Abstract:

The behaviour of rock mass is largely influenced by the in-situ anisotropy and is different from other engineering materials. The assessment of the strength and deformation of rocks is essential for engineering design and analysis. The dynamic modulus of elasticity is very important when dealing with the problems like blasting. Field tests to determine these parameters directly are time consuming and expensive. Therefore, several authors have proposed empirical relations for estimating the rock mass deformation modulus on the basis of classification schemes. This paper presents the study on the strength and deformation characteristics of jointed rock by conducting laboratory tests on cylindrical specimens of plaster of Paris (POP) by introducing artificial joints, under static and dynamic conditions. Cylindrical specimens of POP mixed with Portland cement to simulate rock of higher strength were also tested. The specimens having one to four joints at inclinations varying from 0o to 90o were tested for both static and dynamic properties. The laboratory results were presented as stress-strain plots and were examined to understand the effect of joint frequency and joint inclination on the strength and deformation behaviour of jointed rock mass. In this paper the compressive strength/elastic modulus of the jointed rock mass was estimated as a function of intact rock strength/modulus and joint factor. The joint factor reflects the combined effect of joint frequency, joint inclination and joint strength. Therefore, having known the intact rock properties and the joint factor, jointed rock properties can be estimated. The test results indicated that the rock mass strength decreases with an increase in the joint frequency and a sharp transition was observed from brittle to ductile behaviour with an increase in the number of joints. It was also found that the rocks with planar anisotropy exhibit the highest strength in the direction perpendicular to the anisotropy and the lowest at an inclination of 30o-45o in jointed samples. The anisotropy of the specimen influences the dynamic elastic modulus more than the static elastic modulus. The results were also compared well with the published works of different authors for different type of rocks.

INTRODUCTION

The behaviour of rock mass is largely influenced by the in-situ anisotropy and is different from other engineering materials. The assessment of the strength and deformation of rocks is essential for engineering design and analysis. The deformation modulus of a rock mass is an important input parameter in analysis of rock mass behaviour. Field tests to determine this parameter directly are time consuming and expensive. Therefore, several authors (references) have proposed empirical relations for estimating the rock mass deformation modulus on the basis of classification schemes. There are two elastic moduli, namely, static and dynamic. According to Ciccitti and Mulargia (2004) the values of the static modulus, in general, are 5–10% lower than those of dynamic moduli. The dynamic modulus of elasticity is very important when dealing with the problems like blasting. Cylindrical specimens of POP mixed with Portland cement to simulate rock of higher strength (wall strength) were also tested.

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