Application of rock mass classification techniques to weak rock masses: A case study from the Ruahine Range, North Island, New Zealand

Abstract

Rock mass classification systems were first generated for use in engineering applications, but their potential for utilization in geomorphic studies has since been recognized. These techniques were mostly developed for hard rock environments, and questions remain about their applicability to weak rocks. Here, the applicability of three rock mass classification techniques (rock mass strength (RMS), rock mass rating (RMR), and slope mass rating (SMR)) to weak rock masses was analyzed. Techniques incorporated parameters such as uniaxial compressive rock strength, discontinuity condition and orientation, and groundwater ratings. Rock mass classification values were determined from 14 profiles sited on recently excavated road cuttings on the Saddle Road, in the Ruahine Range, North Island, New Zealand. This is an important transport route across the North Island’s axial ranges, with the road excavated into weak late Pliocene – Early Pleistocene mudstone. Mean slope and minimum slope angle were measured at each profile in concert with the rock mass classification schemes. The three classification techniques all appear to have limited usefulness given the subaerial conditions prevalent at the study site. It would appear that the relative weightings of the different parameters within the RMR, RMS, and SMR classification schemes would need modifying for weak rock masses, but the precise details of this are difficult to determine.