Piling equipment and methods 119prised out by spade when the cutter is raised to the surface. Drilling is effected in coarse-grained
soils by means of a baler or ‘shell’, which is again a simple tube with a cutting edge and flap
valve to retain the soil, the soil being drawn into the baler by a suction action when the tool
is raised and lowered. If no groundwater is present in the pile borehole, water must be poured
in, or a bentonite slurry may be used. This suction action inevitably causes loosening ofthe soil
at the base of the pile borehole, thus reducing the base resistance. The loosening may be
Figure 3.36Pile top rig drilling 3.8 m dia piles for foundation strengthening to the Richmond-San
Raphael bridge, California (courtesy Seacore Ltd).