18-Geol-A5 Rock Mechanics · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — 18-Geol-A5, Rock Mechanics. Closed-book, 3-hour exam; 5 questions of 20 marks each (80 marks total); candidates were instructed to answer only 4 of the 5 — all 5 are answered below. Every page footer of the paper reads “May 2019”.
Reference texts:
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Both groundwater and ground vibration act on pit-wall stability primarily through their effect on the effective normal stress and shear strength along the discontinuities (joints, bedding, faults) that actually control most rock-slope failures, rather than through the intact rock strength itself.
Groundwater — detrimental effects. Water pressure in joints and along potential failure surfaces reduces the effective normal stress ($\sigma_n'=\sigma_n-u$) without changing the total stress, directly lowering the frictional shear resistance available on that plane (Mohr–Coulomb: $\tau_f=c+\sigma_n'\tan\phi$). A rising water table or poorly drained pit wall can also generate outward seepage forces at the toe, add weight to a potential failure mass, and — in weaker or clay-bearing discontinuity infillings — soften and reduce the cohesion and friction angle of the infill itself over time. Any of these can trigger or accelerate a planar, wedge or circular failure that a dry wall of the same geometry would not experience.
Groundwater — beneficial effects (small movements). Where movement is small and progressive rather than catastrophic, controlled dewatering-driven consolidation of a weak zone can increase its effective stress and long-term strength once excess pore pressures dissipate; and limited drainage- induced settlement can also close open tension cracks at the crest, restoring some interlock and reducing the driving surface-water infiltration path for the next rainfall event. In practice this “benefit” is really the far side of proper engineered depressurization (horizontal drain holes, drainage adits), not something to rely on passively.
Ground vibration (blasting/seismic) — detrimental effects. Cyclic dynamic loading adds an inertial (pseudo-static) force component to the static driving force on a potential failure surface, and can progressively degrade the peak shear strength on a discontinuity toward its lower, residual value by breaking down asperities and disturbing interlocking rock bridges — a rock bridge that has survived static loading for years can still fail under repeated blast-induced cyclic stress. Vibration can also transiently elevate pore pressure in saturated, poorly draining material (a mechanism related to liquefaction), further compounding the groundwater effect above.
Ground vibration — beneficial effects (small movements). Low-amplitude, well- controlled blast vibration away from the wall of interest can promote minor densification/compaction of loose, poorly-graded fill or waste-rock material, incrementally increasing its density and frictional strength; and a very small, controlled release of locked-in residual stress at a free face (a form of stress relief) can reduce the driving stress concentration at a stress-sensitive corner or toe, provided the movement stays within the elastic/small-strain range and does not itself open new discontinuities. Both effects are narrow and easily reversed by excessive amplitude, which is why blast vibration near a pit wall is tightly monitored against peak-particle-velocity limits rather than assumed to be net-beneficial.
Ground support in underground mining serves two distinct, complementary functions. Reinforcement acts within the rock mass itself, improving its own ability to support itself — tying loose or potentially loose blocks back to more competent rock beyond the failure surface, increasing the effective shear strength across discontinuities by clamping them together, and creating a self-supporting reinforced arch or beam out of rock that would otherwise be a collection of independent blocks. Surface support instead acts at the excavation boundary, retaining the rock surface between reinforcement elements, preventing individual small blocks or slabs from ravelling/spalling into the opening, and distributing point loads from the reinforcement elements over a larger area of the excavation surface.
i) Reinforcement of excavation rock zones is typically achieved with mechanically- or resin/cement-grouted rebar rock bolts (fully bonded along their length, mobilizing shear resistance across any discontinuity they cross), friction-anchored bolts (Split Sets, Swellex) that develop capacity through radial friction against the borehole wall for fast, simple installation, and cable bolts (multi-strand steel cable, grouted over several metres) for longer-range reinforcement where a conventional 2–3 m bolt cannot reach beyond the failure surface (e.g. large spans, deep-seated block failures, or pillar reinforcement).
ii) Surface support enhancements are typically plain or fibre-reinforced shotcrete (sprayed concrete providing both a physical membrane and, once cured, some tensile/flexural capacity across small blocks), welded-wire or chain-link mesh (retains loosened material between bolts without itself adding much strength), and steel straps or standard/tensioned mesh panels tied to the bolt pattern — these purely retain material and transfer local point loads back onto the reinforcement grid; they do not by themselves increase the rock mass's own strength the way reinforcement does.
