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Foundations

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Geotechnical contractor Norwest Holst Soil Engineering recently installed continuous flight auger piles in restricted headroom as part of a refurbishment contract at Meadowhall shopping centre in Sheffield.

Working for main contractor Marshalls, Norwest Holst used specially adapted Edeco continuous flight auger rigs with shortened masts to work in headroom as low as 6.5m to put down over 300, 450mm diameter CFA piles to a maximum depth of 12m.

The rig was adapted and short auger lengths used. Trials were first carried out under similar conditions on another site to prove the system could produce piles of sufficient capacity.

The site on the east side of Sheffield was once home to the Vulcan Steelworks. With up to 4m of industrial fill, pile positions were probed in advance at full pile diameter, which allowed any obstructions to be removed.

Concrete was mixed on site, which enabled use of a mix tailored to the piling work.

Sensor purpose

Foundation contractors Bachy Soletanche and Bauer Foundations are using an innovative piling method to ensure integrity of continuous flight auger piles for a secant wall in London's Docklands.

The firms are working in joint venture for client Canary Wharf Contractors in a massive 400m long, 384,000m2 cofferdam being formed in the dock around Heron Quays next to the existing Canary Wharf development. Consultant for the contract is Arup Geotechnics.

The wall, which is initially helping to support a section of the cofferdam will eventually form the basement for a new 32 storey office block on plot HQ5. Five rigs are installing the 95m long wall, which comprises 170 male and female piles.

Tolerances are very stringent to ensure the basement is watertight, with verticality set at 1 in 200. Temporary steel casing is being used for the 30m long male piles which are founded in the Thanet Sands to control accuracy, installed by high torque rigs - two Casagrande HT55 and two BG30 rigs - that can handle such heavyweight casing.

The fifth machine is a Bauer BG22 CFA rig which is forming the 900mm diameter female piles. These are 15m long and founded in the London Clay.

One of the main concerns for CFA piling is integrity. Most CFA rigs are fitted with pressure sensors to monitor concrete flow. A positive reading, above groundwater pressure, indicates that concrete is filling the full diameter of the pile. But these sensors are typically mounted at the top of the auger and can only measure a pressure differential that can be affected by concrete flow rates and speed of auger extraction. Traditionally, the only truly accurate measure of integrity is a visual check of the piles once they are excavated.

The Bauer rig is fitted with a pressure sensor mounted at the tip of the auger. This records real, rather than differential concrete pressure and therefore gives a much more accurate indication of pile integrity.

It is linked to the rig's Betronics system, which offers real time monitoring of the rig functions on a display mounted in the cab. Cables running up the hollow stem of the auger have to be segmental to allow new sections of the auger to be added.

Rig at large

Foundation contractor Expanded Piling is using its latest con- tinuous flight auger piling rig to install large piles for a new residential development in London.

Working for client Galliard Homes and main contractor Carillion Building, the firm is using its new Soilmec CM120 machine to install 252, between 600mm and 900mm diameter piles for the six storey building at Cayenne Court in Butler's Wharf.

Expanded says one advantage of the new rig is that it can install large diameter CFA piles to great depth. For this contract, the 900mm piles are being put down to a maximum of 28m, which means a single large diameter pile can be used instead of multiple, smaller diameter, piles.

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