SHEAR BEHAVIOUR OF ROCK JOINTS

Abstract

Rocks in natural state always consist of joints and the presence of intact rock is very difficult because some weathering action is always taking place in nature .Compared to intact rocks the weathered rocks has very less crushing and shearing strength ,thus this requires the study of strength parameter of rock masses. In jointed rocks, the shear displacement is accompanied with vertical movement or dilation and it is one of the most important parameter which should be considered while designing any structure in jointed rocky strata. It is very difficult to take actual sample of rock from field because i) during the extraction of sample, sample get disturbed due to vibration therefore actual sample will not be found ii) it is not possible to take out sample from inside core because process employed is difficult & expensive, therefore for such purposes the artificial rocks are prepared with the help of different material. The material for construction of sample should be such that i) It should be easily available in the local market ii) It should be cheaply available in local market iii) Sample prepared with material should represent best model of actual field condition. There are the various material can be used for sample preparation example Mortar, POP and other adhesive material. But we have taken POP for testing because POP is best suited for testing gives good strength and can also be used for simulating behaviour of hard as well as soft rock. The joints can be prepared at any angle but in this analysis the joints are taken unequal with asperity geometry of 15 -30 and 15 -45 and 45 -15 .The geometry of 15 -30 represent that the facing angle is 30 . The sample are prepared with POP at the moulding water content of 50% by weight and direct shear test is conducted at 14 days of air dried curing and test is done in conventional direct shear test machine. The direct shear test can be conducted in CNL or CNS condition. In CNL condition, the normal stress is kept constant but in actual the normal stress does not remain constant, in field CNS condition occurs. In CNS condition stiffness remains constant ,the result obtained from CNS condition gives higher value of shear stress but this condition of testing is not adopted because the equipment developed in the past for CNS boundary condition was either having difficulty in changing the boundary conditions or it was not useful for wide variety of rock joints. The orientation of rock during testing plays an important role in resisting shear stress. xi Direct shear stress carrying capacity for 15 -45 asperity sample as compared with 15 -30 asperity sample will more and dilation will also more in case of 15 -45 asperity sample as compared with 15 -30 asperity sample. In case of 15 -45 asperity sample and 15 -30 asperity sample, the peak shear displacement increases upto 0.4MPa but it drastically reduced at 0.5MPa because at this value the normal stress tries to prevent the horizontal displacement and finally the result obtained experimentally are compared with well known equation of Barton(1973), Bandis(1982) and Patton’s(1966) and Shrivasta and Rao (2012) criteria.