The Castle Mall Development in the centre of Norwich involves the construction of two storeys of shopping and a basement up to 18m below ground level, alongside a five storey underground car park. The majority of the ground surface of the site will be reinstated as parkland. A plan of the 2½ hectare site is shown in Figure 1.

Ove Arup and Partners' permanent works design consisted of a 900mm diameter contiguous bored pile perimeter retaining wall propped by the floor slabs. Management contractor, Bovis Construction Ltd., were responsible for the temporary support of the perimeter wall during construction and adopted a scheme requiring 1120 ground anchorages. This paper describes the design and construction of these ground anchorages. A companion paper in this conference by Grose and Toone explains why ground anchoring was chosen and describes the overall design of the wall.

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"Theory and Practice of the Single Bore Multiple Anchor System". Author : A.D. Barley. Presented at International Symposium on "Anchors in Theory and Practice", Saltzburg, Austria, October 9th and 10th 1995.

The new concept of installing a multiple of unit anchors in a single bore hole has allowed a considerable increase in anchor capacities in soils and weak rocks. Fixed anchor lengths as much as 30m long can now be efficiently utilised to achieve failure loads of 2000 to 3000kN in clays. Where circumstances demand, the anchors can be double protected for permanent works, or for temporary works the steel tendons can be fully withdrawn after use.

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"The Single Bore Multiple Anchor System". Author : A.D. Barley. Paper presented at the ICE Conference on Ground Anchorages and Anchored Structures, London, March 1997.

A typical anchor tendon with a 6m fixed length in soil or weak rock will, at test load, need to extend some 15 to 20mm at the proximal end of the fixed length before any load will be transferred to the distal end of the tendon. It is unusual for the elastic behaviour of the grouted ground around the anchor tendon to be compatible with the elasticity of the tendon and allow a uniform distribution of the load along the fixed length. Thus, it is widely acknowledged that, in the majority of circumstances, debonding at the tendon/grout or the grout/ground interface must occur as anchor load increases and prior to any load being transferred to the distal end of the fixed length. This phenomenon is commonly known as progressive debonding and is associated with grossly non-uniform distribution of bond stress along the fixed length at all stages of loading. Information has been published by a multitude of researchers on this topic.

Progressive debonding generally results in a highly inefficient use of the in-situ ground strength; in the load condition where the ground strength deep in the fixed length is being utilised, the ground strength above has been exceeded and only a residual strength is available there at the anchor soil interface. However, a system that can transfer the load simultaneously to a number of short lengths in the fixed anchor bore without the occurrence of progressive debonding, will mobilise the in-situ ground strength efficiently and result in a considerable increase in anchor capacity. This is the principle of the single bore multiple anchor (S.B.M.A.)

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"The Design, Construction and Performance of an Anchored Retaining Wall – Newcastle on Tyne". Authors : A. Woodland, C. Lomax and A.D. Barley. Paper presented at the ICE Conference on Ground Anchoes and Anchored Structures, London, March 1997.

The Quayside to the east of the Tyne Bridge in Newcastle-upon-Tyne has stood derelict for many years as a consequence of the industrial decline of the area. In 1987, after the establishment of the Tyne and Wear Developmet Corporation (TWDC), the area was identified as a TWDC 'flagship' site, to be developed for high quality offices, leisure, car parks and housing. In 1989, Ove Arup & Partners was commissioned to design the civil engineering infrastructure require to realise the potential of the area.

Retaining wall 'D', approximately200m long, is constructed of contiguous bored piles, with a cast-in-situ reinforced concrete upper panel. Both the piles and the RC wall are restrained by up to three rows of permanent ground anchorages, thereby imposing no restrictions on site access or requirements for long-term support from the office buildings being constructed in front.

The utilisation of 600kN working load anchors on Phase 1 of Retaining Wall D, and 950kN working load anchors on Phase 2, to satisfy the specified waling load rerquirements, marked a notable increase in the safe working capacity of anchors installed in glacial deposits. The use of Single Bore Multiple Anchors allowed high loads in soils with a proven factor of safety of three and the incorporation of an efficient double corrosion protection system over the entire length of the drilled anchor. The inability to fail any components of the trial anchors suggests that even higher loads are available using this system.

The advancement in anchor design technology which accommodates efficiency in load transfer, and the ability to design each unit anchor to suit ground strength at depth provides a considerable increase in confidence in the design of high load anchors in soils.

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"Multiple Anchors hold down the Millenium Man". Authors : A.D. Barley, Simon Dale.

