Engineering behaviour of jointed model materials

The discontinuities in general and joints in particular are the inevitable part of rock mass encountered in almost all Civil and Mining Engineering projects. Under very low confining pressure or unconfined stress condition, they dictate a major influence on strength and deformational behaviour of the jointed rock mass. The uncertainty in predicting the behaviour of a jointed mass under uniaxial stress condition is essentially due to scale effect and unpredictable nature of the mode of failure of the mass. In nature several combinations of modes of failure are possible and without assessing the probable mode, the reliable prediction of strength and deformability of the mass is not possible. Several studies have been conducted on strength and deformational aspects but most of them are directed towards explaining the behaviour of the mass under confined state. Moreover, no study is available which links the strength and modulus of deformation of a scale free jointed mass to that of intact rock through configuration of joints and failure modes. In the present work, an attempt has been made to link the strength, tangent modulus, modulus ratio and failure strain of jointed block mass to intact rock through failure mode which is governed by configuration of joints. Tests have been conducted on specimens of jointed block mass under uniaxial stress conditions. A suitable model material with average uniaxial compressive strength of 17.13 MPa has been used. The tangent modulus of the material is 5344 MPa and the material is classified as TB' on Deere-Miller classification chart. The specimens are assembled, out of blocks of model material and a specimen consists of more than 260 blocks for the majority of the experiments. The scale free nature of the mass is thus ensured. Various combinations of four types of the geometries of blocks, eight number of joint orientations and 8 number of staggering conditions were used to simulate the most probable configurations of joints in the field. Four distinct modes of failure under unconfined state have been observed during the present study namely: splitting. shearing, rotation and sliding. The modes of failure lie in specific regions defined by the orientation of joints and the interlocking conditions controlled by their staggering. The guidelines have been suggested to assess the probable mode of failure in the field depending on the orientation of joints and interlocking condition of the mass. The strength, tangent modulus, modulus ratio and failure strain are found to be dependent on these modes of failure. The geometry of the block forming the specimen is also observed to be affecting the strength and modulus values of the jointed mass. The mass is found to behave anisotropically in strength and deformational response. The anisotropy curves for strength and tangent modulus are different from usually reported V-shape in the literature whereas for modulus ratio nearer to V-shape of the anisotropy is observed. The interlocking conditions systematically affect the anisotropy of strength and tangent modulus but have no such effect on anisotropy in modulus ratio. A reliable assessment of the strength of the jointed rock mass in the field can be done by using the concept of joint factor, Jf. The joint factor is a weakness coefficient and represents the effect of joint configuration through mode of failure. The methods for computing J1 for various modes of failure have been established. The value of the J1 is computed for each metre depth as: