Set in rolling wooded hills just north of the South Downs, Crest Nicholson's Bolnore Village housing development near Haywards Heath in East Sussex is now in its third phase.
This will see a further 233 units built on a steeply sloping part of the site. But alarm bells rang soon after work began last July when a site investigation, prepared by Crest's geotechnical consultant Southern Testing, identified an acute slope stability problem that would be exacerbated by the earthworks needed to allow development to go ahead.
The site is made up of alternating layers of over-consolidated clay and sandstone, which form the Wealden deposits of the Lower Cretaceous.Where these sheared clays outcrop on slopes, they are prone to landslipping, even at shallow slope angles.
The Bolnore site slopes on average at just 5infinity, although locally inclines are up to 22.5infinity.
Southern Testing, with knowledge of a number of slope failures on similar developments across Kent and Sussex, identified the need for substantial slope retention.
Crest technical executive Kevin Feeney says:'We designed the development's layout around the contours, but there were still massive forces to contend with.' Civil and structural consultant Powell Tolner & Associates (PTA) produced an outline design based around a substantial 5m to 6m high vertical contiguous piled wall, terracing the site into more gently sloping upper and lower areas, and optimising the garden gradients to make them more useable.
The design of the retaining structures had to account for both the stability of the wall resulting from the height of the retained soil, and critically that the wall is positioned within a potential landslip.
Stability was achieved principally through use of ground anchors, designed to tie back the wall and intercept - and so pin together - potential failure surfaces within the slope. A second piled retaining wall was also needed at the base of the slope to support a new access road running alongside a small stream and a proposed balancing pond.
The initial price for PTA's outline scheme came in over budget at £1.2M. But brought in to develop the proposals, Keller Ground Engineering looked closely at the loading, space and aesthetic requirements and was able to introduce a range of alternative retaining systems, reducing the cost of to £0.7M - 60% of the original estimate.
Keller's alternative scheme still includes sections of anchored bored pile wall, but also makes use of conventional cantilever retaining walls and includes a number of reinforced soil solutions including Timbacrib walls, Geolock and Textomur.
Bored pile walls were kept where space was tight and the house loads were close to the retaining wall, but where loads were lower and space was less critical, Keller used Timbacrib walling.
Keller's design made use of a combination of retaining approaches. For example, where earthworks include upfilling above the retaining wall, Keller designed a heavily reinforced piled wall to support the natural ground, but used cheaper reinforced soil techniques to retain the new areas of engineered fill to be built above.
Further savings were made by cutting down the number of ground anchors through increasing pile sizes. This not only reduced the number of piles within the wall, but also speeded up the programme.
Nevertheless around 20, 650kN ground anchors were still needed to provide for both the wall and overall slope stability.
To tackle the deep seated slip planes Keller used the single bore multiple anchor (SBMA) system. In this the load within a single anchor is shared by a number of different tendons, removing the risk of progressive de-bonding along the length. These high capacity anchors were up to 50m long - some of the longest Keller has ever installed.
The piles making up the contiguous walls were also designed to extend through the potential slip zone and needed to be over 18m long. Keller installed around 200, 600mm and 750mm diameter CFA piles over six weeks using a Casagrande CM48 rig. All piles were reinforced to their full depth, typically incorporating cages with 12 t40 steel rebars.
For the new access road being built on raised ground alongside the stream, Keller proposed a cantilevered bored pile wall in the area of the deep seated slip zone. The high groundwater table in this part of the site meant it was not practical to use ground anchors and so a double row of 750mm diameter piles is providing the required lateral restraint.
On less critical areas Keller opted for a combination of earth retaining solutions that included using their Textomur system and their Geolock precast modular block retaining system.
Keller's designs were checked by Southern Testing.
According to Don Bennett of PTA, the early involvement of both Keller and Southern Testing was vital. 'In terms of the value engineering for this complex geotechnical project it was outside our expertise, ' he says.
With ground stabilisation works complete, Keller is now back on site to start piling for the houses. The first occupants should move in by May, Feeney predicts.