GeoZone Applied Geology and Geotechnics

GeoZone - Pile Integrity Testing - Sonic Logging

Any structure founded on a piled foundation relies on the integrity of the installed piles. However, by their very nature it is almost impossible to determine visually whether the pile has been installed to the correct depth or whether any flaws exist within the pile. GeoZone has a strategic partnership with the only pile integrity testing company in the country and the service on offer is considered by all role players in the civil engineering field to add significant value to piling contracts, particularly as such testing provides a rapid and inexpensive means of screening the structural integrity of all piles on a site. The alternative to integrity testing is pile load tests on a few selected piles – a drawn out, expensive and disruptive process.

A good example of poor pile installation came to ligth during one of these Pile Integrity tests. The results showed acoustic anomalies in twenty five percent of the piles and subsequent physical exposure of the piles to the depths indicated by the acoustic anomalies confirmed the results of the testing. The adjacent photographs shows the severe ‘necking' of the piles which had occurred during installation. The structural defects in the piles shown in the accompanying photographs were confirmed when exposed by excavating around the pile shafts.

Two methods are used for testing piles, namely "Frequency Response and Sonic Echo Testing" more commonly known as "Pile Tapping", and "Sonic Logging"

Sonic Logging provides a method of assessing the homogeneity of concrete in piles or other mass foundation such as diaphragm walls. Sonic logging has no inherent limitations on penetration depth of the size or shape of the structure which can be tested. No direct access is required to the concrete surface and the method can be used to test structures with low cut-off levels at an early stage. However to allow for this, pre-selection of units to be tested is required and access tubes need to be cast into the structures to facilitate the test.

The tests are directed towards anomalies identified within a group of results, which may warrant further investigation. For the ultrasonic test at least two tubes, either plastic or steel of minimum diameter 50 mm, are cast in the pile (steel tubes are preferable as these are more durable, are less likely to be damaged and less prone to debonding with the concrete). The tubes must be completely filled with water to act as an acoustic-coupling agent between the probes and the tube wall.

A transmitter, emitting ultrasonic pulses, is lowered into one of the tubes and a receiver into the adjacent until they reach the base of the pile. Both the transmitter and receiver probes are connected to the CHUM software (a ruggedised pen computer), which records the first arrival time (FAT) and the energy attenuation as the probes are simultaneously raised to the top of the pile.

Provided that the FAT and the energy attenuation are relatively constant, one may deduce that the concrete quality is also uniform and the pile is therefore acceptable. If o n the other hand, at some level there is a noticeable increase in the FAT and / or in the energy attenuation, it means that the concrete at this level is inferior or defective. In such a case, the test may be repeated with the transmitter and receiver at different levels, a technique that allows for the determination of both the location and extent of the defect (tomography). Consistent transmission characteristics indicating anomalous zones within the sonic transmission profile are immediately obvious and are noted.

By installing an increased number of tubes on the perimeter of the pile/structure gives an almost complete coverage of the pile/structure cross-section. This method of testing is especially suitable for testing large diameter piles. The piles can be tested after the concrete has gained some strength, usually seven days or more after casting.