Pile Design and Construction Practice, Fifth edition

Piling equipment and methods 115

Figure 3.32Numa hammer with extending under-reaming drill bit for simultaneously drilling and
inserting casing (courtesy Numa Hammers).

drill plate bits and tricone bits can be rotated by the kelly in most drillable ground conditions,
subject to the available power. The kelly may be in sections, or more usefully, telescopic to make
up the required length of drill string. Boreholes can be drilled as open holes, or supported by
either excess hydrostatic head using drilling fluids (see Section 3.3.8) or by casing. The casing
can be installed by oscillators or by the rotary drive with some of the larger rigs. Under-reamers
and belling tools are expanded by an upward or downward force from the rotating kelly.