Mass Wasting Flashcards
Mass Wasting
-Masses of debris (mud, sand, gravel) or bedrock moving downhill driven by gravity
-Caused by slope failure
-Slopes can become unstable when they are oversteepened or comprise of unconsolidated material.
-Part of natural weathering and erosion process
Resisting force < Gravitational force - Friction < Shear Force
Triggers of Mass Wasting
- tectonically uplifted areas
- the oversteepening of a slope
- vibrations from earthquakes or manmade events
- heavy rainstorms
- freeze-thaw action
- volcanic activity
How can slope stability be determined
- the angle of the slope
-material strength
Factors of Mass Wasting
-Nature of material
-Cohesion (Cohesionless materials, loose sediments, can be stable within their ‘angle of repose ‘, depends on size, roughness, water content)
-Shear Strength
-Slope angle
-Thickness of debris over bedrock
-Consolidated vs unconsolidated material
-Water content
-Vegetation- (heavily vegetated vs light or no vegetation)
Nature of Material
-Unconsolidated (any loose and un-cemented material) such as soil, sediments, etc
-Angle of repose → for unconsolidated material, it is the maximum slope at which a pile of loose material will remain stable.
What determines the angle of repose
1) Particle size
2) Particle shape
3) Moisture Content
Example of Stronger Soils
Cemented, vegetated, clay-rich
* Clays can increase cohesion of materials
* Mineral cements (silica or carbonate) hold rock together
* Roots add strength to material by binding loose material together.
Example of Weaker Rocks
Planes of weakness (in bedrock) can reduce cohesion and stability
* bedding planes; foliation; joints
* plane orientation
Classification of Mass Wasting
-Type of material that moves
-Nature of movement (flow or slide)
-How fast Material Moves
What is rock fall
- Sudden and rapid free fall movement of rock
- Occur on steep slopes
- Forms talus piles
- Can be associated with rockslide
What is land-rock slide
Movement along a defined slippage surface
Blocks of rock detach along bedding planes, joints, fractures, etc.
Occur on steep slopes
Triggered by rain falls or ground vibration
Very fast and most destructive type of mass wasting
What is flow (debris flow/avalanche)
- Mixture of rocks, mud, & water
- Moves as a viscous fluid
- Common after heavy rains
- Rapid movement – up to 100 km/h
- Common in semi-arid regions
What happened to the Gyama Gold Mine Area
Happened March 28th 2013. 70 km east of Lhasa, Tibet. This rock avalanche buried 83 people. It was about 3 km long and between 20 m and 50 m thick, with a volume of about 2 million cubic meters.
What caused the Gyama Gold Mine Area Avalanche
- steep V-shaped valley
- earthquake activity
- high meltwater runoff
- lack of stability at its back started the slip
What is slump
– Scarp
– “Hummocky” terrain on and below
