Advances in stress-wave nondestructive testing methods for evaluation of deep foundations

Stress-wave based nondestructive testing (NDT) systems such as cross-hole sonic logging (CSL), low strain integrity testing, and ultrasonic echo devices (e.g. Sonicaliper) are routinely deployed for quality assurance (QA) and quality control (QC) of deep foundation systems. However, there are still a number of scenarios where the current standard of practice for QA/QC can fail to provide sufficient information regarding a deep foundation system. For example, limited information can be obtained regarding in-service pile and drilled shaft integrity. Additionally, conditions beneath drilled shafts excavations are not well characterised unless a significant amount of probing is performed, particularly if the shaft diameter is large. This paper introduces recent advances in stress-wave NDT systems for use in assessments of in-service foundation integrity and drilled shaft construction. A borehole testing system was developed in a laboratory setting as a proof-of-concept study to generate high-resolution images of defects in deep foundations. The probe, emitting and detecting stress waves with ultrasonic frequency content, is capable of non-destructively imaging the outer perimeter/circumference of in-service or newly constructed foundations without any need for foundation instrumentation. The foundation elements in the experimental model were constructed with practical cross-sectional dimensions to simulate real field conditions with defects due to corrosions and poor concrete placement practice. The primary results of the experiment are very promising, even with a simple data processing technique applied on the acquired raw data, as the acquired image was capable of differentiating changes in the section as low as 0.3 cm. The same hardware components were incorporated into a down-hole testing system developed to identify the location of anomalous features beneath drilled shaft excavations in karst. The preliminary results on a relatively large scale cemented sand model were also promising. Numerical modelling was also performed to evaluate the issues involved in scaling the system for field applications and evaluating more complex subsurface conditions using a full waveform inversion technique. A brief overview is provided of both systems including hardware, survey methods, and data processing techniques, followed by a discussion on application of the results to QA/QC of deep foundations.