Pile Testing

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 left 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).

Low Strain Pile Integrity Testing
A small hand held hammer is impacted on the top of the pile. The resulting stress wave reflected back from the pile bottom or defect is measured by an accelerometer attached to the top or side of the pile. Due to energy loses, there maybe no toe reflection in deep piles, also no data are available from below serious defects. The method cannot distinguish between a large cavity and a hair-crack.

Recent projects include:
Emirates Towers (Dubai), Burj Al Arab (Dubai), Festival City (Dubai), 21st Century Tower (Dubai), Dubai Marina, Kuwait Petroleum Complex.