Engineering Geology: Rock Instability Analysis Flashcards
How is uniaxial compressive strength (UCS) measured in rocks?
By loading an unconfined, intact rock cylinder to the point of failure.
Strong rocks have a UCS greater than ___ kg/cm², weak rocks less than ___ kg/cm².
500; 50
What does Rock Quality Designation (RQD) measure?
The percentage of core recovered in sticks >10 cm.
What are the RQD categories for core quality?
100–75%: Good
75–50%: Fair
<50%: Poor
Name five geological characteristics evaluated during rock mapping.
- Type of rock
- Continuity
- Fracture evaluation
- Rock geometry
- Weathering
What site-scale factors influence rock strength?
Rock heterogeneity, bedding/joint/fault orientation, in-situ stress, seismicity.
What causes internal sliding resistance in rocks at the micro-scale?
Grain interlocking → coefficient of friction.
How does high fluid pressure affect rock strength?
It reduces effective stress, lowering rock strength.
What three observational scales are important in engineering geology?
Field scale, hand-specimen scale, microscopic scale.
Which of the following promote slope failure?
A. Downslope dip of bedding
B. Permeable layers
C. Fracturing and low cohesion
D. ‘Slippery’ rocks
Correct: A, C, D
Why are impermeable layers dangerous in slopes?
They obstruct groundwater and form lubricated slip surfaces.
What structural geology aspects are crucial in excavation design?
Rock strength
Dip angle
Fractures/joints/faults
Permeability and weak beds
What effect do joint sets have in tunnelling?
They cause overbreak and form unstable rock wedges in tunnel roofs/walls.
Overbreak is most severe when the tunnel is ___ to the main joint set.
Sub-parallel
How can folding affect slope stability?
It can align planes of weakness (e.g. bedding) in failure-prone directions.
How do boreholes help manage geological uncertainty?
They confirm subsurface layering and identify contacts between strata.
What is a typical use of kinematic analysis in road cutting design?
To assess potential for rock wedge failures on slopes.
What do pole projections help visualise in structural geology?
Orientation of bedding planes, fold axes, and lineations.
What does a small circle on a stereonet represent?
Lineations on a pre-folding plane.
What geological formation underlies the Taf Fechan dams in South Wales?
Sedimentary layers with defined contacts in the foundation.
Name one advantage and one disadvantage of numerical modelling.
Advantage: Can model stress and failure accurately
Disadvantage: Requires high data input and assumptions
What is a DFN model and what data does it simulate?
A model of fracture geometry and block sets; simulates fracture lengths and orientations.
What is a key conclusion regarding scale in rock engineering?
Rock mechanical properties vary with observation scale.
What must geotechnical solutions in rock take into account?
Structural geology and construction type.
