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Painless extraction

Bauer Technologies has come up with an innovative way of removing piles from a Crossrail site in Moorgate, central London. NCE reports.

Bauer Technologies was presented with a complex pile removal operation at Crossrail’s C501 site in Moorgate. Removing the 900mm diameter, 35m deep piles was a particular challenge because of their size and depth.

They also had to be removed without excessive vibration or noise as the site is in a densely built up area of the City of London and is close to sub surface Underground lines.

“In the recent past our industry has become reasonably proficient at destroying piles in situ,” says Bauer Construction director Yvonne Ainsworth.

“But generally these have been old tripod piles of up to 600mm diameter and around 20m to 25m deep. Incremental destruction of piles can be a noisy process and the limitations imposed upon us would not have allowed the 900mm piles to have been removed without causing some nuisance to the surrounding neighbours.

“We elected to adopt a method of de-bonding the piles from their surrounding soils and later ‘disassembling’ them above ground for transport off-site. The real challenge was to find a method that could reliably remove the soil from around the piles.

“We considered many methods of loosening, liquefying or isolating the soils before lifting the piles from the ground and eventually settled on a novel process.”

The Bauer method involves first drilling a threaded bar down the full length of the pile and grouting it into position. A large diameter segmental casing is then installed eccentrically over the pile, using a powerful BG40 drilling rig, adapted to increase its torque capacity for casing installation by 50%. Once the casing is sealed into the London Clay stratum, an open hole of around 900mm is drilled alongside the pile and also within the segmental casing.

“Incremental destruction of piles can be a noisy process”

Yvonne Ainsworth, Bauer Technologies

This is termed the relief hole.

What is known as an annulus cutter, designed and manufactured by Bauer Technologies, is then attached to the drilling rig and lowered over the pile.

Using the rig to rotate the device, the annulus cutter incrementally removes the soil from around the full periphery of the pile, with the soil cuttings falling to the bottom of the relief hole. In a series of increments, the soil is first cut from around the pile, then removed from the relief hole until the tool has passed over the full elevation of the pile.

Bauer recognised early in the development stage of the tool that completely removing the soil from around the pile would destabilise it. In order to deal with this difficulty Bauer designed an elaborate system of load bearing beams to maintain tension in the centrally grouted bar - and thus the pile during the entire soil-removal process.

Once the soil is removed to the toe of the pile, hydraulic jacks within the head works can be used to raise the pile slightly from the ground, to establish its known weight, before a 280t crane ultimately lifts the pile through a working platform, where it is cut apart into sections between 2m and 11m long.

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