Removable anchors for temporary works allow the technique to be used on the most congested urban projects. Tony Barley charts the development of the technique.
Over the past 40 years ground anchors have been used to retain cofferdams around the world.
As cofferdam support is replaced by floor slabs during construction, these temporary anchors become redundant. But until recently they were usually left in place.
Many of these temporary anchors have been installed under highways and buildings where they may prevent or make it difficult for basements to be built or utilities installed later.
This potential 'contamination' means the use of anchors has been considered unacceptable on some recent projects.
The idea of temporary anchors with removable steel tendons was conceived in the early days of anchoring in the 1960s. However, attempts to develop removable anchor systems have had mixed success.
An early concept was to install bar tendons with a coupler positioned at the free length/fixed length junction.
The debonding and lubrication of the free length tendon allowed the rotary removal of this tendon component after destressing of the anchor.
It is a simple idea, but a 100% removal cannot always be guaranteed, particularly when long multiples of coupled bar lengths are required.
The principle was extended to apply to multistrand tendons in which the strand steel at the free length/fixed length junction was deliberately weakened.
After they had performed their temporary function, strands were loaded to achieve explosive failure and then withdrawn.
However, most of the so-called 'removable' systems developed before 1990 only allowed, at best, withdrawal of the tendon from the free anchor length.
The fixed anchor zone, often outside the construction site, remained 'contaminated' with either parallel or criss-crossed steel bars or strands.
Mountain climbers or those who watch sponsored abseiling events know that descent of a mountain face requires recovery of the most essential part of the equipment - the rope.
This is achieved by using a double rope looped through a fixed point in the rock such as a sling or piton. When this part of the descent is complete, one of the ends of the loop is pulled to recover the rope.
This concept of the recovery of a flexible loop led to the 'invention' of a highly successful removable anchor system that allows the entire length of multiple strands to be withdrawn from the full length of the anchor bore with relative ease.
Not just one but many looped strands are installed in the bore during grouting, with the base of each loop located at a staggered depth along the fixed length of the anchor.
Each loop passes around a curved saddle and short plastic compression bar grouted into the bore. This arrangement is a removable, patented single bore multiple anchor system (SBMA).
The loop in each strand is bent before delivery to site and a number of loops banded together to form a prefabricated tendon. The entire length of the strand in each of the loops is lubricated with grease and contained within a close fitting plastic sheath.
Like the abseil rope, once the anchor is finished with, one end of the loop is pulled and the entire tendon easily recovered.
The availability of such a successful totally removable anchor system, supplemented by the high capacity available when using the SBMA concept of staggered load distribution, has influenced the use of modern anchors for temporary cofferdam support.
Over the last decade in the UK totally removable anchors have been specified for use in Edinburgh, Manchester, London, Kent and Croydon.
The technique is now being used on a mixed development in central Leeds. On the site in Whitehall Road, a steel sheet pile cofferdam for an excavation is being supported by a single row of up to 30m long, 400kN working load anchors.
Barr Construction is carrying out the design and build contract for developer KW Linfoot. Keller Ground Engineering is installing the anchors.
Rather than the capacities of the anchors, the attraction is the 100% removal of the strand from the bore to leave only a grout filled hole in the soil.
In soil anchors loads as high as 2000kN in up to six rows have proved successful and more than 100km of strand removed.
The system has been used in Berlin, Graz, Hong Kong and Singapore and is growing in popularity. Some local authorities have even made removal of all temporary anchor tendons mandatory.
Tony Barley is managing director of Single Bore Multiple Anchor.