The enterprising utilisation of a 100year old offshore, cast iron structure demanded reassessment of the foundations when subjected to new loading conditions. The physical dimensions required the provision of the high holding down anchor forces to be limited to only four corner anchors, each founded in the Caisson Base and in the underlying stratum. Strict time constraints prevented the ground conditions being fully investigated. The flexibility of the Single Bore Multiple Anchor System allowed the final "design" to be implemented as anchor drill logs were produced. This was effected by the distribution of the unit anchors within the stronger beds of ground materials encountered.

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"Dublin Port weighs anchors". Author : Dan Simpson.

A single bore multiple anchor system is being used to support massive sheet pile walls for a new berthing dock in Dublin Port.

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"Multiple Choice". Author : Margot Cole.

Engineers are increasingly specifying single bore multiple anchors to enhance the capacity of poor ground.

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"Exhumation and Design of Anchorages in Chalk". Authors : A D Barley, Mothersgille and R B Weerasinghe.

An anchorage test programme carried out on the site of deep basement construction in Norwich, UK, allowed determination of anchorage capacities, creep characteristic data and the exhumation of several anchor bodies in chalk.

This paper reports on the performance of the chalk based anchorages, the observation of exhumed grout body shapes and sizes, extent of grout penetration into dissolution cavities, pressure grouted fissures and the formation of grout cake around the grout body periphery.

Knowledge of construction techniques utilised for drilling, flushing, pre-grouting, redrilling, pressure grouting and the general methods of control implemented to prevent dissolution cavity collapse, has been related to the characteristics of the exhumed anchor bodies. Analysis of these grout bodies has allowed the proposal of a failure mechanism, comprising shaft resistance and shear of grouted fissures.

Tests were carried out on both conventional and single bore multiple anchors (SBMA). Using short fixed length capacities with recently developed efficiency factors allowed accurate evaluation of the ultimate capacity of a typical 10m fixed length conventional anchorage. This design approach confirms that the SBMA system mobilised double the capacity of a conventional 10m fixed anchor. Advancements in SMBA technology has enabled designs of 20m fixed lengths, safely supporting working loads of 3000 kN. .

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"Two Important Recent Developments in Ground Anchor Technology". Authors : D A Bruce and A D Barley.

Abstract :
Two important developments in ground anchor technology have taken place since the middle 1980s, namely the use of epoxy protected prestressing strand for tendons, and the invention of the Single Bore Multiple Anchor (SBMA) system. The former has helped to improve standards of corrosion protection and make them more cost effective, the latter has permitted markedly higher anchor loads to be developed in weak rocks and soils. The popularity of epoxy protected strand, however, has recently been impacted in the U.S. by concerns relating to the adhesion of the epoxy to the steel surface, although recent research would suggest that the problem has been resolved. Conversely, it is only within the last few years that SBMA technology has been employed in the U.S. This paper provides therefore a timely review of the current state of practice as related to each of these highly significant developments.

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"Anchor Advances". Author : Peter Jewell KBR.

Report on a meeting of the BGA at the Institution of Civil Engineers on 13 November 2002.
The main presentation was by Tony Barley, director of Single Point Multiple Anchor and Geoserve Global. He began with an update on the codes of practice for ground anchors (EN1537: 2000 and BS8081) and soil nailing (pr EN1537: 2999 and BS8081) and soil nailing. (pr EN 144490-draft) and clarified the differences between an anchored wall and a soil nailed face.

Convenor (and author of this report) Peter Jewell concluded that there was a clear message that design approaches more in keeping with actual behaviour of ground anchors can readily demonstrate why recorded capacities are often much higher than those predicted from the over-simplistic traditional design method.

Originally published in 'Ground Engineering' April 2003.

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"Chalk Anchorages : exhumation, load transfer mechanism and design guidelines.". Authors : A.D. Barley, D Mothersille and R Weerasinghe published in the Proceedings of the Institution of Civil Engineers, Geotechnical Engineering 156, July 2003.

An anchorage test programme carried out on the site of a deep basement construction in Norwich, UK, allowed the determination of anchorage capacities and creep characteristic data, and the exhumation of several anchor bodies in chalk. This paper reports on the performance of the chalk-based anchorages, the observation of exhumed grout body shapes and sizes, the extent of grout penetration into dissolution cavities, pressure-grouted fissures, and the formation of grout cake around the grout body periphery. Knowledge of construction techniques utilised for drilling, flushing, pre-grouting, redrilling, pressure grouting and the general methods of control implemented to prevent dissolution cavity collapse has been related to the characteristics of exhumed anchor bodies. Analysis of these grout bodies has allowed the proposal of a failure mechanism, comprising shaft resistance and the shear of grouted fissures. Tests were carried out on both conventional and single-bore multiple anchors (SBMAs). Using short, fixed-length capacities with recently developed efficiency factors allowed accurate eveluation of the ultimate capacity of a typical 10m fixed-length conventional anchor. This design approach confirms that the SBMA system mobilised double the capacity of a conventional 10m fixed anchor. Advancements in SBMA technology have enabled designs of 20m fixed lengths, safely supporting working loads of 3000kN.

