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New cross-hole logging system

A NEW cross-hole sonic logging system for piled foundations has been developed by Piletest [Check]. The Pile Integrity Sonic Analyser (PISA) uses new software and data acquisition for real-time graphical detection of pile defects during logging.

Cross-hole sonic logging involves fixing vertical PVC or steel tubes to the pile reinforcing cage before installation. These are filled with water and transmitter and receiver transducers are lowered into adjacent tubes.

The transducers are then pulled up at a constant rate to produce a profile of the pile between two tubes. The process is then repeated between different pairs of tubes to ensure as complete a coverage as possible.

A variation, usually used on smaller diameter piles, involves a single hole with the transmitter and receiver placed one above the other.

The cross-hole method gives a direct assessment of pile integrity but can only detect defects between the tubes. Debonding can occur between the tubes and the concrete and testing in fresh concrete can cause difficulties in interpretation of defects.

The PISA system is based on a lightweight, portable, pen-touch computer that collects data in real time. It can be used for cross-hole and single hole logging, cross-hole ultra sonic logging and sonic echo testing (for small strains). It is modular so it can be adapted for a variety of test methods.

The tube combination is carried out by simply drawing a line between any two tubes on the screen. Real-time graphical presentation of the concrete integrity is provided during data collection. If a suspect zone is detected, the probes can be lowered and raised relative to each other around the area, to further investigate and delineate its extent.

The system was tested on four bored cast insitu piles at the Auburn University in Alabama, US. The 914mm diameter, 11m long piles were each fitted with four access tubes.

Artificial defects, in the form of sand bags, were installed in two of the piles.

Over a year after installation Geosciences Testing and Research tested the piles with the PISA system. The results were then compared with the actual defects.

Three of the four defects were identified at about the correct size but the fourth defect was assumed to be three times the actual size. Two of the defects were identified within 0.6m of their actual positions.

This was considered to be an accurate and reliable evaluation and allowed the operator to estimate the extent of the defect with increased accuracy.

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