The History and Evolution of the Single Bore Multiple Anchor System

"Wetherby based Keller Colcrete Ltd. has won the 1990 Castrol/Construction News Award for outstanding innovation in construction techniques, with an innovative flexible ground anchorage system, giving 50-150 per cent greater load capacity than traditional fixed anchor systems.

The company accepted the award from Sir Clifford Chetwood, Chairman and Chief Executive of George Wimpey Plc, at a ceremony held at London's Savoy Hotel on 15th November."

November 22, 1990 - Construction News. "Anchor Breaks New Ground"

"Every so often, an invention comes along which people will look at and ask the question: "Why has no-one thought of it before?" But designing the obvious: harnessing inovation with simplicity and efficiency is the true mark of a mind with a fundamental understanding of a problem." "...the single bore multiple anchor system (SBMA) had its birth in the mind of Keller Colcrete's contracts director, Tony Barley, some 10 years before he found time to develop it.""By any engineering standards, the SBMA is an impressive development. It performs a routine application cheaper, more efficiently and quicker than conventional techniques. It is a ground anchor system which makes the ground do more of the work, with a resultant saving both in time and money for client and contractor.""...SBMA system has been shown to increase the load capacity of an equivalent anchor site by at least 50 per cent, and as much as 150 per cent. And fixed anchor lengths of up to 25m. have been successfully tested with no evidence of redundancy."

"The...single bore multiple anchor system is being used on the 7 acre Castle Mall development (Norwich) right next door to the castle." "The multiple anchors are being installed in six rows down the piling wall with about 3m between each row.""Preliminary test anchors achieved 1300 kNewtons without failure, acceptable to the code of practice in BS8081."

"Using the patented single bore multiple anchor system that won the contractor the Construction News R & D Award last year, Keller will eventually drive and stress some 1,100 anchors through the retaining wall."

"...single bore multiple strand removable anchors can be destressed and withdrawn from a grouted borehole after use, leaving behind just the grouted column and small steel and plastic components." "Tendon pull out forces are comparatively low, less than 50kN, eliminating risk of strand breakage.""Keller Colcrete has carried out trials in London Clay and in Hong Kong granite. ...for the planned Jubilee Line extension ... These included four and six strand anchors,which were proof loaded to 1300kN then relaxed prior to the tendons being withdrawn.""In the Hong Kong trials 12 strands with a 3m rock bond length were loaded to 1600kN... A complete 26 strand, 28m long anchjor, using 12mm diameter tendons, was then tested to 2500kN, load released and strands withdrawn."

"Of the three SBMA trial anchors installed, two were overstranded to allow testing to three times the 600kN working load on the designed fixed length, while the third, constructed as a production anchor, was installed for load loss testing." "Keller loaded the overstranded anchors up to 2032kN with no indication of failure, says Barley. While in the load loss test the anchor was monitored for 10 days at 1.1 times working load (660kN) to ensure load loss conformed to BS8081, which requires load loss over 10 days less than 8%. At Newcastle only 3% was lost and when loaded to 900kN for 24 hours, load loss was still only about 3%.""Anchor lengths are up to 35m as the anchors pass, in free length form, through up to 15m of placed fill before encountering till."

"...a couple of drilling crews ... are installing what are said to be the highest loaded ground anchors yet designed." "Yet, impressive as working loads of 950kN are, contractor Keller Colcrete's engineering director Tony Barley claims the record will not last long. "Given suitable ground conditions, there is no reason why we cannot achieve working loads of 2000kN and ultimate loads approaching 5000kN," he says.""The 63 sets of ground anchors now being driven an average 45m into the steep sloping riverside site..."

"But using the SBMA system, the firm achieved a jump in the working load to between 50 and 115 tonnes. And Keller hasn't stopped there. "One year ago we were getting an ultimate load of 240 tonnes," says Mr Barley. "Now it is 360 tonnes."

"A further two anchors were installed in completely decomposed granite .... achieved in excess of 2000kN without failure .." "On completion of the tests, 54 of the 56 prestressing strands were fully removed from the 30m to 40m long anchor bores by simple jacking and winching."

"New records for high capacity anchors in soils have been established at the new Hong Kong station ... Working loads of 2000kN have been achieved, ..." "...further pre-production testing was needed to verify the design and system at these loads.Tests carried out ... achieved in excess of 3000kN without failure, confirming that 2000kN working load anchors were safely within the system's capabilities. Trial anchors also demonstrated complete removal of the tendons.

"The working SBMA anchors, each around 25m long, were installed at high levels on the wall as piling progressed, and were stressed once the capping beam was finished. A trial anchor was installed before the main works, confirming that a final design load of 750kN per anchor could be used in the Lias Clay."

"Working loads on the SBMAs are 1300kN, .... the highest ever achieved in permanent works anchors founded in such fine grained soils as the Bracklesham Beds - stiff to hard silty, sandy clay and claybound sand." "Two test anchors proved it when they were taken to 2.5 times the working load (up to 3250kN)."

"All other anchor systems are designed to found the entire anchor in a single stratum and thus normal systems could not have accommodated the variation in the encountered ground nor provided 900kN working load when founded in one of the relatively thin soil materials,..."

Further developments of the SBMA system are documented on the 'Stop Press' page.