Proceeding of the 2018 DFI-EFFC International Conference on Deep Foundations and Ground Improvement, Rome, Italy, (DFI, EFFC)
Laboratory Evaluation of a Proposed Nondestructive Testing System for Detecting Anomalies below Drilled Shaft Excavations
Alireza Kordjazi, Joseph T. Coe, Assistant Professor and Trumer Wagner
Anomalous conditions such as voids, soil seams, or zones of weakness beneath deep foundation elements constructed in situ (i.e., drilled shafts) can greatly affect the development of base resistance. These conditions are seen in many geological environments across the world, including in areas of karst and/or significant residuum. Current standards of practice to evaluate conditions underlying a drilled shaft can have trouble identifying the true vertical and lateral extent of anomalous conditions underlying the base once excavation is complete. Moreover, methods such as rock coring and probing are invasive and can be time intensive. A nondestructive system that can effectively identify anomalous conditions beneath drilled shaft excavations prior in a timely manner prior to concrete placement would be a welcome tool to address drilled shaft capacity in areas with significant karst and weathered bedrock. To address this concern, a nondestructive testing (NDT) system was developed in this study that utilizes seismic stress waves and a series of receivers at the base of an excavation. Elastic stress waves (P-waves and S-waves) are generated by a seismic source at the base of the excavation and the corresponding reflected are recorded and analyzed using geophysical analytical techniques. In this manner, the stiffness of the underlying earth material stiffness is estimated and the extent of anomalous conditions can be profiled. This study consisted of initial laboratory efforts in the development of this NDT system. A large cemented sand model was constructed to represent the bottom of a drilled shaft excavation in karst conditions with softer soil seams present. This paper summarizes the development of the system, including hardware components, system operation, as well as the construction of the laboratory soil model to represent field conditions. Two-dimensional seismic stress wave data was acquired for the model and analyzed to present a depth profile of the base conditions. A discussion is provided regarding data reduction, analysis, and the resulting interpretation. Finally, the paper also discusses development issues related to anticipated future applications of the system in the field.
|article #2960; publication #1040 (ROME-2018)|