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Slopes - Balance between:
•FORCE (EXOGENETIC)
•RESISTANCE (ENDOGENETIC)
STRESS > STRENGTH = MASS MOVEMENT
FORCES
Slope Failure is due to:
1.Increase in Stress (i.e. the
forces attempting to pull a
mass downslope)
2.Reduction in Internal
Resistance (Shear Strength)
Factors contributing to a High Shear Stress
1. Removal of lateral support
Fluvial or glacial erosion
Wave action
Faulting
Weathering: frost action > weakening rocks
Mass Movement events
Man-made quarries and pits
2. Overloading
Weight or rain, snow, talus (scree), vegetation
3. Transitory Stresses
Earthquakes, movement of trees in wind
Factors contributing to a High Shear Stress
4. Removal of underlying support
Undercutting by rivers, waves
Weathering – eg sub surface solution
Mining
5. Lateral Pressure
Freezing of water
Swelling of Clay (Hydration)
Dilitation
WEAK
RESISTANCE
Factors contributing to a Low Shear Strength
1. Compostition and texture
Weak materials - eg Sedimentary layers
Loosely packed materials
2. Weathering Effects
Disintegration of granular rocks
Hydration of Clay minerals
Dissolution of cementing materials in rock / soil
3. Changes in porewater pressure
Saturated pores > push grains apart >
decreases friction due to reduction in cohesion
Factors contributing to a Low Shear Strength
4. Changes in Structure
Spontaneous liquefaction – eg in
Clays
5. Vegetation
Removal of trees
Removing tree roots
OCR 7.3.11 “Slope Processes”
Mass Movement Types
1. Heave
(Wetting and Drying / Freezing and Thawing)
2. Flow (Material does not retain its form)
1. Soil Creep
2. Solifluction
•Increasing Speed
3. Earthflow / Debris Flow
•Increased Water Content
4. Mudflow
3. Slides
1. Landslide
2. Rotational Slump
3. Debris / Rock Avalanche
4. Rockfall
1. Sheetwash
•When the volume of water is too
high for the infiltration capacity of
the soil - the water flows over the soil
surface. Sheetwash is a very fast
‘Flow’ event
•Case Study Example > Lynmouth Flood, 1953
2. Rills and Gullies
•Small river
Sheetwash.
channels
resulting
from
•Small ephemeral channels (seasonal
temporaray - do not flow all year round)
/
•Caused by a significant increase in
precipitation - I.e. an extreme rainfall event
•As the water table rises - the water flows on
the surface of the soil - as the regolith has
become saturated
3. Heave
•A slow form of mass movement
•Occurs due to periods of:
•Wetting and Drying - eg ‘swelling of clay
minerals’
•Freezing and Thawing
•Hence favours Cold, Wet conditions - eg Tundra
Environments - eg White Mountains, Alaska
•A significant form of mass movement in
environments where flows and slides are not
common
Terracettes (the small terraces) on the grassy side of the
Frenchman River valley in Grasslands National Park
near Val Marie, Saskatchewan. Often due to Creep
4. Mudflow
•A type of Flow - involving a significant amount of
water
•Very rapid - flowing at over 1km/hr
•Similar to Earthflows - except they are thinner in
consistency - a higher water content
•Where are they likely to occur?
•In mountainous areas after heavy rain
•In periglacial areas at the time of maximum
summer thaw
•Jokulhlaups – eg Southern Iceland, 1995
•On slopes of erupting volcanoes
5. Debris Flow / Earthflow
•A type of “Flow”
•Occurs on slopes 5-15 Degrees
•Rates between 1mm/day > 1m/ sec.
•Produces Flow Tracks
•Thicker in consistency than mudflows > but
having a lower water content
•Common in humid areas - where deep
regolith
The Aberfan Disaster
21 October 1966
The Slumgullion earth flow in the San Juan
Mountains of south-western Colorado.
This landscape in Franciscan rocks of the
California Coast ranges (USA) is
primarily eroded by earthflows
6. Gravity Falls
•Extremely fast form of mass movement
•Weathering Limited Slope
The speed at which the slope develops over
time is only ‘limited by the rate at which that
slope weathers - as the mechanism of transport
(eg via gravity) is always available to transport
(or erode) weathered material - eg a Free-Face
•Lines of weakness occur along the joints in the
rocks
•Case Study Example : Wast Water Screes, Lake District
Rockfall - showing material (Talus) that fell from the cliffs above
7. Rotational Slumps / Rotational Slip
•A type of Slide
•Rotational movement along a curved / arcuate slip
plane
•Often found in areas of mixed geology
•Eg a permeable chalk layer overlying an
impermeable clay layer.
•The permeable chalk layer can become saturated,
heavier and the stress on the slope outweighs the
strain > leading to a slide
•The build up of water at the base of the chalk layer
also acts as a lubricant, reducing the friction >
promoting a Slide
•Eg Vaiont Dam, Langarone Italy 1966
Police Point landslide occurred in May, 1967 in the valley of battle Creek in the
Cypress Hills of south-eastern Alberta. The upper part of the landslide consist of a
series of rotated slump blocks.
Rotational failure and earthflow in shaly
rocks in the California Coast ranges (USA)
8. Solifluction
•A Type of Flow> material therefore does not retain its
form
•Common in Cold Climates - eg White Mountains,Alaska
•Can occur on slopes with shallow gradients
•Regolith - becomes saturated - eg due to:
•High precipitation
•Seasonal melting of snow
•Often forms Solifluction Lobes - eg Cheddar Gorge
•NB: Where this process occurs in Periglacial Areas - I.e.
due to the Seasonal melting of permafrost layer over a
frozen subsoil = Gelifluction
9. Landslide / Debris Slide / Landslip / Rockslide
•A Type of “Slide”
•Where the entire mass of material moves along a Slip Plane therefore tends to keep its shape (as opposed to a flow)
•Often in places where there is active undercutting at the foot
of slopes
•Endogenetic factors important:
•Eg Amount and degree of jointing in the rock
•Strength of the Rock
•Angle of slope
•Water lubricates these joints / planes > reducing the
strength of the slope
•Speeds vary between 1mm/day > 10m / sec
Debris avalanche tracks eroded into mountainous slopes
in the Alaska Range in Alaska ending in debris cones.
The original steep relief was created by glacial scour.
Frost weathering weakens the bedrock, triggering the
debris avalanches which scour the avalanche chutes.
A rockslide in the Front Ranges of the Rocky
Mountains in south-western Alberta.