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The advent of anchoring techniques some two and a half decades ago offered new solutions for soil retention and slope stabilisation. Their use is now well accepted and proven. This paper describes five recent case histories where anchorages were founded in difficult ground or rock conditions and required development of new techniques to achieve the most effective and economic solutions.

• Landslip stabilisation at Risca Bypass.

• Alexander Quay, Southampton

• Slope stabilisation at Barton-on-Sea

• Retaining of landslip debris during construction of Palm Beach Flats, Sandgate, Kent

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"Protection of Public Beaches in Gibraltar by Stabilisation of Quarried Cliffs Using Rock Anchors". Authors : A.D. Barley; P. Burke. Paper Presented at the ICE Conference on Ground anchorages and Anchored Structures, London, March 1997.

Cliff stabilisation has to date been more commonly carried out using rock bolting systems of varying quality with respect to corrosion protection. The severity and potentially fatal consequences of a cliff collapse at either Camps Bay or Little Bay inlets, which provide recreational areas in Gibraltar, demanded the use of double-protected high quality rock anchors complying with BS8081. These bay areas had originally been created by extensive quarrying activities carried out to win stone for local construction. Crude excavation and blasting techniques had left steep and shattered rock faces up to 75m in height.

120 rock anchors with working loads up to 500kn and lengths up to 20m, were required to enhance the stability of large buttresses of rock, some partially separated from the main cliff. Specially designed cliff face drilling rigs were winched into position and fixed to the face with dowels during drilling and grouting operations. All operatives were secured independently, using abseil sit harnesses and dual rope safety systems. All construction operations in this potentially hazardous area were executed in accordance with a well defined safety related method statement.

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"Properties of Anchor Grouts in a Confined State". Author : A.D. Barley. Presented at the ICE Conference on Ground Anchorages and Anchored Structures, London, March 1997.

During the loading of a ground anchor the load transfer mechanisms between the tendon and the grout, and the grout and the borehole wall, demand the utilisation of extremely high values of shear and compressive stresses within the body of the confined grout. Normal uncinfined compressive tests on anchor grouts provide no information on the stresses available for mobilisation in the anchor bore grout, but a simple new test such as that described, simulating the confinement of a grout column in dense to very dense sand or moderately weak rock, provides an indication that compressive stresses in the range of 200 to 800N/mm2 are avaiable in the anchor grout body. These values are an order of magnitude greater than UCS values. Hence it should be appreciated that UCS test values, while entirely appropriate for basic Quality Assurance, are in no way reflective of the actual strength available in-situ. The presence of such high values is supported by the very satisfactory performance of removable multiple anchors, which mobilise high stresses of both direct compression and shear.

Other simple tests on confined grout indicate that direct shear strength may range from 12 to 20N/mm2 in normal anchor grout and 25N/mm2 in proprietary encapsulating grouts. these values cosiderable exceed the bond stress generally available at the grout/ground or rock interface. This highlights the enormous benefits to be obtained by the pressure grouting of weak fissured rocks (chalk and mudstone) since the load contribution of a grouted fissure (the grout of which must be sheared prior to pullout) may equate to the capacity of the non-fissured bore lengths of 50 to 100 times the fissure width.

In water permeable strata, the application of pressure while grouting has been shown to reduce the w/c ratio of the anchor grout from the 0.45 to 0.50 range tp 0.30 to 0.34 range. This occurrence further increases the in-situ strength of the anchor grout such that compressive and shear capacity may be in excess of those values reported above.

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"Ground Anchor Tendon Protected Against Corrosion and Damage by a Double Plastic Layer". Author : A.D. Barley. Presented at the ICE Conference on Ground Anchorages and Anchored Structures, London, March 1997.

This paper describes the research, development and design of the ground anchor tendon protected against corrosion and amage by a double plastic layer.

The double plastics encapsulation system ensures total insulation of the load carrying steel strands from the corrosive conditions of the geotechnical environment. Even in the event of damage or perforation of the outer plastic during carriage or installation, the protection against corrosion is still fully provided by the inner duct.

