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Anchors take the load on Northampton clay

KELLER GROUND Engineering has used its patented single bore multiple anchor system (SBMA) to tie back a bored pile retaining wall for a Lias clay slope on a new industrial development in Northampton.

Keller technical director Tony Barley claims the system 'effectively doubles the load available from conventional anchors and has been safely used in clay soils to provide working loads well in excess of 1,000kN.'

Some 15,000m2 of industrial space is being built by developer Wrenbridge Brackmills on the site at Brackmills Business Park. The site has an 8m high slope running across it and sits next to a country park with planning height restrictions. To squeeze the planned development in and comply with planning height restrictions, a large scale cut and a 7m high retaining wall were needed around three sides of the site.

Soil conditions across the site are generally water-bearing Lias clays. The site has a history of slope failure, and site investigations revealed the slope was soliflucted with reworked soils. And although alternative solutions, such as soil nailing would perhaps have provided a more economical solution, consultant Brian Cole & Associates was concerned over the slope's long term stability.

It was decided that the most cost effective solution would be to use an anchored bored pile wall to withstand the potentially substantial lateral forces. Although Keller claims there is no record of ground anchorages ever being used in Lias clays, it proposed the use of SBMA, which has been successfully used in clays on other sites. The company also says that using the system has led to savings in pile diameters and reinforcement.

With ground levels rising up from both the north western and eastern boundaries, a series of level piling platform 'plateaus' had to be formed. Keller elected to use bored piles instead of the originally planned CFA piles, first to allow pile cutoffs to vary from 0m to 2m across each platform, and also to give a good quality finish. 'The site will form a natural amphitheatre, with exposed piles as an additional feature,' says Keller piling contracts manager Steve Longdon. This meant that a smooth, regular line was essential, which Keller achieved by using a single guide wall on the inner face of the wall.

The four-week contract started in early spring 1998. Keller used two rigs, a Soilmec R412 and a Watson 5000, to install some 473 piles to depths up to 15m. Pile diameters range from 600mm to 900mm depending on the retained height, which varies around the site. 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.

Keller also installed inclinometers in some of the piles which were monitored throughout excavation and after contract ended. This data will be used to refine design of the system for future contracts.

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