Civil Engineering Reference
In-Depth Information
Figure 8.7
Arrangement of sliding mass (2000 kg) for stress wave integrity test. (Photograph courtesy
of PMC Limited, Durham.)
The analysis of the progress of the compressive stress wave as it travels down the pile
and its subsequent return as a tensile reflected wave, modified as it is by the restraint
of the soil is complex. The difference between the upward force in a pile in free space
and the actual upward force represents the soil restraint.
To obtain a realistic measure of this force, all the soil around the shaft has to
simultaneously attain the condition of ultimate resistance. Although this may be the
situation at the top of a pile when the blow is struck, it is unlikely to be so towards
the base of the pile and at the toe. By creating a computer pile/soil model the actual
deflections and forces measured can be compared with the computer predictions, but
because of time-dependent soil properties this is not straightforward. In the early
methods (e.g. by the Case Organization in the United States and the Institute TNO) an
empirical damping factor (
J
) is employed. Later workers (e.g. SIMBAT - Testconsult
