PAP0054-01.pdf

SERVICE PERFORMANCE OF TEMPORARY REMOVABLE MULTIPLE ANCHORS AND CASE

HISTORIES

Tony Barley,Single Bore Multiple Anchor Ltd, Harrogate, UK

Email: [email protected]

Goncalo Veloso,

Keller Ground Engineering, Wetherby, UK / Single Bore Multiple Anchor Ltd, Harrogate, UK


Dr. Devon Mothersille,Single Bore Multiple Anchor Ltd, Harrogate, UK


Abstract

Ground anchors have been used for more than three decades for king post, sheet pile wall, diaphragm

and bored piled wall retention. However, research and development in specialist anchor technology over the last

fifteen years has made available temporary ground anchors with supplementary benefits: reduction in cost per

kN of retention force provided by anchors in soils or weak rocks, in conjunction with the availability of a ground

anchor system which on completion allows the complete removal of the entire steel tensile member from the

ground.

The removal of anchor steel tendons from the ground has made the use of temporary anchors more

acceptable and attractive to contractors and owners of adjacent properties and highways. Since the remaining

grout filled bore after anchor removal presents little or no obstacle, a development of an adjacent land can

therefore be carried out without major physical restrictions.

Introduction

Asking an adjacent landowner of a city site if you can drill under his plot leads to a number of

questions, probably the two most important are: what is the purposes of the anchors and what is left in the

ground when you finish? Well the purpose as we all know is to temporary support the new retaining wall,

however, if you are proposing to leave steel in the ground under his property ad infinitum, then he may be

understandably very reluctant to authorize this permanent ground contamination. For instance, if in the future a

deep excavation for his own new basement is required, the presence of the buried steel would influence the

earthworks and if the installation of piles or other services was required it could compromise its installation.

Fixed Length Design Philosophy

Multiple anchors consist of a number of short unit anchors founded at staggered depth in a single anchor

borehole (Barley 1997). Typical unit anchor fixed lengths are varied from 3.0m to 5.0m depending on the

envisaged strength of the ground and the bond capacity anticipated in that ground at that unit anchor founding

depth. The use of a multiple of short fixed lengths reduces the effect of progressive debonding encountered in

long conventional anchors and allows more efficient load transfer and hence mobilization of higher anchor loads

therefore enabling us to achieve very high loads on soils and weak rock.

Tendon Systems

Research work on totally removable anchors was started in the mid eighties by the investigation of the

capacity of a looped strand around a small diameter saddle (Figure 1). Inspection of a loop after testing did

indicate the expected damage to the sheathing but the sheath in the fixed anchor constitutes a barrier to prevent

tendon bond to the cement grout and not a protective medium (Figure 2).

Figure 1. Research work on strand Loading around a steel saddle (Barley 1989)

Other Systems which allow removal of the steel tendon from the free lengths (unbonded) have been

available for more than a decade, albeit the steel still remains in the fixed length. However, part of the

development of the high capacity multiple anchor system has involved the utilization of totally unbonded strand

loops within the bore. A plastic coated strand of 12mm or 15mm diameter is pre-bent to 180° looped strands at

its midpoint to internal an diameter as little as 60mm. This loop, complete with a “saddle” placed with the loop

is installed to the base of the anchor bore. A second looped strand is installed to a depth 3 or 5m from the base

of the bore and third, fourth and fifth etc., at the progressive distances from the base following the SBMA

principle

.

Figure 2. Example of a looped strand after removal from test arrangement

The strand ends pass through their own hollow ram jack and are loaded simultaneously and since all the

jacks are coupled to the same hydraulic power pack, the “unit” anchor loads are always equal. On completion of

usage of the temporary anchor the lubricated unbonded strands can be easily withdrawn from the grouted

boreholes by the use of a 10T winch (Figure 3) or a crane or where access space is limited by the use of a long

stroke stressing jack.

Figure 3. Tendon removal by Winch Method

Performance in Service

Working loads of up to 1200 kN and test loads of 3000kN have been achieved in the UK in medium

dense sands without failure. The looped strand temporary removable ground anchors performance follows the

same criteria as non removable anchors and are also all tested to acceptance test up to 1.25 x working load as per

the criteria on BS8081.

To evaluate the behavior under load of a temporary removable anchors, a typical example of a proving

test up to a test Load of 720 kN is shown in Figure 4.

Figure 4. Proof loading of a temporary removable anchor to Test Load of 720 kN involving four

simultaneously stressed unit anchors.

