Certain geophysical techniques can be employed by the civil engineering industry to test bored pile design and integrity.
Caliper Testing
A 3 arm caliper is lowered to the bottom of the pile bore before cement emplacement and used to measure the diameter of the
bore. This is useful to ensure that the bore is of the correct diameter or larger and to check on the rugosity of the bore wall. A rugose
bore causes more friction between the pile and the surrounding soil / rock. Pile bores of up to 1500mm diameter can be tested.
Strain Gauge Testing
Strain gauges are attached the pile cage at various levels before it is lowered into the bore. These are monitored at regular intervals
during load testing and the results used to determine the load distribution during loading. This technique is used to determine that
the load is taken up by friction along the length of the pile and that end bearing resistance does not occur. Digital settlement gauges
are also available, which can be connected to a laptop computer. These allow settlement readings to be taken at a safe distance
from the pile during loading. The example to the right shows that there is no strain at the bottom of the pile at 45m and that the load
is all taken up along the length of the pile.
Sonic Coring
The pile integrity can be accurately checked after cementation by lowering a sonic transmitter and receiver to the bottom of
adjacent, narrow tubes attached to the pile cage. As the transmitter and receiver are pulled slowly to the surface, sonic pulses are
produced by the transmitter. These travel through the cement and are detected by the receiver opposite. The amplitude and
velocity of the pulses are recorded simultaneously on a computer at small intervals.
Solid concrete of good integrity will produce a sonic signal of high amplitude and high velocity. Anomalies such as voids, pile
necking, honeycombing or collapse of the bore during cementation will produce lower amplitude signals of lower velocity. The
number of tubes needed to test a pile depends on the pile diameter, for instance, it is recommended that 5 tubes are used for a pile
of 1500mm diameter. Data from all available pathways between tubes are acquired. Retaining walls and other structures can be
tested using the same technique. The sonic coring results at left exhibit a large anomaly at 28m - shown by vastly increased arrival
time (red) and a much reduced amplitude (blue). |
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| Conducting a pile load test |
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Carrying out sonic
coring test |
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