Pile Design and Construction Practice, Fifth edition

118 Piling equipment and methods

purpose-built crawlers and excavate a square hole, Ell-, Tee-, or rectangular slots under

bentonite or other support fluid. The ‘Hydromill’or ‘Hydrofraise’rig developed by Bachy-

Soletanche is a reverse circulation down-the-hole milling machine with two contra-rotating

cutter drums powered by hydraulic motors mounted on a heavy steel frame. The cuttings are

removed from the slot in a bentonite or polymer slurry by a pump fitted above the drums to

the de-sanding and cyclone plant at the surface for reconditioning and reuse. Over-break is

minimal and the absence of vibration makes the system suitable for urban sites and close to

existing buildings. Standard width is 600 mm but greater widths are possible for depths to

60 m. Walls have been constructed to 150 m deep, and a low head-room version is available.

3.3.7 Tripod rigs

Small-diameter piles with diameters of up to 600 mm installed in soils which require con-

tinuous support by lining tubes can be drilled by tripod rigs. The drilling is performed in

clays by a clay-cutter, which is a simple tube with a sharpened cutting edge, the tube being

driven down under the impact of a heavy drill stem. The soil which jams inside the tube is

Figure 3.35Wirth PBApile top rig (courtesy Wirth Maschinen und Bohrgerate-Fabrik GmbH).