Determination of rock quality designation (RQD) using a novel geophysical approach: a case study

Rock mass integrity is mainly assessed by geotechnical parameters. Rock quality designation (RQD) is a reliable geomechanical parameter, extensively used in geotechnical engineering to obtain proper design of large-scale engineering infrastructures. RQD is traditionally obtained by the rock cores from the boreholes. However, the conventional methods are time-consuming and costly, offer only point-scale measurements, and have topographic constraints. Hence, the conventional determination of RQD often causes ambiguities in the evaluation of subsurface geotechnical model. Geophysical methods, alternatively, are more economical, faster, user-friendly, and non-intrusive, provide volumetric measurements, and efficiently reduce the gaps between a true geotechnical model and the insufficient drilling tests. In this study, we estimate RQD via empirical integration of ERT with inadequate well data. The estimated RQD provides the in-depth subsurface valuation of engineering rock strength covering the whole area where even no borehole is accessible. Subsurface geology was contained in a geotechnical model of three distinct layers, namely, highly fractured/weathered layer via RQD 0–50% and resistivity < 400 Ωm, partly fractured/weathered layer using RQD 50–75% and resistivity between 400 and 1000 Ωm, and integral/fresh or unweathered layer with RQD varying from 75 to 100% and resistivity > 1000 Ωm. The obtained models reduced the geological uncertainties caused by subsurface heterogeneities and the insufficient well data. Our proposed research provides a thorough appraisal of rock mass integrity for smooth advancement of engineering structures.