Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 9 FOUNDATION ENGINEERING
pile (practically 0.5 m) with others positioned near stratum boundaries and at intermediate levels. A
minimum of two and preferably four gauges should be provided at each level where practicable.
c) Lateral loading tests
The common types of internal instrumentation used in a lateral loading test are inclinometers, strain
gauges and electro-levels.
The deflected shape of a pile subject to lateral loading can be monitored using an
inclinometer. The system consists of an access tube and a torpedo sensor. For cast-in-place
piles, the tube is installed in the pile prior to concreting. For displacement piles such as H- piles,
a slot can be reserved in the pile by welding on a steel channel or angle section prior to pile
driving. The tube is grouted into the slot after driving. During the test, a torpedo is used to
measure the slope, typically in 0.5 m gauge lengths, which can be converted to deflections.
Care needs to be exercised in minimising any asymmetrical arrangement of the pile section or
excessive bending of the pile during welding of the inclinometer protective tubing. In
extreme cases, the pile may become more prone to being driven off vertical because of these
factors.
Strain gauges with their stems orientated in line with the pile axis can be used for measuring
direct stresses and hence bending stresses in the pile. They can also be oriented horizontally to
measure lateral stresses supplemented by earth pressure cells.
Electro-levels measure changes in slope based on the inclination of an electrolytic fluid that
can move freely relative to three electrodes inside a sealed glass tube (Price & Wardle, 1983;
Chan & Weeks, 1995). The changes in slope can be converted to deflections by multiplying the
tangent of the change in inclination by the gauge length. The devices are mounted in an
inclinometer tube cast into the pile and can be replaced if they malfunction after installation.
Earth pressure cells can also be used to measure the changes in normal stresses acting on the pile
during loading. It is important that these pressure cells are properly calibrated for cell action
factors, etc. to ensure sensible results are being obtained.
9.2.5.5 Interpretation of Test Results
a) General
A considerable amount of information can be derived from a pile loading test, particularly with an
instrumented pile. In the interpretation of test results for design, it will be necessary to consider
any alterations to the site conditions, such as fill placement, excavation or dewatering, which can
significantly affect the insitu stress level, and hence the pile capacity, after the loading test.
b) Evaluation of failure load
Typical load-settlement curves, together with some possible modes of failure, are shown in
Figure 9.7. Problems such as presence of a soft clay layer, defects in the pile shaft and poor
construction techniques may be deduced from the curves where a pile has been tested to
failure.
March 2009 9-25