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"Post grouted Single Bore Multiple Anchors at Hodenpyl Dam, Michigan". Authors : M E Bruce, R P Traylor, A.D. Barley, D A Bruce and J Gomez, presented at GeoSupport 2004, Orlando, Florida, February 4-7,2004

Despite the installation of a row of anchors in 1996, the right downstream retaining wall of Hodenpyl Dam, MI, continued to move inward and downstream. The remediation design called for installation of additional anchors. However, there were concerns about creep-induced relaxation of the anchor load over time given the existence of high-plasticity clays at the site. The solution was the installation of post-grouted Single Bore Multiple Anchors (SBMAs) along the base of the retaining wall.

One sacrificial anchor and 13 production anchors were installed. An innovative testing setup and program were developed to allow extended creep testing of the sacrificial anchor followed by load testing to 2.8 times the design load without anchor failure. This paper describes the design and construction of the SBMAs, the load testing setup , and the results of extended creep load testing.

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"Enhancement of Ground Anchor Design based on Tests of Multiple Anchors". Authors : A.D. Barley, C. Eng. and M.E. Bruce, P.E., presented at The International Association of Foundation Drilling "Geo-Quality Assurance and Control" Conference, Dallas, Texas, USA November 6th 2005

A single conventional trial anchor provides adequate information to substantiate the design of a production anchor that uses the identical construction technique in a particular soil stratum. Values of failure load and average bond stress at failure may be established and these data may also be considered appropriate to substantiate the design of anchors installed at separate locations in similar or identical soil conditions.

The installation and testing of one or two trial multiple anchors can provide more extensive data, such as values of failure loads and ultimate bond stress of several unit anchors installed at various depths in the soil. Unit anchors may be in the same or in differing soil strata.

The information gained may be used in conjunction with a design approach that acknowledges and quantifies the effect of progressive debonding, which can provide considerable advancement in the knowledge, the understanding and the design capabilities for use in anchor technology.

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"Case History of Satisfactory Performance, Shortcomings and Failures in Sservices - Freeport Mine Soil Anchors in Service".Authors : Paul McBarron, Philippe Vincent, Austress Menard - Sydney, Australia, for presentation at International Conference on Anchors November 2007, ICE London

The Freeport Mine in West Papua, Indonesia is the largest copper mine in the world and has been operating since the early 1970's. Located in the highlands of Papua the mine site is found at 2400m above sea level whilst the crusher plants were found at approximately 3760m above sea level.

Crusher plant no 6 was installed in 1997 and crusher no 7 in 2001. The plant was designed around 27m high vertical retained earth walls, and 36 meters high operating mine dump structures, allowed 200 tonnes dump trucks to tip the mined ore load into the crushers located at the wall base.

Inspection of the walls in 2002 revealed that the fill material used (mine cuttings and tailings) in the Reinforced Earth wall construction, combined with acid water percolating into the backfill, had led to severe deterioration of the steel facing panel and of the buried tensile members essential to the sound structural behavior of the wall.

An anchored piled wall 23 meters high was designed to allow the demolition and rebuilding of the Reinforced Earth walls 2 to 4 that support the crusher 6 dump slabs.

The performance criteria of the anchors and the anchored structure in service during the limited life span were fully satisfied in that face movement did not exceed the specified limits and the multiple anchors satisfactorily maintain their load capacities even when founded in an unusual back fill material of mine cuttings and tailings.

The remote location, the project environment and the site conditions provided demanding logistical and engineering challenges. These demanded the use of established anchoring methods and some innovative practices to be adopted in this unique location.

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Removable Multiple Anchors

"Field Trials on Four High Capacity Removable Multiple Anchors Founded in Marine Sand Fill and in Completely Decomposed Granite". Authors : A.D. Barley; P.L. McBarron. Presented at the ICE Conference on Ground Anchorages and Anchored Structures, London, March 1997.

The future construction of multistorey buildings spanning the new Central Station on Hong Kong Island, demands that any tempoarary anchors installed during the station works must ensure complete removal of the entire length of the steel tendons from the ground. The 20m depth of the cut and cover sections of the station approach from the tunnel works under Victoria Harbour required retention by up to six rows of extremely high capacity anchors founded in both marine sand fill and completely decomposed granite (dense silt). A preliminary field trial programme to establish the capacity of the anchors and demonstrate their removability was essential. The proposed anchors involved the single bore multiple anchor system, particularly appropriate for high loads in soils, and with modifications to ensure tendon removability. The initial anchor proposalsa were based on the utilisation of working anchors with service loads of 1400kN. The trial anchor location was selected on the basis of available area to construct a reaction block and install two inclined trial anchors. This was some 300m from the actual production area and it was not possible to install a site investigation borehole at the trail site until after the reaction block construction. This investigation, and dredge records, revealed adequate depth of marine sand fill, but inadequate depth of alluvium or completely decomposed granite (CDG), in which to install trial anchors. Thus at this location one trial anchor with unit fixed lengths of the same order as those to be used on production anchors was installed into the sand fill. It was acknowledged that, even with the maximum number of tendons installed, there may not have been adequate capacity to fail the unit anchors at the grout/ground interface. For this reason the second trial anchor utilised shorter unit fixed lengths, in an attempt to establish ultimate bond capacities such that in production anchors optimum values of factors of safety could be accommodated with a safe but economic design approach.

