Secant piled walls are generally a preferred solution for inner city projects as they are more flexible in shape, the process is virtually vibration-free and, as they negate the need for support fluids, it allows for a faster, more compact operation. CSP can be used on various wall types, either contiguous, interlocking or secant and on deep or load bearing foundations.
This technique was developed to combine the more cost-effective CFA technique with temporary casings, which are more traditionally associated with rotary bored piling methods. The introduction of the temporary casing around the CFA auger improves the pile verticality and also increases drilling speeds through stronger materials.
CSP also has several specific advantages, from offering better tolerance, appearance and reduced overbreak compared to an uncased CFA secant wall, to minimising the risk of ‘flighting’ in poor soils. As a cased system, it also reduces the risk of distress to adjacent structures in particular soils such as sands or gravels.
The single flight auger allows rapid continuous drilling and while the rotary drive units for the casing and the augers are connected, they rotate in opposite directions, simultaneously drilling into the ground. The spoil travels up the auger string and is deposited from the top of the casing into a series of telescopic tubes, safely expelling it at ground level.
Concrete is then pumped through the hollow stem of the auger as it is retracted in combination with the casing and the cages are installed when the pile is complete.