CIVE1105 Rock Slope Stability
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Rock slope failure mechanism:
 Continuum
 Discontinuum

Basic rock failure mechanism
 a.) circular slip
 b.) plane sliding
 c.) wedge sliding
 d.) toppling
 (page 21, week 9 lecture)

Stabilisation measures for failure mechanisms
 Key parameters: slope geometry, joint strength, joint water pressure (water conditions).
 Also failure mode, slope geology & structures, material properties,
 (p 32 week 9)

Rock slope stabilisation methods
  sprayed concrete
  weep holes
  dowels
  dentition
  rock bolts, rock anchors
  drain holes
  mesh

Design considerations
  ensure slope stability
  reduce excavation cost
  optimize design

What are the parameters/factors that control the stability of the rock slope?
 Slope geometry
 Joint strength
 Water pressure

Internal and External loads
 Internal loads: Gravity and hydraulic pressure
 External loads: can be restraining/resisting/stabilizing forces, (e.g. cable, bolt, anchors) or disturbing/sliding/destabilizing, (e.g. additional load from traffic ).

Forces acting on unstable block
 Weight of block  need to know the crosssectional area, the density of the rock
 Hydraulic forces  depend on the distribution of groundwater in the slope
 Shear resistance  depends on the shear strength

Factor of Safety against sliding
 =
 Forces resisting sliding / Forces causing sliding

Tension crack
It is not uncommon to find a Tension crack developing behind the crest of a slope as it begins to fail. It isn't always there and it isn't always visible even if it is there. Need to consider the possibility because water in this crack will push the blocks out of the slope and allow water to get under the base of a sliding block.

Measurement of shear strength of joints
 τ = σ tan(φ) = c + σ'tan(φ)
 σ is the stress applied normal to the shear plane
 φ is the angle of shearing resistance (in soil mechanics, φ is called the internal friction angle)

Simple model
 interface: N = W cosΨ + T sinβ
 S = W sinΨ  T cosβ
 FoS =
 (W cosΨ + T sinβ) tan φ/W sinΨ  T cosβ