JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV ROCK EXCAVATION ENGINEERING Previous Question Paper (July/August - 2022 - R18)

The geological properties of rock masses significantly influence excavation performance, stability, and the selection of equipment. Key properties include:

• Lithology and Mineralogy: The rock type (igneous, sedimentary, or metamorphic) and its mineral composition determine hardness, abrasivity, and chemical stability. Harder minerals like quartz increase tool wear.
• Structural Discontinuities: Features such as joints, faults, bedding planes, and shear zones create planes of weakness. These dictate the block size, fragmentation patterns, and the overall stability of the excavation face.
• Rock Strength: Measured by Uniaxial Compressive Strength (UCS) and Tensile Strength, these properties determine the resistance to penetration and the energy required for breakage.
• In-situ Stress State: The existing tectonic and gravitational stresses within the rock mass affect excavation stability and the potential for rock bursts or squeezing ground conditions.
• Weathering and Alteration: The degree of chemical or physical weathering reduces rock strength and increases permeability, often leading to slope instability or water inflow issues.
• Groundwater Conditions: The presence, pressure, and flow rate of groundwater influence excavation methods, stability, and the potential for erosion or pore pressure buildup.

The cutting principle of a ripper is based on the application of high concentrated force through a hardened steel tip to exceed the tensile strength of the rock. The process involves the following stages:

• Penetration: The ripper shank is lowered, and the tip is forced into the rock mass. The hydraulic system of the tractor applies a downward force combined with the machine's weight to initiate penetration into existing cracks or joints.
• Fracturing and Splitting: As the tractor moves forward, the ripper shank exerts a horizontal force. Because rock is significantly weaker in tension than in compression, the wedge action of the ripper tip induces tensile stresses that propagate existing fractures.
• Heaving and Breaking: The geometry of the ripper tip creates a lifting effect. As the tip moves through the rock, it acts as a lever, causing the rock mass to heave and break along the lines of least resistance.
• Efficiency Factors: The effectiveness of this principle depends on the rippability of the rock, which is governed by the spacing of discontinuities, seismic wave velocity, and the hardness of the rock material. Ripping is most efficient in stratified or jointed rock where the tool can exploit natural weaknesses rather than attempting to crush the intact rock matrix.