The Effects of Stress Redistribution on the Propagation of Stress Waves beneath the Bottom of Drilled Shaft Excavations

摘要

The presence of voids or karst features beneath rock-socketed drilled shafts can significantly diminish end bearing. However, very few methods exist to verify the presence of such anomalous features. Development of a stress wave non-destructive testing (NDT) method for in situ evaluation of conditions beneath drilled shaft excavations would improve quality control of these deep foundation systems in karst. However, development of such a system would need to address the potentially complicated stress conditions present at the base of excavations. The propagation of stress waves in soil and rock is influenced by the state of stress in the material, as manifested in their governing equations. Material modulus, velocity of waves, and the travel path of stress waves are in particular affected by the stress condition. Studies on the effect of anisotropy have been principally performed on small samples of soil and rock and there is limitation on the understanding of the effect of stress irregularity on wave propagations at larger scales. In this study, the state of stress and wave propagation underneath the bottom of drilled shaft excavations were numerically modeled to acquire a clearer understanding of the effect of stress redistribution on stress wave propagation. The initial results demonstrate the complicated state of stress at the bottom of excavations, where the gradient of stress changes rapidly up to the depth of one shaft diameter beneath its base. Details regarding the development of the numerical models, including material property selection, stress conditions, and wave propagation are provided. For the purpose of tomographic imaging, changes in the fundamentals of wave propagation can cause misleading results if unaccounted for when determining the velocity field beneath the drilled shaft excavation. A brief discussion on how the stress redistribution could potentially affect the imaging technique is therefore provided.