Measuring Rock Strength While Drilling Shafts Socketed into Florida Limestone

The focus of this research was the real time assessment of rock strength during drilled shaft installations in Florida. Measures of unconfined compressive strength, a function of rock strength used commonly in shaft design, are provided through five monitored drilling parameters: torque, crowd, rotational speed, penetration rate, and bit diameter. This study is part of a larger project in which laboratory and field investigations were performed. This paper covers the laboratory portion of the investigation in which synthetic limestone blocks were cast at four design strengths representative of Florida limestone and drilled in a controlled environment. The laboratory drillings revealed that torque provides a better correlation with unconfined compressive strength than crowd force, which was found to be the case independent of drilling parameter groupings, and when groupings based on variations in bit diameter and the penetration rate to rotational speed ratio were enforced. During the analysis, multiple drilling equations were investigated, and a unique relationship was developed with Teale's specific energy equation for nonpercussive rotary drilling using a rock auger. (c) 2017 American Society of Civil Engineers.