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Caught at slip

Foundations Slope stabilisation

A grown-up minipiling rig has made light work of a slope-stabilising retaining wall in Hythe.

The South Kent coastal towns of Folkestone and Hythe are built on cliffs riddled with deep-seated landslides.

The geotechnical hazard this represents is clearly illustrated by the difficulty housing developer Larkshire has experienced in the last 18 months.

Larkshire obtained approvals for a small housing development on a site on Hythe's Hillside Street, which was formerly occupied by a garden centre. But when Larkshire's contractor Bees got on site in October last year, a modest cut into the slope triggered significant movements on the site itself and within several gardens up-slope of it.

Not surprisingly, all hell broke loose. As the first stage of a rescue plan, Shepway District Council and local residents brought in Southern Testing who sensibly recommended the immediate replacement of excavated materials, the installation of surface drainage and further site investigation.

Bees meanwhile had contacted Kent-based consultant Weeks to plan remedial works.

Weeks was also independently approached by Shepway to comment on Southern Testing's recommendations and as Weeks director Phil Parnell puts it, it seemed 'sensible that we carried on with the whole thing.'

The company completed the detailed investigation in January 2002. This confirmed the geology to be as mapped by the British Geological Survey - with landslipped Atherfield Clay over-lying landslipped Weald Clay and the Hythe Beds escarpment up slope.

Weeks identified the principal ancient basal landslip surface to be at a depth of 8m, but concluded the up-slope damage had resulted from a shallow translational slip.

The site slopes down at an average of 11 degrees, but locally is much steeper. The houses are designed as 'balanced rafts' sitting on platforms cut into the slope. They are balanced in the sense that the weight of the houses is equal to the weight of the removed soil. However, as the slope was at the point of failure, it was impossible to build the houses without first stabilising the slope.

Weeks has three decades of experience on slope stabilisation projects in the area.

According to Parnell 'they are always tricky, but this one was particularly constrained.'

First, Hillside Street is narrow, steep and winding, making access difficult. Furthermore, access over the site, difficult at the best of times, was even more precarious after the failure. Additionally the site was surrounded by generally very angry householders, which meant negotiating permission to install ground anchors below adjacent properties was basically a non-starter.

That left the only realistic option to be a substantial cantilevered retaining wall, designed - because of the history of landslips - using effective residual strength soil parameters. This meant assigning a very low phi value for the clay, which combined with a high water table meant substantial piles would be needed to retain a cut of up to 4m.

To top it all, Shepway wanted wall deflections to be limited to 50mm - a reasonable target given the sensitivity of the work - and as a consequence the retaining wall would need substantial steel reinforcement.

Clearly a solution was needed that would not only fulfil these requirements but that could also be constructed safely and without risk of triggering further slope movements.

While Parnell was pondering how such a wall could be constructed, he read an article in NCE sister magazine Ground Engineering about a foundation contractor, Piling Solutions, which specialised in difficult access work. It had imported a minipiling rig from Brazil that could form 600mm-diameter, high-capacity continuous flight auger piles - far greater diameters than other minipiling rigs available.

The key thing about the Clo Zironi manufactured rig is it has a very low centre of gravity compared to a conventional unit. As a consequence Piling Solutions' CA40 rig can be mounted on a smaller and lighter carrier making it particularly suitable for difficult and limited access work.

When fully flighted, the CA40 has 13.6m of augers - and a 6m Kelly bar extension is available for greater depth - yet the rig sits on a tracked Komatsu base, just 2.7m wide.

The low centre of gravity is achieved by rotating the auger through an hydraulic drive box at the base of the mast. This grips and rotates the auger through the innovative use of rectangular 'keyways' or slots cut into the auger flights. As David Wandless, Piling Solutions contracts manager, puts it: 'This puts a big chunk of weight down at the bottom of the rig, instead of at the top of the mast'.

The rig fitted Parnell's needs exactly. Its relatively modest proportions meant a firm and level piling platform could be safely formed at the top of the slope - and by using larger diameter piles he could produce a 'sensible' design. 'If we hadn't found Piling Solutions we would have needed an awful lot of 450mm diameters, ' he says.

The resulting £250,000 scheme includes 141, 600mm diameter piles at 1m centres, up to 16m long. Piles are reinforced to their full depth with up to 9T40bars.

Where the excavation was deepest, Wandless suggested a novel design modification, incorporating blocks of piles immediately downslope of the wall. These act as buttresses to reduce deflections.

Piling Solutions carried out the work during the summer and, given the potential for problems, it went remarkably smoothly, aided by dry weather.

The proof of the pudding with any retaining structure is when the wall is exposed and loaded.

To date, Bees has removed soil from in front of the wall along a third of its length, without drama. A year behind its original schedule, the housing developer is at last making progress on the new houses, no doubt extremely grateful for the rising housing market.

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