Quantitative Profiling of Rock Strength and Rock Mass Quality with In Situ Drilling Speed at Three Sites in Shenzhen and Hong Kong, China

Abstract Accurate and real-time profiling of rock strength and rock mass quality is critical for ensuring safety and efficiency in underground space construction and avoiding potential geohazards. This paper examines the correlations between drilling speed and (a) uniaxial compressive strength (UCS) of rock and (b) P-wave velocity in rock mass, and (c) rock quality designation (RQD) of rock mass at two specifics granite and marble sites in two megacities of Shenzhen and Hong Kong, China. Laboratory and in situ tests are carried out for the factual data of the correlations. Drilling speed is calculated from drilling process monitoring (DPM) data. Results show that UCS inversely correlates with drilling speed, enabling empirical prediction of in situ UCS profiles. Intact granite exhibits lower drilling speeds and higher P-wave velocities, while fractured zones correspond to increased drilling speeds, allowing precise fracture zone identification. Although average drilling speed poorly reflects RQD, a weighted drilling speed demonstrates a strong correlation with a high coefficient of determination $${R}^{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:math> of 0.908. It leads to the development of a refined DPM-based RQD indicator for continuous strength profiling along drillhole depth. Consequently, we can use real-time data from the DPM system to improve current methods for rapidly profiling rock strength and rock mass quality with the need for no coring or a small number of core extractions, aligning with the carbon-neutrality objective. The drilling powers are intended to be kept stable in this paper, indicating that the primary cause of high variability in drilling speeds is the variable rock strength and rock mass quality encountered.