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Secrets of the system

Bachy Soletanche's displacement piling technique relies on imparting high torque rotary energy directly down to the drill bit - here a large conical screw. This requires the firm's highest powered rig - a Casagrande C90 that provides 55tm torque through an HT55 rotary head - to be modified so that a strengthened kelly bar can be passed through the rig's rotary drive and be powered directly by a sleeve in the centre of the casing adapter.

The rig's full torque, normally used to install casing, can then be redirected to the kelly bar and down to the cone. The adapter also holds and rotates the casing at the same speed as the kelly bar. A fine water mist is sprayed into the casing to help suppress any dust.

The diameter of the cone is only fractionally less than that of the casing through which it protrudes. The two units are not directly connected, and the cone can move some 700mm vertically in relation to the follow-on casing. But the aim is for both advance together so no soil can enter the casing.

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