Ground investigations, contracts and testing 525The settlement of the pile head under load can be measured optically by means of a
surveyor’s level reading onto graduated scales fixed to the pile in four positions or by laser
beam producing an image on a photo-electric detector. Using a digital optical theodolite set
up 3 to 10 m from the pile and a suitable target the movement of the pile can be measured to
an accuracy of 0.5 mm which is more than adequate for most piling tests. An alternative
method is to set a dial gauge on each of four reference points on the pile head. The dial
gauges are clamped to a datum frame securely mounted well clear of the ground around the
pile or the reaction support system, and the gauges are calibrated to read to 0.1 mm. This
order of accuracy is not realized in practice since wind, temperature effects, and ground
vibrations can cause the datum frame to move by much more than 0.l mm. However, it is
helpful to be able to read to such an accuracy when making each increment of jacking force
since the time/settlement curve can then be plotted accurately and the rate of decrease of
movement is readily obtained. Levels should be taken on the datum frame before and after
the loading test to check that the frame has not been displaced during the test. A linear
potentiometer can be used to obtain the pile movements, which are read on a dial or print-out
mechanism at an instrument station well clear of the pile. The kentledge support system
must be carefully designed to give technicians safe access into the confined space under the
kentledge to install and read the dial gauges.
As reported by Fleming(11.20), it is now usual practice to record pile head loads and
settlements directly on a portable computer to plot load/settlement and time/settlement
curves as the test is in progress. The data can be reproduced in the format of the test report
and used to analyse the pile behaviour throughout the whole range of loading.
Figure 11.10Patented arrangement for a 5800 tonne static load test.