The preliminary trials fully demonstrated the high capacities attainable using the patented single bore multiple anchor system. The system modifications to allow complete removal of the entire length of prestressing strands from the grouted borehole, were also demonstrated.

Supplementary trials during initial stages of the works confirmed that working loads of 2000kN can be safely utilised in soils. This represents a new dimension in the working capacities of soil anchors.

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"Cofferdams Supported by Removable Temporary Anchors". Author : A.D. Barley. Reproduced from NATM, May 1999.

The use of ground anchors to retain well-designed cofferdam walls has thirty years of historical success, only with the occasional blemish. The Code of Practice for ground anchorages demands the testing of each individual anchor to 1.25 or 1.50 times the designated working load (1.33 is the new "Eurocode") prior to lockoff, to ensure that the performance of every anchor is demonstrated prior to acceptance of the works.

The introduction of a new anchor system with more than double the working capacity (800 to 2000kN) of normal anchors in soil and weak rock, has provided a supplementary economic benefit in the use of anchor systems; an advantage beyond that of the provision of open uninhibited working space for construction within the cofferdam.

High load and low load capacity anchor systems now exist which allow the total removal of the steel tendon from the anchor bore and from the the ground. This removal after temporary provision of safe support of the cofferdam makes the use of anchors more "environmentally friendly" and more acceptable to adjacent property owners.

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"A Reinforced Soil Mix Wall Cofferdam Supported by High Capacity Removable Anchors". Authors : A.D. Barley, W.D. Payne; P.L. McBarron. Presented at Value of Geotechnics in Construction – AGS Seminar, ICE, London, November 1998.

The temporary works construction of a deep walled cofferdam unhindered by the presence of internal struts provides a distinct benefit for the bottom-up construction of an underground structure. Extensive planning and trial work were required prior to the installation of deep soil mix walls and support by several rows of high capacity soil anchors. Total removal of the steel anchor tendons from adjacent areas was carried out as backfilling proceeded upwards around the completed structure.

The 38m deep soil mix wall surrounding the 240m x 75m x 20m deep cofferdam, fulfilled its intended purpose in providing both an efficient cut-off trench and a 27m deep structural retaining wall. The wall support system which required up to six rows of soil anchors, was extremely demanding in load requirements and in overcoming physical difficulties in construction; the penetration of the existing granite sea walls was achieved where required and the head of groundwater, which was 18m above the level where the lower row anchors penetrated the wall, was satisfactorily controlled throughout drilling and grouting and during the anchor working life.

The Single Bore Multiple anchors allowed utilisation of working loads of 1400 and 2000kN in unusual soils, namely marine sand fill and completely decomposed granite. Furthermore, the system allowed on completion of the temporary works the full extraction of the strand tendons from both the fixed and free length of the grouted bore to prevent obstructions to any future development.

These temporary works, on a fast track programme, effectively utilised two relatively novel systems. Their successful use adjacent to an existing 25 storey building and in a critical environmental area demonstrate an excellent example of engineering innovation.

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"Six Rows of High Capacity Removable Anchors Support Deep Soil Mix Cofferdam". Authors : A.D. Barley; W.D. Payne; P.L. McBarron. Presented at X11th European Conference on Soil Mechahnics and Geotechnical Engineering – Amsterdam, June 1999.

The use of high capacity soil anchors in lieu of internal props to support the cofferdam walls allowed rapid bottom-up construction of new rail tunnels with unimpeded progress. Innovative techniques in the use of a 38m deep soil mix wall and 2000kN working capacity fully removable anchors were utilised.

These temporary works operations on a fast track programme effectively utilised two relatively novel systems despite the encounter of constructional difficulties.

• the soil mix wall provided an efficient cut-off trench and a high quality retaining wall

• the soil anchors provided probably the highest working capacities ever used in anchors founded in fine sands and dense silts

• the wall movement that occurred during excavation did not exceed 50% of the predicted value of 70mm

• the ability to remove the steel tendons from the free and fixed anchor length after use, indicated a further advancement in ground anchor technology.

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