JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III ROCK MECHANICS ENGINEERING Previous Question Paper (August - 2022 - R18)

The determination of mechanical properties of rocks in the laboratory is essential for civil, mining, and petroleum engineering applications. Standardized procedures, primarily governed by ISRM (International Society for Rock Mechanics) and ASTM standards, are used to evaluate the strength, deformation, and failure characteristics of rock specimens.

The primary mechanical properties and their determination procedures are as follows:

1. Uniaxial Compressive Strength (UCS): A cylindrical rock specimen with a length-to-diameter ratio of 2:1 is placed in a compression testing machine. An axial load is applied at a constant rate until failure. The UCS is calculated as the maximum load divided by the original cross-sectional area.
2. Deformability (Young's Modulus and Poisson's Ratio): During the UCS test, axial and lateral strains are measured using strain gauges or LVDTs. Young's Modulus is determined from the slope of the stress-strain curve (typically the tangent modulus at 50% of the ultimate strength), and Poisson's ratio is the ratio of lateral strain to axial strain.
3. Tensile Strength: Since direct tension tests are difficult to perform, the Brazilian Test is commonly used. A thin rock disc is compressed diametrically between two platens. The tensile strength is calculated using the formula:
   σt = 2P / (πDt)
   where P is the failure load, D is the diameter, and t is the thickness of the disc.
4. Triaxial Compressive Strength: This test simulates the confinement experienced by rocks at depth. A cylindrical specimen is placed in a triaxial cell, subjected to a constant confining pressure (σ3), and then loaded axially until failure (σ1). This is repeated at different confining pressures to develop the Mohr-Coulomb failure envelope, which defines the cohesion and angle of internal friction.
5. Point Load Strength Index: This is a rapid index test performed on irregular or core specimens by applying a concentrated load between two conical platens. The point load strength index (Is) is calculated and converted to an equivalent UCS value using empirical correlation factors.
6. Shear Strength: Direct shear tests are conducted by applying a normal load and a shear load to a rock sample across a specific plane of weakness or through the intact rock to determine shear stress versus displacement characteristics.

All laboratory tests require careful specimen preparation, ensuring smooth, parallel end faces for compression tests and adherence to specified loading rates to minimize the influence of strain rate effects on the measured rock properties.