8. Rock Slope Failure

Rock mass stability is determined by ... especially ....
Joints, but especially stress-release joints resulting from unloading
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Slope failure occurs when
Shearing force > shearing resistance
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Types of rock slope failure
1. Fails and topples: rockfalls, topples, rock avalanches 2. Slides: Translational slides, rotational slides, block gliding. 3. Rock creep: cambering, rock mass deformation
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Who classifies rock-slope failure
Selby, 1993
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Rockfall mechanisms:
Stress release (joint development), Freeze-thaw activity, Heating and cooling, Build-up of joint water pressures
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Rockfall retreat rates: (Alpine and British)
Difficult to measure rate. Alpine mounts rate is 0.1 - 3 mm/yr (seems little, but a lot over time). British mountain rate is 0.01 - 0.05 mm/yr
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Rockfall talus slopes
Deposit of slide rock at foot of hill in cone shape. Top of talus is straight.
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2 Talus slope elements:
1. Longer the rocks have to fall, the faster they will travel and further they will go - KINETIC ENERGY. 2. NEGATIVE FEEDBACK SYSTEM
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Negative feedback system
(Destroys itself progressively through time) Built in - as builds up the distance that rocks fall becomes less and less thus velocity less and less so they travel less.
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Straight slope develops progressively at the expense of the
Basal Concavity
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Talus modified through other processes:
1. Snow avalanches 2. Debris flow
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Occurs when weight and height are larger that SR
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Causes of toppling
Expansion stress release joints, Fluctuation in joint-water pressure, wedging of chockstones, basal erosion, earthquakes
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Rocks that fall down cracks
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Rock avalanches result from:
Progressive joint development
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Rock avalanches also triggered by:
Earthquakes, and joint-water processes
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If momentum is sufficient, rock avalanche may develop into
Sturzstrom (excess run out landslide)
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Turtle Mountain. So much kinetic energy that it travelled 4km across valley and 140m uphill.
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Causes of run out:
Inertial grain flow
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Inertial Grain flow process:
As rocks come down hill, kinetic energy increases, though momentum slows as rocks bash ground and there is friction. However, rocks behind all crashing into one another so as long as landslide at back has kinetic energy this is transferred to front
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Translational Rockslides occur when?
Occur when joints dip outwards at steep gradients
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Causes of translational rockslide
1. Progressive failure - slow shearing along joints. 2. Build up of joint-water pressures 3. Earthquakes
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R.. rockslides
Rotational Rockslides
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1. Progressive stress release 2. Oversteepening 3. Undermining
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Block gliding:
Similar to rotational sliding but without rotation: blocks of intact rock are detached from scarp and 'glide' outwards on weak clays or shales.
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Rock Mass deformation
Gravitational slope deformation without development of continuous failure plane.
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What does it involve?
Creep, folding and faulting of rock
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1. Stress release 2. Slope steepening 3. Triggered by earthquakes
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Upward squeezing of ductile rock under the weight of overlying strata
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Cause of cambering:
flow of unsaturated clay during thaw of permafrost
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Type of failure depends on:
Lithology and structure
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Main long-term cause of failure
Joint development by stress release leading to critical conditional stability of rock mass
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Short term trigger effects:
Earthquakes, build up of joint-water pressures, and undermining or slope steepening.
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Card 2


Slope failure occurs when


Shearing force > shearing resistance

Card 3


Types of rock slope failure


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Card 4


Who classifies rock-slope failure


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Card 5


Rockfall mechanisms:


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