Integrity testing - Piling Engineering - page 295

Figure 8.9 Idealized response curve from a vibration test.

The distance between resonance peaks at higher frequencies is related to the effective
pile length, l , by the expression:

f 2 − f 1 = V c /

2 l

where V c is the velocity of longitudinal wave propagation in concrete. Hence l , may
be estimated. A value for the cross-sectional area of the pile, A c , may be obtained from
the mean mechanical admittance of the pile head, N , over the higher frequency range,
from the relationship

N = 1 / p c × V c × A c

where p c is the average density of the pile concrete.
A development of this test is to record the transient response of the pile to a single
blow, and to carry out a Fast Fourier Transform analysis of the waveform. A test of
this type is now offered commercially by Testconsult Ltd.
An advantage of this type of test is that it can be carried out only 4 days after
casting. The pile head is prepared 24 hours in advance of a test by rough levelling
using a mortar screed where necessary, and final levelling with a thermosetting resin
just prior to the test. It is usual to carry out the test with the pile head freely accessible,
but in some soil conditions it may be possible to carry out the test from the pile cap.
The advice of the specialist testing house should be sought if the cap has been cast and
the pile is to be tested in this condition. Expert interpretation of the results of the tests
is required, but as with the seismic test, there is by now a great deal of experience in
the use of the test, to guide the interpretive process.
There are some soil type/pile length-diameter limitations with the test. In stiff clays,
a minimum length to diameter ratio of 20:1 may be found, with a maximum of the