Predicting rock strength from investigative drilling data: a practical industrial solution based on simulated rock drilling in concrete

Investigative Drilling (ID) is a measurement-while-drilling (MWD) technique increasingly applied in Australian site investigations. Although preliminary studies demonstrate its potential for estimating rock strength, a reliable predictive equation remains unavailable. This study developed a practical equation for predicting unconfined compressive strength (UCS) using a purpose-designed field testing program involving six ID boreholes drilled into two concrete piles with controlled strengths of 35 and 70 MPa. Penetration rate, percussion pressure, and rotation rate were systematically varied under fixed drilling conditions. Based on established drilling mechanics, a power-law equation was developed and evaluated using drilling test data, third-party granite data (220 MPa), and field project data. The equation achieved relative errors below 9% for the controlled drilling tests. Evaluation against granite data revealed increasing strength underestimation at lower rotation rates, with errors ranging from -7.7% to -29.5%. Field validation against point-load-test (PLT)-converted UCS values, following ASTM standards, achieved an R2 of 0.624, RMSE of 30.20 MPa, and sMAPE of 20.5%. Direct UCS prediction, however, required consideration of rock structural effects. The proposed equation offers a practical means of generating continuous rock strength profiles from ID data within defined operational and geological conditions, potentially reducing uncertainty associated with discrete laboratory testing.