Geomechanics and Ground Control
Geomechanics asks whether an opening will stand and what support it needs. The exam tests rock strength criteria, rock-mass classification, stress distribution around excavations, pillar design, subsidence and slope stability. Numerical questions are usually one formula deep, so the marks turn on correct units and on knowing which strength criterion or classification scheme applies.
Rock properties and classification
- Strength set: uniaxial compressive strength, Brazilian tensile strength, triaxial strength; Hoek-Brown criterion links them through the geological strength index.
- RQD (Deere) counts intact core pieces over 100 mm from double-tube drilling; RMR (Bieniawski) and the Q system (Barton) convert ratings into support recommendations; higher values mean better rock in both.
Stress around openings
- The Kirsch solution gives the elastic field around a circular hole; in a hydrostatic field the tangential stress at the boundary is twice the far-field stress, which is why walls spall.
- The ground response curve relates wall convergence to support pressure; support is designed to intersect it before the stand-up time expires.
Worked example
Depth H = 300 m, unit weight 0.025 MN/m^3, square pillars 20 m wide with 5 m roadways. Tributary-area pillar stress = unit weight x H x (w + w_o)^2 / w^2 = 0.025 x 300 x (25/20)^2 = 7.5 x 1.5625 = 11.72 MPa. If the pillar strength formula returns 30 MPa, the factor of safety is 30/11.72 = 2.56, comfortably above the usual design band of 1.5 to 2.0.
Traps
- Entering unit weight in kN/m^3 (25) instead of MN/m^3 (0.025) in a formula expecting megapascals, giving an answer 1000 times too large.
- Forgetting that the tributary area uses (w + w_o), the pillar centre-to-centre spacing, not the pillar width alone.
- Mixing subsidence terms: angle of draw, angle of break and critical width are not interchangeable.