A further study (Barley, 2007) has shown that load loss checked after a year in these type anchors range

from zero to 5% of the original lock-off load. Hence no unit anchors indicated a loss of load greater than the

10% normally accommodated in anchorages complying with the acceptable load change requirements of BS

8081.

These sound performances substantiated by very nominal loss of load ,or in other instances minimal

creep displacement, are entirely typical of multiple anchor systems. Conventional anchors which concentrate

load over a relatively short single section of the fixed lengths are more susceptible to creep than multiple

anchors which distribute load over a multiple of short lengths.

In the removable anchors, strands have been inspected after tendon withdrawal and exhibited no

indication of corrosion or corrosive products or indeed any damage to the seven individual wires making up

each of the removed looped strands.

Removable Single Bore Multiple Anchors in Doha, Qatar

Qatar like many countries in the Middle-East is experiencing tremendous economic growth and this is reflected

in its massively expanding infrastructure. The Doha Convention Centre and Tower project comprises some

105No. storeys and is typical of the type of construction that exists in a constantly changing skyline.

In a bid to satisfy environmental and physical constraints imposed on the project it was proposed that removable

anchor be used to provide temporary support to section of diaphragm wall which had a total perimeter of

approximately 1300m and up to three storeys deep. The excavation was carried out in weak limestone (referred

to as ‘Simsima Limestone’) and was characterised by heavy fracturing and the existence of cavities.

Trained site operatives carried out the tendon fabrication, grouting, installation and testing single bore multiple

anchors (SBMAs) in Qatar (Figure 5). In highly weathered and fissured Simsima Limestone tested loads of up to

1920kN were achieved without failure. The programme of works also involved the testing of removable

SBMAs. After satisfactory proof testing in accordance with PTI (2004) the tendon were completely removed in

safe and controlled operation.

Figure 5 Trial anchors installed in weak Limestone, Doha, Qatar

Removable anchors in Hyde Park London, UK

The new multi-million pound penthouses at 1 Hyde Park, London, UK required temporary high capacity

removable anchors in London Clay for basement support.

Over 100 anchors were installed in 2006 and subsequently removed using a 10T winch. No problems are being

encountered in removing all the looped strands from the Single Bore Multiple anchors.

Figure 6. Removing anchor pice detail

Removable anchors in Canary Wharf, London, UK

In the heart of the financial district of London, construction of the basement of a new offices building required

temporary ground anchors to support a diaphragm retaining wall. Due to the constraints with underground

tunnels and public and privately owned property, the use of temporary removable anchors was chosen.

141 anchors were installed along different sections of the diaphragm wall. Anchors with working loads ranging

from 500kN to 1200kN were installed within the Thanet dense Sands and tested to acceptance up to 1.5x

working load. Trial anchors were installed to confirm design assumptions and test loads of up to 2800kN were

achieved.

Summary and Recommendations

The use of removable tendons in temporary work anchorage is becoming more common owing to the

modern advancement and development of ground anchorage technology. In many cities throughout the world

including major conurbations such as London, Hong Kong and Singapore it is considered that the abandonment

of used prestressing tendons within the ground is unnecessary and unacceptable long term contamination of the

subsoil. This removal after temporary provision of safe support of the cofferdam makes the use of anchors more

“environmentally frienly” and more acceptable to adjacent land owners.

National and International Codes of Practice for Anchorages should fully acknowledge the extensive

development of such available systems (Herbst 1997) and incorporate acceptance criteria for tendon removal

from temporary work anchorages.

References

British Standard, Code of Practice for Ground Anchorages, BS8081:1989

Trummer F., Stockhammer P., “Der wiederausbaubare Litzinanker, System Keller” Int. Conf on Anchors in

Theory & Practice – Salzburg, October 1995

Barley A.D. (1989)”Summary of initial saddle tests” Keller Ground Engineering Confidential Report

Barley A.D., “The Single Bore Multiple Anchor System” Int. Conf on Ground Anchorages and Anchored

Structures, ICE London, November 1997 – Pages 65 to 75

Herbst T., “Categories of Removable Anchors” Int. Conf on Ground Anchorages and Anchored Structures, ICE

London, November 1997 – Pages 197 to 205

Barley A. D., Edwards M, "Service Performance of Multiple Anchors in Difficult Ground Conditions for both

Permanent Anchors and Temporary Removable Anchor usage". International Conference on Anchors November

2007, ICE London

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