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"The Execution of Ground Anchor Works". The European Standard prEN1537. Authors : Dr C.M. Merrifield, A.D. Barley, U Von Matt. Presented at the ICE Conference on Ground Anchors and Anchored Structures, London, March 1997.

The development of EN1537:1997 has resulted in the articulation for the first time of a coherent and simple limit state design approach for ground anchors which may be adopted throughout Europe. Whilst not identical in method of execution, nor in outcomes, the test methods adopted are reasonably rigorous in achieving an acceptably low level of risk of failure. On the other hand, recognising the existence of disparate practices of both corrosion protection and anchor testing across Europe, the Standard presents cogently and simply the alternative courses of action which the Client's Technical representative may take, on the clear understanding thast there is no suggestion of parity between either the corrosuion protection systems nor the testing methods.

Whilst providing a balanced standard of reasonable prescription and sensible recommendation EN1537, however, underlines the responsibility of the industry to make decisions on performance and durability based on evidence currently available.

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"The Failure of a 21 Year Old anchored sheet Pile Quay Wall on the Thames". Author : A.D. Barley. Ground Engineering March 1997.

A description of the failure of an anchored sheet pile wall. Reasons for the failure were investigated and a stability analysis carried out.

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"The Development, Installation and Testing of Ground Anchors below Water Level but in a Dry Regime Using a Limpet Dam". Authors : M.A. Hodgson, A.D. Barley. Paper presented at the ICE Conference on Ground Anchorages and Anchored Structures, London, March 1997.

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"Recent Advances in Ground Anchor Technology with Reference to the Development of the Art". Authors : A.D. Barley; Dr C Windsor for presentation at GEO 2000 International Conference on Geotechnical and Geotechnical Engineering, Melbourne, 19th – 24th November 2000

The recent advances in ground anchor technology and the related techniques of cable bolting and rock bolting are reviewed. Collectively the technology associated with the three techniques enable design engineers to address stability problems over a range of scales and in a range of geomechanical environments. The techniques have similar aims but have developed into separate disciplines with unique attributes. The procedures for designing and creating ground anchors that meet the stringent requirements associated with modern civil infrastructure are discussed. This contrasts the very different design approaches being developed in rock bolting and cable bolting where standards are less exacting but the design problems can sometimes be more complex.

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"Efficiency of Prestressing Heads". Author : A.D. Barley Published in 'Foundation Drilling', magazine of ADSC, May 2005

A technical note illustrating the benefits of grease filling for anchor head protection based on results of anchor exhumation and inspection.

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"Inspection of Anchorages Supporting Waterside Structures". Authors : A.D. Barley; D Mothersille for presentation at International Conference on Anchors November 2007, ICE London

From 1969 ground anchorages were installed at Aberdeen Harbour to accommodate increases in quay wall depth brought on by accommodating a fully tidal harbour. For the majority of these anchorages, the corrosion protection measures employed would be judged inadequate by today's standards. The paper assesses the results of visual inspections and metallographic examinations, undertaken in 1991 and aimed at identifying the nature of corrosion on samples of strand that failed during testing.

Observations are compared with a similar programme of inspections, carried out on 35 year old anchorages on the River Thames in London, and undertaken in 2005. Observations confirmed that the anchor head protection on both contracts was not fit for extended use and were all in various states of corrosive degradation. The paper also describes the rating system developed to create objective consistency in the inspection process and the procedure used to extend the service life of the existing anchorages.

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"Investigation of the Failure of Permanent Anchorages adjacent to a Tidal Surge Barrier". Authors : A.D. Barley; D Mothersille; P Charnley for presentation at International Conference on Anchors November 2007, ICE London<

To provide protection against flooding of densely populated areas, a tidal barrier and anchored sheet pile walls were constructed as part of overall flood protection scheme across a tidal estuary in England. The piled structure, completed in 1979, were supported by thirty four 48m long permanent ground anchorages installed below the capping beam at an angle of 45 degrees to the horizontal.

