Civil Engineering Reference
In-Depth Information
the case as the concrete at the pile head in a cast-in-place pile may be less dense. The
relationship is given by
V
l
=
V
r
, where
C
is approximately equal to 1.90. In some
circumstances, the surface waves are not well defined and
V
l
may be assumed to be
of the order of 4000m/s. The distance to the base of the pile or a discontinuity (
l
)is
given by 2
l
C
×
V
l
, where
t
denotes the delay time.
The recording system comprises a triggering device to start the time trace, with the
accelerometer connected to an oscilloscope by way of an amplifying filtering system.
Signals from the accelerometer are digitally processed such that background 'noise' is
filtered out and a clean trace, suitably amplified, may be obtained by striking the
pile head a number of times with a small nylon-headed hammer. It is necessary
only to remove loose or weak embedded material from the surface of the pile in
order to obtain an acceptable signal and therefore under ideal conditions, a single
operator can test 200 to 300 piles per day. The analyzing equipment is compact,
comprising a small robust portable data-logger and computer with an integral dis-
play (Figure 8.5). For each pile tested, a fully-annotated record is produced which
records the estimated length of sound pile. Data recorded in the field can be down-
loaded onto a PC operating Windows for further analysis. Equipment commonly in
use is that marketed by IFCO of the Netherlands, who are specialists in this form of
integrity testing.
Whilst the travel time of the stress wave gives a measure of the length of the pile
under test, or the distance from the pile head to a distinct break, the form of the
trace provides information on the geometry of the pile, such as reductions or increases
in cross-section. Results of the test are either presented in the time domain or the
frequency domain. Both use the same pile response data and the difference is only
in the method of presentation. Additional data is recorded in the frequency domain
method of presentation only because it is necessary in this method to eliminate the
characteristic components of the hammer/pile blow response from the plot. Typical
outputs are shown in Figure 8.6. The test may be carried out after the concrete in the
pile has cured for at least 4 days.
=
t
×
Figure 8.5
Seismic integrity test (sonic echo method) in operation.
