7. Slope Failure in Soils

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3 modes of sediment transport:
1. Rapid, short-lived movement of sediment (landslides, floods) 2. Prolonged, slow movement of sediment (soil creep, transport of dissolved solids). 3. Storage of sediments (e.g. alluvial fans, talus accumulations, river terraces)
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2 basic types of slope process:
1. Transportation by running water 2. Mass movement (mass wasting)
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5 mechanisms of mass movement:
1. Fall 2. Sliding 3. Flow 4. Creep 5. Heave
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Fall
Detachment of rock or soil from steep faces
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Sliding
Slip of rock or soil over a sheer surface
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Flow
Movement of soil or rock as a flowing mass
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Creep
Slow downslope movement of soil or rock
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Heave
Repeated swelling and contraction of soil
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Types of slope failure:
1. Shallow translational slides 2. Deep rotational slides
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Shallow translational slides
Surface moves over bedrock
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Deep rotational slides
Upper slump zone, flows into foliage zone, at toe of hill.
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Slope stability depends on:
1. Shearing Force (SF) 2. Shearing Resistance (SR)
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Failure occurs when:
SF > SR
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a
gradient
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w
soil weight
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Downslope component of weight of soil on shear plane:
( = W sin a)
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Normal component of weight of soil on shear plane
( = W cos a)
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Downslope component and normal component?
Downslope ( W = sin a) Normal ( W = cos a)
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What is the downslope component of soil weight termed?
Shearing force (SF)
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Sooo, formula will always be:
SF = ( W = sin a) or SF = ( W cos a)
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Shearing resistance (SR) depends on?
1. Cohesion (c) 2. Normal component of soil weight ( W cos a) 3. Pore-water pressure (u) 4. Friction angle (φ) is decreased
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Shearing Resistance is expressed as:
SR = C + ( W cos a - u) tan φ
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Slope failure occurs when.. (+ formula for this)
W sin a > C + ( W cos a - u) tan φ
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What is model termed?
The Limit Equilibrium Model.
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So slope failure occurs when (4):
1. Slope angle (a) is increased 2. Water pressure (u) is increased 3. Cohesion (C) is decreased 4. The friction angle (φ ) is decreased
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Factors triggering soil failure (5):
1. Rainstorms 2. Shocks and vibrations 3. Changes in gradient 4. Weathering and progressive failure 5. Vegetation removal
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Rainstorms:
Increase in pore water pressure (u) causes a decrease in (W cos a-u). in clay soils C and φ are also reduced.
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Critical build up depends on:
Intensity of rainstorm, duration of rainstorm, and antecedent soil moisture conditions.
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Shocks and vibrations:
Destroys grain-to-grain contacts
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Causes
1. Loss of cohesion (C) 2. Reduction in the friction angle (φ) 3. And sometimes liquefaction of the soil
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Changes in gradient
Slope steepening cases increase in sin a ( increase in SF) and decrease in cos a ( decrease in SR)
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Weathering and Progressive failure
Mainly involves over consolidated clay soils. They swell, crack and deform when exposed at the surface, causing a slow reduction in C and φ.
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Vegetation removal, impacts and causes:
Vegetation binds soil (increasing C). Vegetation reduces build-up in pore water pressures (u) by interception. Vegetation removal (especially deforestation) reduces shearing resistance (SR)
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Debris Flow
Downslope movement of debris mixed with water. Referred to as 'mudflows'
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Causes of debris flow:
Translational slide becomes a viscous flow. Rupture of sediment dams in gullies
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Types of debris flow:
1. Hill slope flows 2. Valley-confined flows 3. Catastrophic flows (including lahars)
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Conditions for hillslope and valley confined debris flow:
1. Steep slopes (> 30o) 2. Abundant loose sediment on slope 3. Intense rainstorms
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Environmental conditions for hill slope/valley confined debris flow
1. Hot, semi-arid areas. 2. Polar and Alpine mountains 3. Maritime mountains.
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Catastrophic debris flow:
1. Low water content 2. High potential (kinetic) energy 3. long runout (several km's) 4. High velocities
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Example of catastrophic debris flow:
Mt Huascaran, 1970. >20,000 fatalities.
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Lahars
Catastrophic debris flow triggered by volcanic eruptions.
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Card 2

Front

2 basic types of slope process:

Back

1. Transportation by running water 2. Mass movement (mass wasting)

Card 3

Front

5 mechanisms of mass movement:

Back

Preview of the front of card 3

Card 4

Front

Fall

Back

Preview of the front of card 4

Card 5

Front

Sliding

Back

Preview of the front of card 5
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