Assessment of shear strength from measuring while drilling shafts in Florida limestone

Abstract

The focus of this research is the real-time assessment of drilled shaft capacity based on the unconfined compressive strength (q u ) obtained from measuring while drilling (MWD). Measures of q u , a function of rock strength commonly used in drilled shaft design, are provided through five monitored drilling parameters: torque, crowd, rotational speed, penetration rate, and bit diameter. Monitored shaft installations took place at three separate locations on drilled shafts, which were subsequently load tested. Using the q u values obtained from MWD, side shear was estimated in portions of each shaft where instrumented segments indicated the side shear was fully mobilized for direct comparison. To consider all of the current side shear equations used in Florida drilled shaft design, the estimation of tensile strength (q t ) in real time was also needed. This led to a theoretical approach to establish the q t /q u relationship that was later verified empirically and provided new correlations between material and mechanical properties of Florida geomaterials. A comparative analysis indicated that the results from multiple established side shear equations, used with q u from MWD, align well with the results obtained from load testing. This suggests that estimating drilled shaft capacity from MWD is viable to reduce spatial uncertainty.