A geo-engineering classification for rocks and rock masses

Abstract

In this article, an attempt is made to assess the reliability of predicting the uniaxial compressive strength and the corresponding modulus of a rock mass by current approaches. These two basic engineering properties, when estimated from rock mass rating (RMR), Q and geological strength index (GSI), indicate hardly any change in the modulus ratio with the change in the quality of the rock mass from very good to very poor. However, the modulus ratio obtained from the relations involving the joint factor, J_f, indicate a definite decrease in the modulus ratio with a decrease in the quality of the rock mass. The strength and modulus in the unconfined and confined states, the modulus ratio and failure strain in the unconfined case were linked to J_f in earlier publications based on a large experimental database. Some of these relations were adopted to verify the response of jointed test specimens, the response of the rock mass during excavations for mining and civil underground chambers, in establishing ground reaction curves including the extent of the broken zone, and the bearing capacity of shallow foundations.

The joint factor is now linked to RMR, Q and GSI. The prediction of compressive strength and modulus of the rock mass appears to be more suitable. For classifying the rock, based on these properties, the Deere and Miller engineering classification, applicable to intact rocks, has been suitably modified and adopted. The results of different modes of failure of jointed specimens establish definite trends of changes in the modulus ratio originating from the intact rock value on the modified Deere and Miller plot. A geo-engineering classification is evolved by considering strength, modulus, quantifiable weathering index and lithological aspects of the rock.

Introduction

Rocks have been classified on the basis of their origin, mineralogical composition, void index, fracture/joint intensity, joint inclination, flow rate of water, velocity of propagation of shock wave, weathering, colour or grain size. When rocks and rock masses are classified for geotechnical purposes, they need to be classified on the basis of strength and/or modulus to give an indication of their stability and/or deformability. A rock classification has to provide a common basis to communicate, to identify a rock mass within one of the groups having well-defined characteristics, and also to provide basic input data for engineering design. For effective and successful usage of a classification system, it has to be simple, easy to understand, remember and apply.

Only the significant and intrinsic parameters of the rock should be considered which will influence the engineering behaviour most, and each parameter must represent itself exclusively. The parameters should be easily measurable and be linked in such a way that the quality of the rock mass is reflected in terms of its strength and modulus. It is imperative, while classifying a rock mass and to understand the mass response, to obtain an indication of the extent of reduction which has taken place in the strength and modulus of intact rock. This is desirable because field tests are time consuming, costly and often adequate in terms of estimating realistic design parameters.

Presently, there are four approaches available to estimate the uniaxial compressive strength and corresponding modulus, namely: rock mass rating (RMR), Q, J_f and geological strength index (GSI). Each of the approaches gives different values and each one can be tested for its reliability by considering the modulus ratio. The approach of J_f is based on many experimental data and suggests a continuous decrease of modulus ratio with the decrease of rock quality—unlike other approaches. It also enables one to estimate the strength and modulus under any required confining pressure.

A rock, either intact or jointed, should be classified in its simplest state of existence, i.e., in the unconfined condition. The influence of in situ stress (i.e. confining stress) and other environmental factors (such as seepage pressure, etc.) should be considered appropriately in the analysis for assessing the stability of the rock mass, e.g. in terms of effective stress, as is the practice in saturated soils.