JNTU B.Tech III ROCK MECHANICS Previous Question Paper (August - 2022 - R18)

Rock mass classification systems integrate geological, physical, and mechanical parameters to assess the quality and stability of rock masses for engineering purposes. The primary factors considered include:

• Intact rock strength: Compressive and tensile strength of the rock material.
• Discontinuity characteristics: Spacing, orientation, persistence, aperture, roughness, and infilling material of joints and fractures.
• Groundwater conditions: Presence, pressure, and flow rates of water within the rock mass.
• In-situ stress conditions: The state of stress existing in the rock mass prior to excavation.
• Rock Quality Designation (RQD): A measure of the degree of jointing or fracturing.

One widely used system is the Rock Mass Rating (RMR) system, developed by Bieniawski. It assigns ratings to six parameters: uniaxial compressive strength, RQD, spacing of discontinuities, condition of discontinuities, groundwater conditions, and orientation of discontinuities. The sum of these ratings provides a total score (0-100), which classifies the rock mass into five classes ranging from Very Poor to Very Good, providing guidelines for excavation methods, support requirements, and stand-up time.

Rock Quality Designation (RQD) is a quantitative index used to estimate the quality of a rock mass based on drill core samples. It is defined as the percentage of the total length of a borehole run consisting of sound rock pieces that are 100 mm (4 inches) or longer in length, measured along the centerline of the core.

RQD provides a quick, simple, and inexpensive method to assess rock quality. It is highly sensitive to the degree of jointing and fracturing; a higher RQD indicates a more competent rock mass. The classification based on RQD is as follows:

• 0–25%: Very Poor
• 25–50%: Poor
• 50–75%: Fair
• 75–90%: Good
• 90–100%: Excellent

While useful, RQD has limitations: it does not account for the orientation, roughness, or infilling of discontinuities, and it is dependent on the direction of the borehole relative to the joint sets.