MASS WASTING
Mass wasting (mass movements) is the movement of surface material by gravity. This can be slow or fast, consolidated or unconsolidated material. Mass wasting slowly works to level the Earth's surface.
Any type of Earth material can be subject to mass movements. Rocks falling off a cliff, mudslides, any surface material.
Factors that control mass movements:
1. Lithology and stratigraphy:
A) consolidation of Earth material
B) permeability
C) bedding planes: direction, number
D) rock cleavage
E) fractures, joints, faults
F) strength
Surface material: is it consolidated or unconsolidated? Both will move under the right circumstances. Is it porous or nonporous? Porous material allows pore pressure to build encouraging movement. Porous material becomes saturated and allows water to flow to layers below. Is it slippery with pronounced bedding planes? Bedding planes provide a plane for movement to occur. Is the rock fractured and weak? Weak and fractured rock will easily crumble and encourages movement. Are there numerous joints and faults? Any cracks in the rock surface allows water to enter and can become a plane of movement. How is the material arranged...permeable over impermeable? Water build up between units and acts as a lubricant. Strong over weak? Weak will crumble out from under the strong and encourage movement of overlying material.
2. Slope angle:
Is the slope oversteepened? Are bedding planes steeply inclined? Bedding planes can be planes of movement and the more steeply inclined the more likely movement will occur. Is Rock cleavage present? This forms a plane for movement. What is the angle of repose? Unconsolidated material forms a stable slope at the angle of repose, this angle depends on the particle size of the material. Larger particles can support a steeper slope, sand has an angle of repose of 34 degrees, slopes greater than this are oversteepened and movement will occur.
3. Climate:
Alternating freeze thaw? Freeze-thaw acts to expand/contract rock units causing them to separate which encourages movement. Torrential rains saturate material? Pore pressure will encourage movement and water also acts as a lubricant. Frost heaving? This causes unconsolidated material to be loosened encouraging movement.
4. Vegetation:
Vegetation will anchor slopes. Vegetation also acts to absorb water reducing saturation.
Factors that can trigger movement:
Earthquake activity is a common trigger for mass movements.
Weather events
Weight added to a slope: buildings, ice, snow ect.
Undercutting, terracing or oversteepening for construction often causes stable slopes to become unstable.
Urbanization: road cuts, clear cutting
Different types of movements:
Flow: unconsolidated material moves downslope much like a fluid, usually water soaked, not a coherent mass.
Slide: sudden slipping of consolidated material as a unit along a shear plane. Can also include unconsolidated material if it moves as a unit along a shear plane.
Fall: free downward movement, sudden fall of material.
FLOW
FAST
1. Debris Flow (Debris Avalanche): unconsolidated material (usually larger in size) moves rapidly downslope tumbling, mixing with air and/or water. A "river" of unconsolidated material.
2. Mudflow: fine grained unconsolidated material, saturated with water moving rapidly downslope. Has the consistency of newly mixed concrete. Usually flows in a channel.
3. Earthflow: downslope movement of soil, rocks and fine sediment without channelization, usually not as fluid as a mudflow.
SLOW
1. Creep: slow downslope movement of unconsolidated material. Rock creep and talus creep refers to slow movement of rock slopes.
2. Solufluction: movement of unconsolidated material over permafrost.
SLIDE
FAST
1. Rockslide: sudden movement of a rock slab along a plane of weakness, fault of bedding plane.
2. Debris slide: sudden slipping of unconsolidated material (usually larger sized) along a shear plane. Material moves as a unit even though it is unconsolidated.
3. Slump: sudden slipping of unconsolidated material (usually smaller sized) along a shear plane. Material moves as a unit and slump is characterized by a backward rotation.
FALL
1. Rock Fall: rock dislodged and falls from higher elevation.
Talus: pile of rock debris at the bottom of a slope usually in a wedge shape.
Strategies for preventing mass movement:
Road cuts are common areas for mass movement, this movement can be prevented or controlled.
Lowering the angle of the road cut can keep material from becoming oversteepened.
Terracing along slopes where bedding planes are steeply inclined.
Bolting rock planes together, this is effective in areas where rock cleavage is a problem or thinly bedded rock units.
Fencing a slope face to keep rock falls and slides from the roadway is effective in some areas.
Retaining walls can be used to stabilize a slope with unconsolidated material.
Proper drainage helps relieve pore pressure behind retaining walls.
Building on terraced slopes adds weight to slope encouraging movement.
