A Novel Approach to Evaluate the Nature and Rock Mass Quality of Near-Surface Granitic Strata

Abstract

Summary The nature and rock mass quality (RMQ) of near-surface strata in part of southern Penang Island, Malaysia, were investigated for infrastructural and groundwater development, using a novel approach that combines seismic refraction tomography (SRT), ERT, borehole drilling, and regression analysis. The seismic P-wave velocity (Vp) and resistivity models revealed three distinctive strata, namely, residual soils, very poor-to-good (weathered) granite, and fresh (excellent) granitic bedrock. Through regression analysis, we developed an empirical relationship that is effective for predicting rock quality designation (RQD) from Vp data in tropical granitic environments, with a prediction accuracy of 96%. The results accurately classified the strata beneath the area into Classes I–VI, based on the predicted RQD values. According to the Vp and resistivity models, the massive stable non-rippable strata (with 90% RQD and Vp of >2100 m/s) extending from the central to north-central parts of the area are potential sections for building placement, and their foundations should be piled to the stable strata. Deep-weathered and fractured zones of depths >35 m, beneath profile 1, towards the central part of the area, with resistivity and Vp values of 100–900 ohm-m and <1900 m/s, respectively, were identified as potentially water-containing zones for sustainable groundwater abstraction.