Quality assurance of concrete foundation elements using an ultrasonic evaluation technique

In-situ cast concrete deep foundation elements are common in Hong Kong for supporting high-rise buildings. If the quality and structural integrity of these foundation elements are in doubt, the safety of buildings will be jeopardised. Traditionally, a static loading test is the only effective way to verify the loading capacity of a foundation element. In this test, a pile is required to withstand up to two times its designed loading capacity to pass the acceptance criterion. However, the time and expense involved in this procedure can only be justified for a small number of foundation elements and, as such, it is not representative of all foundation elements. Drilling or coring is sometimes applied to evaluate the integrity of concrete foundation elements. Again, there are some restrictions in adopting these methods as these tests are expensive, timeconsuming and cause delays to subsequent works. Furthermore, only limited fractions of the foundation elements are covered in these processes. Recently, technology has advanced so much to allow various low-cost non-destructive testing methods to be carried out on completed foundation elements. Among these methods, Cross-hole Sonic Logging (CSL) is one of the most effective quality assurance methods for testing concrete foundations in terms of assessing the homogeneity and integrity of in-situ concrete foundation elements.