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Ground Stability, Foundations and Substructures 115
(b)
Photograph 3.5 (Continued)
analysis of the subsoil strata is obtained from the soil withdrawn. A disadvantage of piles
cast in the ground is that it is not possible to check that the concrete is adequately compacted
and whether there is adequate cover of concrete around the reinforcement.
Figure 3.49 illustrates the drilling and casting of a bored cast-in-place pile. Photograph
3.6a and b shows the piling rig and steel piling tubes used for the pile excavation. Soil is
withdrawn from inside the lining tubes with a cylindrical clay cutter that is dropped into
the hole, which bites into and holds the cohesive soil. The cutter is then withdrawn and
the soil knocked out of it. Coarse-grained soil is withdrawn by dropping a shell cutter (or
bucket) into the hole. Soil, which is retained on the upward hinged flap, is emptied when
the cutter is withdrawn. The operation of boring the hole is more rapid than might be
supposed, and a pile can be bored and cast in a matter of hours. Concrete is cast under
pressure through a steel helmet, which is screwed to the top of the lining tubes. The
application of air pressure at once compacts the concrete and simultaneously lifts the
helmet and lining tubes as the concrete is compacted. As the lining tubes are withdrawn,
protruding sections are unscrewed and the helmet refixed until the pile is completed. As