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

摘要

The focus of this research is the real-time assessment of rock strength (unconfined compressive strength, qu) during drilled shaft installations in Florida limestone, where measures of rock strength are provided through five monitored drilling parameters: torque, crowd, rotational speed, penetration rate, and bit diameter. To complete the study, both a laboratory and field investigation were required. This paper covers drill rig instrumentation, measuring rock strength during field drilling, and the comparative analysis of rock strength with conventional methods. Real-time measurements were recorded for each drilling parameter and graphically displayed on an in-cab monitor and wirelessly transmitted to an external computer. Measures of rock strength were estimated using a laboratory-developed equation with the monitored drilling parameters for real-time field assessment. Measuring while drilling in the field took place at three separate locations where drilled shaft load testing occurred. Comparative analyses between the monitored shaft installations and core samples subjected to unconfined compression indicated the results aligned well when recoveries were good. As recoveries diminished, the mean strengths were comparable, but more variable.