Civil Engineering Reference
In-Depth Information
Figure 8.1
Schematic diagram of a single-tube sonic logging apparatus.
The sonde is generally lowered to the base of a pile, and raised at a steady rate.
Ultrasonic signals from the transmitter radiate in all directions, some arriving at the
receiver via the pile medium. If this is sound concrete, the delay time from transmitted
pulse to the received signal is small. The compressive wave is received first, followed by
the shear wave. If there is a void, porous concrete or an inclusion of soil, the delay time
is increased. At the receiver, a transducer converts the pulses to electrical energy which
is received at the surface as an amplitude modulated (a.m.) voltage. For interpretation,
the a.m. signal is used to produce a variation in intensity of an oscilloscope scan. A peak
in amplitude of the signal corresponds to a brightening of the trace, and a trough to a
darkening. A voltage which is proportional to the depth of the sonde is derived from
the winch unit. The final display may appear as shown in Figure 8.2. In this example,
defects are indicated at depths of 4 and 7.5 m, as signified by the increased time of
transmission. The profile from a sound pile shaft is also shown.
The cost of drilling access holes is high, and is approximately equivalent to the cost
of forming the original pile, metre for metre. The prior installation of an access tube
in a cast-in-place pile is cheaper, but then a pre-selection has to be made. Steel tubes
should not be used, as steel is a better transmitter of acoustic energy than concrete.
For the same reason the tube or drill hole should not be adjacent to a reinforcing
bar. Plastic tubes can fail under the hydrostatic head of wet concrete combined with
the elevation of temperature that occurs in a massive concrete section as the concrete
hydrates. High quality ABS tubes may be necessary, rather than uPVC.
