anchored at the base of the pile in a sleeve, and by jacking the tension member against the
pile head the ability of the pile shaft to carry the design compressive load can be checked.
It is possible to recover the tension members after each test.
Drilling, either by open hole methods using a percussion drill or rotary rock roller bit, or
by rotary coring, can be used in piles of medium to large diameter, but it is difficult to keep
the drill hole within the confines of the shaft of a small-diameter pile. If it is possible to
flush an open hole clear of dirty water an inspection can be made by TV camera to look for
cavities or honeycombed concrete. Heavy water losses when the drill hole is filled with
water also indicate defective concrete. A cored hole provides a better indication of sound-
ness, and compression tests can be made on the cores, but the method is more costly than
open hole drilling. It should be noted that cores are only likely to be obtained from sound
concrete and any defective zones may not be recovered for testing. Calliper logging down a
drill hole gives an indication of overbreak caused by weak concrete or cavities.
Sonic pulse equipment can also be lowered down drill holes, when irregularities in the
trace of the sonic log indicate a defective shaft. Pairs of ducts can be formed down a
pile shaft at the time of placing the concrete and various logging devices used to scan the
concrete between the ducts. These include sonic pulse measurements, gamma-ray logging
and neutron emissions. The latter methods are believed to be reliable indicators of density
changes and water content respectively, but are costly since they involve the use of skilled
technicians and the transportation to site and operation of nuclear testing devices with
their elaborate safety precautions. The seismic method consists of dropping a weight
on the pile head and observing the time of return of the seismic wave reflected from the
toe or anomaly in the pile. This method is quite widely used and has been shown by
536 Ground investigations, contracts and pile testing
Figure 11.21SIMBAT test using a mini rig for small diameter dynamic tests (courtesy Test Consult).