Integrity testing - Piling Engineering - page 294

CEBTP Ltd.) have used the response of the pile to different strain rates employed in the
impact. The end result is a calculation of the pile capacity in terms of shaft adhesion
and end bearing.
8.2.5 Dynamic response tests
In the vibration test, an electrodynamic vibrator is placed on the prepared pile head,
and the pile is subjected to a continuous vibration over a broad frequency band.
Although the method of pile excitation is different, the results of the dynamic response
test are considered generally similar to those from the seismic (sonic echo) test. Both
tests provide similar information regarding the basic pile geometry. The differentiation
between the tests lies largely in the electronics involved in the signal processing. The
response of the pile is monitored via a velocity transducer. Resonance peaks are pro-
duced, which may be used to deduce the effective length of the pile and to reveal
apparent defects. The method was developed by CEBTP, and has been widely used.
A block diagramof the test arrangement is shown in Figure 8.8. The vibrator supplies
a constant-amplitude sinusoidal input over a frequency band of, say, 20 to 1000Hz.
The voltage from the velocity transducer is fed to the y axis of an x - y plotter which
is provided with a signal on the x axis proportional to the frequency, f . By dividing
the magnitude of the pile head velocity, v ,by F 0 , the applied force, the mechanical
admittance of the pile head is obtained. In the idealized response curve for a pile
shown in Figure 8.9, the slope of the early part of the curve, m , is indicative of the
stiffness of the pile-soil system as presented at the pile head. This may be related to the
behaviour of the pile under load, but an apparent relationship can occur for various
reasons unrelated to static conditions and absolute correlation is not to be expected.
Figure 8.8 Schematic diagram of the test arrangement for the vibration method of pile integrity testing.