In September 1991 a 25m length of the pile wall was subject to a lateral deflection of up to 450mm at the pile head. The paper describes the visual examination of the wall, exhumation behind the wall to anchorage level and inspection of the exposed anchorage components. These included failure of the anchor tendon and pull out of the tendon from the anchor head. In addition, there were indications of the use of inappropriate construction techniques employed at the time of installation.

Given anchored structures of similar type and age exist at many UK locations, there is concern that similar standards of construction and design specification may have been adopted elsewhere. By highlighting the failures faced at this site, the paper gives an insight into the issues that could potentially be presented in other structures and provides recommendations on forward strategies that could be adopted to prevent failures in the future.

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"Ground Anchor Tendons in Carbon Fibre-Reinforced Polymers". Authors : A.D. Barley & John Hartley for presentation at International Conference on Anchors November 2007, ICE London

Carbon fibre plates and flat bar have been used extensively for structural repairs for over a decade. Carbon fibre is inert in the majority of natural environmental condition and does not deteriorate in the alkaline environment of cementitious materials. As a tensile member it has numerous advantages to offer as a replacement of high grade steels which themselves are highly vulnerable to corrosion, and over a time period loss of section.

This research is indicative that carbon fibre anchor tendons can comply with the requirements of anchor codes and be installed as individual or as a multiple of bars each with a load capacity of between 400 and 500 kN. The use of such light weight components may allow fabrication at site level and avoid factory application of corrosion resistant barriers as demanded in the use of steel tendons

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"Engineering assessment of the grease-filled annulus of sheathed prestressing strand for use in permanent ground anchorages". Authors : A.D. Barley & Devon Mothersille for presentation at International Conference on Anchors November 2007, ICE London

The intent of this paper is to briefly review the processes currently adopted to manufacture greased and sheathed strand. The paper also assesses the success of the greasing operations applied to the free length of strand tendons and reports on tests, carried out to investigate the efficiency of greasing, on samples produced by three different manufacturers.

The test work has revealed an unacceptable variation in product quality. Codes of practice recommend that the annulus between the individual strand and the surrounding sheath should be "completely filled with a corrosion inhibiting compound." Where specialized equipment and factory controlled conditions were employed, the efficiency of grease infilling ranged from 76% to 100%. Investigation of strand greasing from one particular site indicated a range of 0% to 100. A practical acceptance criterion > 80% is recommended and that all sheathed strand suppliers should provide factual data confirming compliance with this criterion that can also be checked routinely in the field.

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"Repeated Lift-off Testing of Single Bore Multiple Anchors for Dam Retaining Wall over a 5-Year Period". Authors : Mary Ellen C. Bruce, P.E.; Jesús Gómez, Ph.D., P.E.; Robert P. Traylor for presentation at International Conference on Anchors November 2007, ICE London

The right downstream retaining wall of Hodenpyl Dam in Michigan underwent significant lateral displacement due to global instability of the in situ soils. After a failed remediation attempt with traditional tiebacks in 1996, a remediation effort was implemented in 2002 to limit the rate of movement of the wall while additional studies were conducted. The remediation design called for installation of additional anchors bonded within a high plasticity, stiff clay layer. The length of the bond zone of the anchors was limited by the presence of an artesian aquifer underlying the clay layer; therefore, the design unit bond stress was relatively large. To reduce the potential for creep and increase the unit bond strength, the contractor selected the Single Bore Multiple Anchor (SBMA) system with comprehensive, selective post grouting.

The anchors were effective in reducing significantly the rate of movement of the wall. Presently, the wall is being monitored for displacement and periodic lift off tests of the 2002 SBMA tiebacks are being performed while possible additional remediation efforts are devised for future implementation as needed. The monitoring and lift off test data provided a unique opportunity to correlate the response of the anchors with the wall movement. This paper describes the design and construction of the SBMA anchors, testing of a sacrificial anchor, lift-off testing procedures and results, and conclusions.

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