Questions
8 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Geomechanics and Ground Control form the backbone of safe mining and civil engineering operations, focusing on how rock masses behave under stress. This topic is essential for PSU exams as it tests both fundamental soil/rock mechanics principles and the practical application of empirical classification systems. Mastery requires understanding stress-strain behavior, stability criteria, and the mathematical empirical formulas used in structural design.
Rock Properties and Classification
Rock properties define the physical behavior of materials in situ versus laboratory settings. Exam focus is primarily on the distinction between intact rock strength and rock mass strength, and how anisotropy affects load-bearing capacity.
- Unconfined Compressive Strength (UCS) is the primary design parameter
- Point Load Strength Index (Is) = P / De^2
- Ratio of UCS to Tensile strength is typically 10:1 to 15:1
- Secant Modulus vs. Tangent Modulus application
- Porosity (n) and Void ratio (e) relationship
Rock Mass Classification (RQD, RMR, Q)
These empirical systems convert geological observations into quantitative data for support design. You must memorize the input parameters for each system to answer calculation-based questions quickly.
- RQD = (Sum of lengths of core pieces > 10 cm) / (Total length of core run) x 100
- RMR (Rock Mass Rating) includes 6 parameters like UCS, RQD, and joint spacing
- Q-system = (RQD/Jn) x (Jr/Ja) x (Jw/SRF)
- Q-system values range from 0.001 (exceptionally poor) to 1000 (exceptionally good)
- Relation: RMR = 9 ln Q + 44
Support Design and Pillar Mechanics
Pillar design is critical for underground stability and preventing collapse. Exam questions often involve calculating the Factor of Safety (FoS) for square or rectangular pillars under tributary area loading.
- Tributary Area Method: Stress (sigma) = (depth x gamma) / (1 - R)
- Extraction Ratio (R) = 1 - (w^2 / (w+B)^2)
- Obert-Duvall Pillar Formula: Strength (S) = k * (w^0.5 / h^0.75)
- Roof bolting acts primarily through beam building and suspension effects
- Factor of Safety = Pillar Strength / Pillar Stress
Slope Stability and Subsidence
This section covers the mechanics of failures in open-cast mines and civil cuts. Focus on the geometry of failure surfaces and the influence of groundwater pressure on slope stability.
- Factor of Safety (FoS) = Resisting force / Driving force
- Bishop's Method of Slices is standard for circular failure analysis
- Subsidence trough geometry: Draw angle and Limit angle
- Vibration limit is usually measured in Peak Particle Velocity (PPV)
- PPV = K * (D / sqrt(Q))^-n (Scaled Distance Law)
Formula Sheet
RQD = (Sum of pieces > 10cm / Total length) * 100
Q = (RQD/Jn) * (Jr/Ja) * (Jw/SRF)
RMR = 9ln(Q) + 44
Extraction Ratio R = 1 - (w/(w+B))^2
Pillar Strength S = k * (w^0.5 / h^0.75)
PPV = K * (D / sqrt(Q))^-n
FoS = Resisting Force / Driving Force
Exam Tip
Memorize the specific range categories for RQD and the five primary parameters of RMR; these are frequently tested as direct recall questions.
Common Mistakes
- Confusing the input parameters for RMR vs Q-system indices.
- Neglecting to convert units (e.g., MPa to kPa) when calculating Pillar Stress or PPV.
- Misinterpreting the Extraction Ratio (R) in pillar strength formulas.
More Revision Notes
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