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A secant wall is using two piling techniques to aid verticality on a development in the heart of London's Docklands.

Site workers are helping to drive forward a mixed use waterside development at the former site of the London Arena by using two separate piling techniques. The Crossharbour development is by the water's edge of Millwall Inside Dock.

'A secant wall solution was used for this inner city project because it causes minimal vibration and comparatively low noise levels, ' says Bachy Soletanche contracts manager, Barry Osborn.

'The Crossharbour site has varied ground levels and the interlocking piles form a closed structure, which acts as a barrier in water bearing soils.'

The wall is being built using a cased CFA piling method to cater for strict tolerances required by client Ballymore Properties. It speci ed a vertical tolerance of 1:200 to ensure all piles are installed without gaps that might compromise its watertight structure.

The steel casing used jointly with the CFA method enhances the vertical accuracy while satisfying the client's request that the wall has a smooth nish.

Bachy says it is 577m long and comprises about 970 piles. Two drilling techniques are being used to create it.

Female piles with a diameter of 750mm consist of a softer concrete mix than the male piles.

For the 14m long female piles, a Bauer BG-30 rig pre-bores about 1m into the ground and then rotates down 8m temporary steel casing.

Once in place a smaller Soilmec CM-48 rig drills a CFA pile down to depth inside the casing.

Bachy is installing the male CFA piles with the same diameter, also using a cased CFA method. However, the single Bauer BG-28 rig being used for these has a dual motor head that can drive a temporary steel casing down while simultaneously drilling the CFA pile inside - thereby removing the need for two separate rigs.

The BG-28 is boring through a strata that has layers of made ground down to very soft clay with limestone at about 14m down.

'The 485 male piles are drilled to a depth of 14m as a band of clay appears at 10m to 13m depths, which helps to form a natural seal for the toe of the wall, ' says Osborn.

The contractor will install a capping beam to tie the wall together at the top. Originally awarded only the contract for a secant wall, Bachy is now installing piled foundations for an additional contract.

Aside from those for the wall, about 980 CFA bearing piles are going in. These 750mm or 900mm diameter foundations will be bored to depths between 11.5m and 26.5m.

Work on the secant wall began in October last year and was due to be completed as GE went to press. The main ground engineering work to install CFA piles is scheduled to nish within the next month.

The development at Crossharbour will comprise 1047 residential apartments, 87,126m 2 of ofce space, a hotel and 26,556m 2 of restaurants, retail and leisure facilities.

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