JNTU B.Tech II MINING GEOLOGY Previous Question Paper (November/December - 2020 - R15)

The abrasive resistivity and durability of rocks are critical parameters in mining and civil engineering, determining the wear rate of drilling equipment and the long-term stability of rock aggregates. These properties are assessed through standardized laboratory tests.

The Cerchar Abrasivity Index (CAI) is the standard method for determining abrasive resistivity. A hardened steel stylus with a conical tip is drawn across a freshly broken rock surface under a constant load of 70 N for a distance of 10 mm. The wear on the steel tip is measured using a microscope, and the resulting index (ranging from 0.1 to 6.0) classifies the rock from extremely low to extremely high abrasivity. For example, Quartzite typically shows high CAI values due to its high quartz content, whereas Limestone exhibits low abrasivity.

Rock durability is primarily evaluated using the Slake Durability Test (SDT). This test simulates the weathering effect of wetting and drying cycles on weak or argillaceous rocks. A representative sample of rock lumps is placed in a wire mesh drum, which is rotated in a water bath for 10 minutes at 20 rpm. The process is repeated for two cycles, and the durability index is calculated as the percentage ratio of the final dry mass of the rock retained in the drum to its initial dry mass.

Durability is classified based on the index: rocks with values above 95% are considered very high durability, while those below 25% are very low. For example, Shale and Mudstone often show low slake durability due to the presence of clay minerals that disintegrate upon saturation, whereas Granite maintains high durability due to its interlocking crystalline structure and resistance to chemical weathering.

The tensile strength of rock is a critical mechanical property, typically determined in the laboratory using the Brazilian Test (Indirect Tensile Strength Test) due to the difficulty of performing direct pull tests on brittle rock specimens. The test involves applying a diametrical compressive load to a cylindrical rock core until failure occurs by tension across the loaded diameter.

The test procedure involves the following steps:

1. Prepare a cylindrical rock specimen with a length-to-diameter ratio of approximately 0.5.
2. Place the specimen horizontally between two parallel loading platens of a testing machine.
3. Apply a continuous compressive load at a constant rate until the specimen splits into two halves along the vertical diameter.
4. Record the maximum load at failure.

The tensile strength is calculated using the formula:

Where σt is the tensile strength, P is the maximum load at failure, D is the diameter of the specimen, and t is the thickness of the specimen.

A neat sketch would depict a cylindrical disc positioned vertically between two flat, parallel steel loading plates. Arrows indicate the compressive force P acting downwards from the top plate and upwards from the bottom plate. The failure plane is shown as a vertical line passing through the center of the disc, representing the tensile fracture induced by the compressive stress concentration at the center of the disc.