting compound after 11 years in service . ENGLAND .RIVER THAMES. Severe corrosion up to 10mm deep recorded on protective steel cap after 28 years in a marine environment . Severe corrosion and deterioration of bitumen painted steel cap after 30 years in service . Anchor head after removal of cap showing corroded barrels and remnant grease after 30 years in service . Severely corroded anchor head showing strand slippage and protective cap loss after 30 years in service . RIVER CLYDE. SCOTLAND . Anchor heads subjected to severe exposure within the tidal range after 21 years in service . Exhumed anchor head in concrete deck protected by grease impregnated tape after 21 years in service Note Grease has dried out leaving the tape material ineffective in protecting the anchor head . Strand loss and slippage at exposed anchor head after service of 33 years . Sample of strand with barrel and wedges attached showing 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% in peripheral wires after 30 years in service . MAINTENANCE AND REPAIR . Sequence of remedial measures for corroded anchor head after loss of protective cover in service . Glass fibre reinforced protective cap . FAILURES OF GROUND ANCHORS IN SERVICE BY TENDON CORROSION 1934 TO 1980 FIP (1986) . e. bar.FINDINGS FROM 35 CASE HISTORIES Corrosion is localised and independent of tendon type i. strand or wire Period of service ranges from a few weeks to many years Short term failures due to stress corrosion cracking or hydrogen embrittlement . FAILURE LOCATIONS 19 incidents at or within 1m of anchor head 21 incidents in the free length 2 incidents in fixed length . Typical anchor head detail (BS 8081:1989) . Absence of protective sheathing immediately below stressing head . GUIDELINES . RESPONSIBILITIES OF DESIGNER highlight value and necessity of inspection/monitoring produce a performance specification and maintenance manual provide access for investigations stipulate record keeping ensure appropriate reporting . MAINTENANCE TESTING involves inspection of the condition of anchor materials and components + where appropriate. testing to determine the nature and severity of the condition ESSENTIAL QUESTIONS Has anchor suffered corrosion or mechanical damage? Are the conditions recorded within acceptable limits? . residual load and anchor head displacement ESSENTIAL QUESTIONS Are the trends in movement/deformation acceptable? Have individual anchors maintained their design load in compliance with acceptance criteria? .g. overall movement and local deformation + individual anchors e.SERVICE BEHAVIOUR MONITORING focuses on performance of anchored structure e.g. TESTING AND MONITORING OF IMPORTANT ANCHORS EXTENT AND FREQUENCY annual visual inspection of outer head protection of all anchors. or at least a representative sample annual intrusive inspections of 5-10% of anchors to assess anchor head condition and monitoring by load lift-off checks . 1 year. 1 month.Frequency of visual survey Monitoring of individual anchors No. 1 year. open air car parks. essential services or excessive structural damage to buildings Weekly (up to end of maintenance period) and every 6 months thereafter 15% of first 50 anchors 12% of second 50 anchors 10% of additional anchors 2 weeks. 6 months. rural feeder and local distributor roads that are not sole accesses Monthly (up to end of maintenance period) and annually thereafter 7% of first 50 anchors 3% of additional anchors 2 weeks. 1 month. and economic risk relating to failures affecting country parks. 18 months. and every 2 years thereafter Two anchors 2 years. 5 years and every 5 years thereafter One anchor 2 years. 1989) .The same anchors are to be monitored each time. roads with high traffic density and public waiting areas. 5 years and every 5 years thereafter Negligible risk category meaning risk to life where failures affect country parks. lightly used open recreational areas. 2. of anchors1 Frequency of anchor inspection and residual load measurement Extent and frequency of special grease checks2 High risk category meaning risk to life where failures affect occupied buildings and economic risk relating to failures affecting urban trunk roads. 3 months. 1 year. 2 years.Different anchors are to be selected each time for special grease checks so that the original undisturbed grease can be sampled. 2 years. roads with low traffic density and storage compounds for non-dangerous goods. 3 months. 6 months. 9 months. and economic risk relating to failures affecting rural or primary distributor roads that are not sole accesses and temporary loss of essential services Fortnightly (up to end of maintenance period) and annually thereafter 10% of first 50 anchors 7% of second 50 anchors 5% of additional anchors 2 weeks. and every year thereafter Three anchors 2 years. 