K. Seshagiri Rao

Weathering effects on the strength and deformational behaviour of crystalline rocks under uniaxial compression state

Uniaxial compression tests were performed on different categories of weathering of three lithological units: Malanjkhand granite; Nagpur basalt; and Delhi quartzite, occurring in central and northern parts of India. The deformational behaviour is studied in terms of variation in tangent modulus (Et50) and initial modulus (Ei) due to weathering. The power relationship between uniaxial compressive strength (σc) and Et50 shows strong correspondence for weathering sequence of common rock types. This relationship has been established by regression analysis and significant correlation parameter (coefficient of determination, r2=0.87) for crystalline rocks. It is shown that there is a systematic decrease in stiffness ratio, that is, ratio of tangent modulus and uniaxial compressive strength with increased weathering state. Comparison of Et50 and Ei values has shown that Et50 decreases more gradually than Ei, and reduction is more drastic for Ei values with an increased degree of weathering in all the three rock types. The mode of failure has been found to be influenced by weathering extent in rocks. A brief account is given of the intrinsic characteristics of fresh and weathered rocks and mineralogical changes produced by weathering investigated quantitatively. Correlation drawn between the petrographical and mechanical indices has shown that mechanical properties are apparently dependent on the intrinsic characteristics of weathered rocks.

Shear Behaviour of Rock Joints Under CNL and CNS Boundary Conditions

Shear behaviour of rock joints can be studied under both constant normal load (CNL) and constant normal stiffness (CNS) boundary condition. CNS condition is suitable for non planar and reinforced rock joints whereas CNL condition is suitable for planar and non reinforced rock joints. In the present study shear behaviour of modelled rock joints with different asperity have been experimentally investigated under both CNL and CNS boundary conditions. Test results indicate that CNS boundary conditions gives higher shear strength as compare to CNL boundary condition when other parameter of testing is kept same. But, this effect tends to diminishes with increase in normal stress on the shearing plane and at high normal stress both CNL and CNS boundary conditions gives same shear strength. A new shear strength model is proposed for both the boundary condition and the proposed model is validated by comparing the predicted shear strength with present experimental results and results available in the literature for natural and artificial rock joints with different asperity and roughness. The new study and model will be useful for safe and economical design of underground openings in jointed rocks, stability analysis of rock slopes, design of foundation on rock and design of rock socketed piles.

Development of a Large-Scale Direct Shear Testing Machine for Unfilled and Infilled Rock Joints Under Constant Normal Stiffness Conditions

In the past, numerous studies have been undertaken to understand the shear behavior of unfilled and infilled rock joints by conducting tests on conventional direct shear apparatus, where normal load is kept constant during the shearing process. This is suitable for planar joints, but rock joints are seldom planar, and, in reality, normal stress on the shearing plane is not constant and testing such rock joints by conventional direct shear apparatus will give inappropriate results. The estimation of correct shear strength is important for safe and economical design of underground openings in jointed rocks, stability analysis of anchored rock slopes, risk assessment of underground waste disposal repositories, design of foundations on rock, and socketed piles in rock. Hence, in the present study, a servo-controlled large-scale direct shear testing machine is designed and fabricated to test the rock samples under varying normal load, i.e., constant normal stiffness (CNS) conditions. Direct shear tests have been performed on physically modeled unfilled and infilled rock joints with asperity angle 30°–30°, at different initial normal stress (Pi). The test results indicate that the CNS conditions greatly influence the shear behavior of rock joints.

A Polyaxial System for Testing of Jointed Rock Mass Models

A polyaxial loading system was designed and developed at the Indian Institute of Technology, Delhi, India for laboratory testing of the mechanical behavior of rock mass. Large-scale rock mass models of different joint geometry can be tested under polyaxial stress state simulating in situ stress conditions using this true-triaxial system. The system consists of a 1000 kN capacity vertical frame, a biaxial frame of 300 kN capacity fitted with 2 pairs of hydraulic jacks and platens, constant confining pressure unit for applying, monitoring, and maintaining horizontal stresses on specimen faces, an 8-channel data acquisition system, and a personal computer to record all load and deformation data. Successful working of the system was verified by conducting true-triaxial testing on several models specimens of sand-lime blocks having 3 sets of orthogonal joints.