The cross-hole sonic integrity testing is one of the most advanced and economical tools for testing the integrity of drilled shafts. Currently, there is no better method for non-destructively identifying any defect in the drilled shafts. The test determines the quality of concrete between pairs of tubes pre-installed in bored piles cast-in-situ concrete piles, or other types of concrete foundations using the Cross-Hole Analyser (CHATM). The CHA measures the time it takes for an ultrasonic pulse to travel from a signal source in one access tube to a receiver in another access tube. In uniform, good quality concrete, the travel time between equidistant tubes will be relatively constant. Longer travel times indicate the presence of irregularities such as poor quality concrete, voids, honeycombs and soil intrusions. The technique to be adopted shall be in accordance with ASTM D 6760 standard specification.
Several factors can influence the results and shall be used for the interpretation:
- Because the access tubes may not be perfectly straight and parallel, a variation in FAT is expected through the pile profile and may not be indicative of material properties or flaws.
- An increase in FAT may indicate that the concrete has not cured or it is of low quality/grade. The indication of a decrease in FAT is rather obvious.
- Loss of signal may be due to de-bonding of the access tubes from the concrete. It could also be uncured concrete, honeycombing, soil deposits accumulation, inclusion of foreign materials and contamination of pipes etc…).
- The top 0.5 – 1.0 meter of the piles may show loss of attenuation due to loss of available medium for the transfer of the signals.
- For readings between the sonic pipes, many sources of measurement error may affect the FAT delays at the time of testing, such as concrete variability with depth, FAT picking error, sampling error and hardening time effect, position of sonic pipes etc…
- All the access tubes used for the testing have a diameter of 5 cm or more. Since the sonic probes have a diameter of 25 mm, they may move laterally inside these tubes during the test, which means an extra travel time in water (wave velocity of 1.5 mm/microsec). Hence, the importance of this factor on the FAT is immediately obvious especially with small pile diameters, knowing the distance between the sonic tubes and assuming a concrete wave speed in the range of about 4,000 m/s.