DEVELOPMENT OF CORRELATION BETWEEN ROCK CLASSIFICATION SYSTEM AND MODULUS OF DEFORMATION

Abstract

Rock Classification methods are important for the evaluation of different rock parameters to be adopted for Civil Engineering works. The classification of rock mass also helps to optimise detailed investigation requirements of a large area. During preliminary design stage of a project, the classification of rock mass in accordance with one or more systems can be used to establish engineering characteristics of the rock mass. This also helps in estimating the strength and deformability of rock mass. A number of correlations have been developed by various researchers to correlate the rock mass rating values derived from different systems. Usually, rock mass classification data are not always available in a format that can immediately be applied to a specific engineering problem. Therefore, correlations may prove very useful to quickly derive different design parameters. Furthermore, the availability of the correlations between classification systems facilitate quick means of verifying resultant rock mass rating values, without re-calculation of the values. In this research, four main and well known rock mass classification systems i.e. Rock Mass Rating (RMR), Tunnel Quality Index (Q System), Rock Structure Rating (RSR) and Geological Strength Index (GSI) have been applied to the data obtained from Diamer Basha Dam and Kohala Hydropower Project sites and the rocks have been categorized according to the numerical values. New correlations among these classification systems have been developed which can be used for the rocks of northern area of Pakistan. Generally for a large civil engineering projects; i.e. a tunnel or a dam, modulus of deformation is required at many locations to understand the behaviour of the rock. However, sometimes it is not possible to perform several in-situ tests due to time and funds constraints. Hence it is essential to establish some relationship between rock mass classifications and modulus of deformation. Another purpose of such studies is to authenticate the existing correlations being used worldwide. Due to the abovementioned constraints, it may be uneconomical to conduct tests in all critical areas of a single project, especially for a large project having highly random rock characteristics. In such kind of situations, a few large-scale in-situ tests are conducted and correlations are made between the modulus of deformation values obtained from these tests and different classification systems. These kinds of correlations can be used for extrapolating the modulus of deformation which may be a representative of a rock mass condition for other areas of the project. However the selection of locations of the tests should be done very carefully. Empirical correlations between rock mass classification systems and deformation modulus are useful if a range of in-situ modulus values is desired to be established. Also the estimated values can be provided for the design. The correlations also indirectly shape the bases to identify the weak areas in the foundation rock that may affect the structural behaviour. In this research, data obtained from Plate Load tests and Flat Jack tests performed at Diamer Basha Dam and Kohala Hydropower Project have been analyzed to develop the correlations of modulus of deformation with four rock mass classification systems i.e. RMR, Q System, RSR and GSI. The Plate Load tests performed at Basha were on large size plate and deep deformation measurements were made with borehole extensometer installed underneath the plate. Based on the rock mass classifications in the four systems, the rock existing at Basha dam site mainly comprises Fair to Good quality igneous rock while at Kohala site it is classified as Poor to Fair quality of sedimentary rock units. The correlations developed among various rock mass classification systems have good regression coefficients ranging from 0.835 to 0.901 indicating good correlations. During the research the correlations have been developed between deformation modulus and four (4) rock mass classification systems. Two different sites of different quality of rocks have yielded different range of moduli. The correlations developed during present study have been compared with existing correlations and it has been found that generally these correlations are in good comparison with the other correlations. The research will benefit in the design of future hydropower projects of Pakistan in the region, as the developed correlations may be used to estimate the modulus of deformation at early design stages.