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

Chapter 9 FOUNDATION ENGINEERING
In a compression or tension test, measurements should be taken by four dial gauges evenly spaced
along the perimeter of the pile to determine whether the pile head tilts significantly. The measuring
points of the gauges should sit on the pile head or on brackets mounted on the side of the pile with
a glass slide or machined steel plate acting as a datum for the stems. Care should be taken to
ensure that the plates are perpendicular to the pile axis and that the dial gauge stems are in line with
the axis.
In a lateral loading test, dial gauges should be placed on the back of the pile with the stems in line
with the load for measuring pile deflection (Figure 9.5). A separate system involving the use of a
wire, mirror and scale may be used as a check on the dial gauges. The wire should be held under
constant tension and supported from points at a distance not less than five pile diameters from the
test pile and any part of the reaction system.
Rotational and transverse movement of the pile should also be measured.
LVDT can be used in place of dial gauges and readings can be taken remotely. However, they
are susceptible to dirt and should be properly protected in a test.
The reference beams to which the dial gauges or LVDT are attached should be rigid and stable. A
light lattice girder with high stiffness in the vertical direction is recommended. This is better than
heavy steel sections of lower rigidity. To minimise disturbance to the reference beams, the
supports should be firmly embedded in the ground away from the influence of the loading
system (say 2 m from piles or 1 m from kentledge support). It is recommended that the beam is
clamped on one side of the support and free to slide on the other. Such an arrangement allows
longitudinal movement of the beam caused by changes in temperature. The test assembly should be
shaded from direct sunlight.
In an axial loading test, levels of the test pile and reference beam supports should be monitored by
an optical levelling system throughout the test to check for gross errors in the measurements. The
optical levelling should be carried out at the maximum test load of each loading cycle and when the
pile is unloaded at the end of each cycle. The use of precision levelling equipment with an accuracy
of at least 1 mm is preferred. The datum for the optical levelling system should be stable and
positioned sufficiently far away from the influence zone of the test.
In loading tests using O-cell, rod extensometers are connected to the top and bottom plates of the
O-cell (Figure 9.6). They are extended to the ground surface such that the movement of the
plates can be measured by dial gauges or displacement transducers independently.
9-20 March 2009