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Taming landslips Terrace firma

Foundations - An armoury of techniques has been deployed on a landslip-prone site in East Sussex.

Nestled among the rolling hills of the South Downs in East Sussex, piling specialist Keller is taming a landslip-prone area for a new housing development.

Phase three of Crest Nicholson's Bolnore Village near Haywards Heath began in July. But alarm bells soon sounded when geotechnical consultant Southern Testing identified that the earthworks could exacerbate the already precarious ground conditions.

The sloping ground profile is made up of alternating layers of over-consolidated clay and sandstone, which form the Wealden deposits of the Lower Cretaceous. But the clay layers can contain highly polished shear surfaces which are prone to slipping even on the Bolnore's 5infinity slopes, requiring substantial slope retention works.

'We designed the development's layout around the contours, but there were still massive forces to contend with, ' says Crest technical executive Kevin Feeney.

Civil and structural consultant Powell Tolner & Associates' outline design included improving drainage and a 6m high vertical contiguous piled wall to divide the site into terraces to make the gradients more useable.

A second piled retaining wall was needed at the base to support an access road.

The original project cost of £1.2M was reduced to £700,000 through value engineering. Keller's alternative includes the vertical wall, conventional cantilever retaining walls and reinforced soil solutions. These include Timbacrib interlocking walls filled with gravel to form a gravity retaining wall, a precast modular block retaining system, Geolock and Textomur steel mesh formwork and geosynthetic reinforcement.

To tackle deep seated slip planes, high capacity anchors up to 50m in length were used - some of the longest Keller has ever installed.

The vertical piles within the contiguous piled wall are up to 18m long. Keller installed around 200, 600mm and 750mm diameter CFA piles over a six week period using a Casagrande CM48 rig, reinforced with 12 T40 steel rebar cages.

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