Evaluation of RMR for Meta-Sedimentary Rocks in Hong Kong: Challenges and Potential Solutions

Abstract

Meta-sedimentary rocks are distributed over several areas of Hong Kong, including Sai Kung and Shatin, but are especially prevalent in the Northern New Territories. With the proposed development of the Northern Metropolis in the Northern New Territories in its preliminary stages, there is an increased awareness of the complexity of these meta-sedimentary rocks and how their rock mass and material properties may affect foundation design. The Rock Mass Rating (RMR) system, developed by Bieniawski, has been adopted in GEO Publication No. 1/2006 for estimating allowable bearing pressure and deformation modulus for foundation design in Hong Kong. Over the years, the RMR system has been proven useful for piling works and applicable to various rock types, including meta-sedimentary rocks. Meanwhile, as the use of the RMR system in meta-sedimentary rocks is becoming prevalent in Hong Kong for estimating the allowable bearing pressures, some practitioners encountered challenges when evaluating RMR for these rocks. These difficulties arise primarily from the inconsistencies in the assessment of the rating of RMR parameters by different practitioners, taking cognisance of the limitations of rock joint descriptions in conventional borehole logging practices. This paper examines the challenges encountered in RMR assessments for meta-sedimentary rocks in Hong Kong by making reference to the findings of a reviewing exercise conducted by different geologists using conventional borehole logging approaches. It aims to quantify the impact due to inconsistent interpretations of RMR parameters and inadequate rock joint descriptions. Suggestions tailored to address these challenges and inconsistencies in evaluating RMR for foundation purposes are presented for discussion.