9 months. 18 months. 5 years and every 5 years thereafter Low risk category meaning risk to life where failures affect densely used open spaces and recreational facilities. 6 months. The number may be increased if necessary to provide a representative sample. 2 years. 3 months. 5 years and every 5 years thereafter 1. 9 months. 18 months. GUIDE TO EXTENT AND FREQUENCY OF MONITORING OF ANCHORS (AFTER GEOSPEC 1. 1 month. If the load in the anchorage approaches the original proof load. When an anchor load gain is measured. monitoring commences at short intervals of 1-3 months and later at intervals not greater the 2 years. . depending on the results. monitoring should continue until the load stabilises. or more if desired. the anchorage is destressed to the design working load.USA Depending on the number of anchors and the importance of the measurements. typically 3% to 10% of the anchors. are monitored for service behaviour on any given project (PTI . additional anchors are installed and the overall anchored structure is monitored until the overall system stabilises. In general.2004). Where the purpose of monitoring is the detection of failure due to corrosion e. unprotected anchorages.UK (BS8081:1889 clause 11. .3): Duration and frequency of monitoring. testing should be carried out at not greater than 6 month intervals for a period of 3 years and thereafter at long regular intervals of not greater than 5 years throughout the entire life of the structure.g.5. Closing Remarks Concern about the condition of existing anchored structures constructed circa 30 years ago Remedial measures employed at the anchor head will serve to prolong the effective service life of existing anchorages The condition of existing ground anchors supporting waterside structures can only be assessed by implementing programmes of inspections. monitoring and testing . RECENT CHANGES TO EUROPEAN STANDARDS . BS8081:1989 British Standard Code of Practice for Ground Anchorages Essentially covering three main aspects: Design Construction Testing . BS8081:1989 will be replaced by three documents: EC7 (EN 1997-1. Ch. 8): Design of Anchors Harmonisation EN 1537: Execution of Ground Anchors EN ISO 22477-5: Testing of Anchors . Countries regularly represented on committee CEN/TC288/WG14 • Austria • Belgium • Denmark • France • Germany • Netherlands • Norway • Portugal • Sweden • Spain • Switzerland • United Kingdom . DESIGN IN ACCORDANCE WITH EC7 . Basis of the EC7 anchor design approach The fundamental basis for the limit state design approach adopted in EC7 is that actions (the anchor force) are resisted by internal resistances (the steel tendon strength) and external resistances (the ground/grout interfacial bond). Partial factors are assigned in the design process . EXECUTION IN ACCORDANCE WITH EN1537 . 1: For anchors in these cases.4 Stressing Cl.8.4.5: Stressing of anchors with staggered free lengths Cl.4: ‘If monitoring is to be carried out a minimum of 5% of the anchors should be monitored on a regular basis during their design life. special consideration shall be given to the stressing operation to avoid overstressing of each individual tendon unit.4.10.10 Monitoring Cl. CLAUSE 9.CLAUSE 8.9.6: ‘The monitoring should include the inspection of corrosion protection of the accessible parts of the anchor head. [BS8081 recommends 10% or three anchorages whichever is greater for projects with < 100 anchorages and at least 5% of the excess over 100.] .10.5. Cl.9.8. TESTING IN ACCORDANCE WITH EN ISO 22477-5 . EN ISO 22477-5 Proposals have been made to reduce the proof load factor from 1.25 for both temporary and permanent anchors .5 to 1. It anticipated that full publication of the harmonized documents will not emerge until 2011/12.Closing remarks The national foreword to EN1537:2000 states that it supersedes those parts of BS8081:1989 that deal with the construction of ground anchors. . Until the publication of EN ISO 22477-5 all aspects of BS8081:1989 that deal with testing also still apply. Until the publication of EC7 all aspects of BS8081:1989 dealing with design still apply. RECENT DEVELOPMENTS . RESEARCH INTO THE USE OF CARBON FIBRE TENDONS FOR GROUND ANCHORS . Carbon fibre tendons THE USE OF REAL TIME MONITORING TECHNOLOGY ON ANCHORS Anchored slope in South Korea Fibre – optic sensors used in tendon as part of a real time monitoring system INTERNATIONAL CONFERENCE ON GROUND ANCHORS LONDON 2017 . Final Remarks Millions of anchors installed over the past 70 years with relatively few recorded failures No room for complacency Ongoing need to maintain high standards in anchor design and construction Need for rigorous maintenance inspections and service behaviour monitoring to ensure satisfactory performance in the future . THANK YOU FOR YOUR ATTENTION .