Pile testing - Piling Engineering - page 344

Figure 9.32 The TNO dynamic pile test equipment. (Photograph courtesy of Institute TNO, Delft.)
of excess pore pressures generated during installation. Case studies have shown
that, particularly in soft cohesive soil, the pile capacity may increase by a fac-
tor of 4 to 5 following installation, over a time period of several weeks or
months, depending on the consolidation characteristics of the soil and the pile
diameter (Randolph et al ., 1979). Where the pile capacity is to be estimated
from dynamic measurements, it is necessary to allow for such set-up. This
may be achieved by 're-striking' the pile after an appropriate time delay. It is
important to ensure that the full pile capacity is mobilized in such a re-strike
within the first few blows, so as not to reduce the long-term performance of
the pile.
2 One of the major pitfalls to be avoided is trying to estimate the capacity from
hammer blows of insufficient energy to fail the pile. This is quite common during
re-drive tests and, in such cases, analysis of the data will only yield an estimate of
the maximum static resistance mobilized during that blow.
3 Use of the Case formula to estimate the static capacity of the pile is not
recommended without a full numerical matching of the stress wave data as cor-
roboration. Even where this has been done, and a reasonable estimate of damping
parameter, j c , is available, care should be taken to ensure that the conditions
assumed in deriving the relationship are met in practice. Two particular condi-
tions are (a) that there are no major changes in cross-section (or impedance) of the
pile along its length, and (b) that the pile velocity remains positive (downwards)
over the major part of the return time of the stress wave. In many case, the latter
condition is not fulfilled, and the Case formula will underpredict the pile capac-
ity. A correction may be made, taking account of the proportion of pile that was
rebounding during the return passage of the stress wave, and using the earlier part
of the blow to estimate the soil resistance in that region.
4 The two main causes of differences between static and dynamic performance of
the pile are (a) viscous damping and (b) inertial damping. The dynamic shaft
capacity of a pile may exceed the static capacity by a factor of 2 or more, due to