Pile Design and Construction Practice, Fifth edition

Ground investigations, contracts and testing 499

information on the compressibility of the soil or rock strata with depth, thus enabling
calculations to be made of the settlement of the pile groups in the manner described in
Sections 5.2 and 5.3. If the piles can be founded on a strong and relatively incompressible
rock formation the drilling need not be taken deeper than a few metres below ‘rockhead’(the
buried interface between overburden or superficial sediments and rock), to check that there
are no layers or lenses of weak weathered rock which might impair the base resistance of
individual piles. However, before permission is given for the drilling depth to be curtailed
in this manner there must be reliable geological evidence that the bearing stratum is not
underlain by weak compressible rocks which might deform under pressures transmitted
from heavily loaded pile groups, and that large boulders have not been mistaken for bedrock.
Rockhead contours formed due to erosion prior to the deposition of the overburden may be
unrelated to current topographical surface. It is sometimes the practice, when preparing
borehole records, to define rockhead or bedrock as the level at which auger or cable per-
cussion boring in weak rock is terminated and core drilling in the stronger rock commences.
This practice is quite wrong. The decision to change to core drilling may have nothing to do
with the perceived strength of the rock. It may be no more than a routine changeover of
equipment at the end of a working shift.
Particular care is necessary in interpreting borehole information where the site is underlain
by weathered rocks or by alternating strong and weak rock formations dipping across the site.
Without an adequate number of coredboreholes and their interpretation by a geologist,
wrong assumptions may be made concerning the required penetration depth of end-bearing
piles. Two typical cases of misinterpretation are shown in Figure 11.2.
Where piles are end bearing on a rock formation it may be desirable, for economic
reasons, to obtain a detailed profile of the interface between the bearing stratum and the
overburden, so enabling reliable predictions to be made of the required pile lengths over
the site. Cased light cable percussive rig (‘shell and auger’) borings followed by rotary core
drilling to prove the rock conditions can be costly when drilled in large numbers at the close
spacing required to establish a detailed profile. Geophysical exploration by seismic refraction
on land and by continuous seismic profiling at sea are economical methods of establishing
bedrock profiles over large site areas. However, the success of these indirect methods
depends on there being a sufficient contrast in seismic velocity between the rock stratum

Pile depth assumed from

preliminary investigation

2/3 D

D

B

1 in 4

Depth of borehole1.5

B

Figure 11.1Required depth of boreholes for pile groups in compressible soils.