Mass Wasting
Factors Influencing Mass Wasting

        Mass movement/wasting involves down slope movement of rock, regolith, and soil under the direct influence of gravity. The earth materials behave as solids or viscous masses, and may be consolidated (compacted and cemented) or unconsolidated(loose and uncemented). Movement occurs when the force or gravity exceeds the resisting force (slope stability).

Slope stability is determined by:

1. the strength and cohesiveness of the slope material(s)
2. internal friction between grains
3. any external support of the slope

1. gravity, a vertical force that can be split into vectors parallel to (tangential) and perpendicular to a surface(normal)
2. friction on the surface or between grains
3. shear strength, a measure of material strength and cohesion

a. Slope gradient

This is the most important control on mass wasting. Movement occurs when slopes are steeper than the natural angle of repose of the material. The angle of repose is the steepest angle that a slope can maintain without failing, and is typically 25-40 degrees for unconsolidated materials. Slope can be over steepened by:

1. Natural causes such as stream and wave erosion undercutting the slopes
2. Human activity (e.g., road cuts, hillside construction)

1. Weathering disaggregates and disintegrates bedrock, reducing its shear strength and promoting mass wasting
2. Climate controls the type and rate of weathering, e.g., rockfalls are more likely in areas subject to frost wedging

       Increasing water content decreases the stability of slope materials because it:

1. Adds weight
2. Reduces the cohesion of material through displacing air and destroying surface tension
3. Reduces the cohesion of material through pore pressure pushing apart grains
4. Allows material to move easily by lubricating surfaces

         Removal of vegetation can destabilize slopes. Plants stabilize slopes because they:

1. Adsorb water, decreasing the water saturation limit of the slope material.
2. Bind together soil particles and hold soil to bedrock with their roots.

        Overloading involves an increase in weight which may increase the tangential force on the plane, or may increase water pressure and decrease friction causing failure. This factor is almost always the result of human activity, including:

1. Weight of buildings
2. Dumping, filling, or piling up material

        Rocks inclined (dipping) in the same direction as the slope are more prone to mass wasting than rocks in other orientations. Bedding planes and fractures serve as zones of weakness along which weathering and movement can take place

g. Triggering mechanisms

        Most mass movements are triggered by some event:

1. Excessive amounts of water
2. Earthquake
3. Volcanic eruptions
4. Vibrations
5. Noise

        Several types of movement are recognized. Mass wasting events may involve only one type of movement, or combinations of movement types. Mass movements are classified by their dominant behavior according to:

1. the type of motion
2. the rate of motion, and
3. the type of material involved

        Material moving down slope may behave as an elastic solid, a plastic substance, or as a liquid. In some mass movements, the material may exhibit more than one type of movement:

1. Slide involves movement of coherent blocks of material along a well-defined surface (e.g., rock slide, glide, avalanche, debris slide, or slump)
2. Fall involves free fall of material (no contact with any surface except to bounce) as in a rock fall
3. Flow involves continuous movement of material as a viscous fluid (e.g., mudflow, debris flow, earthflow, solifluction)
4. Heave represents expansion of material at or near the surface at right angles to the slope. Gravity pulls material vertically downward, so material moves slightly. It is typically caused by repeated freezing/thawing or wetting/drying cycles, and may result in creep
5. Complex motion involves various combinations of the above movement types

        Movement during mass wasting ranges from slow to rapid:

• Slow movements may be almost imperceptible, but operate continuously over long periods to move much more material than more spectacular types of mass movements. Slow movements primarily affect unconsolidated material at depths less than 1 meter, where movement rates average from 1 mm/yr to 1 mm/day:

1. Creep is a slow downward movement of surface material by heaving. It can be caused by wetting/drying or freezing/thawing (frost heaving) cycles, and is increased by burrowing animals, decaying roots, and loading. Creep is evidenced by tilted telephone poles, fence posts, bent tree trunks, etc., and represents the most extensive type of mass wasting.
2. Solifluction is slow down slope movement of surface material by flow. The surface material is saturated with water and may be highly affected by frost heaving. The process is most important in regions of permafrost or tundra (permanently frozen subsoil). The active layer of soil above the permafrost thaws in summer and refreezes in winter. Poor drainage makes the active layer water saturated, so it easily flows, even on gentle slopes. About 20% of the Earth's surface is underlain by permafrost.

• Moderate velocity movements have movement rates that average from 1 cm/day to 1 cm/s. Mass movements of this type include:

1. Slumps (slope failure) where there is a downward and outward movement of blocks of material by sliding along a curved surface. A steep scarp surface exists at the top of the moving mass, and movement may be initiated by slope over steepening and aided by water infiltration.
2. Earthflows where water-rich unconsolidated material moves by slumping and plastic flow. Earthflows produce a tongue-shaped mass with a hummocky surface and lobed ends. They are commonly associated with grass-covered slopes in humid areas. With increased water contents, earthflows move faster and grade into mudflows.
3. Debris slides where relatively dry unconsolidated material moves downslope. Debris slides, which involve coarser material than earthflows, are also characterized by a hummocky surface and lobed ends. They grade into debris flows with water saturation.

• Rapid movements, which have movement rates ranging from meters/second to 100 km/hr, occur on steep slopes.

1. Rockfalls involve the sudden fall of large rock fragments along steep cliffs or ridges where rock is jointed. Weathering (mechanical or chemical) produces the talus or rock fragments, which rarely have slopes that exceed 40 degrees.
2. Mudflows represent a well-mixed mass of water (up to 30%) and earth (at least 50% clay- and silt-sized particles) that moves down slopes as a fluid. They are typically triggered by heavy rainfall in semi-arid areas, but also occur in mountainous and volcanic areas. Quick clays are silt- and clay-sized particles produced by glacial grinding and deposited in a marine environment. Quick clays are stable in salt water, but become unstable and liquefy when disturbed and exposed to fresh water.
3. Debris flows are composed of coarser material than mudflows but contain less water. They move more slowly than mudflows, but still can carry large objects. Debris flows typically occur in semi-arid mountainous areas after heavy rainfalls.
4. Rock slides or rock avalanches are the most catastrophic type of rock movement. Glides are where movement occurs along a nearly planar surface (usually a bedding plane, e.g., the Saidmarreh 305 x 14,000 x 5,000 m block (50 billion tons) moved about 18 km). Speeds can exceed 100 km/hr, and millions of tons of material can be transported down slope by sliding along a nearly planar surface. Factors contributing to sudden movement include steeply dipping rock strata, fractures, clay-rich units underlying more resistant rocks, undercutting, heavy rains, and earthquakes. The speed of the slide is increased by momentum transfer (momentum of material at the back of the slide is transferred forward through collisions). Rock slides are usually preceded by warning signs.

        Areas prone to mass wasting can usually be identified. To evaluate the potential for mass movement, site hazard assessment studies involve:

1. Identification of former landslide areas.
2. Geologic assessment of rock/soil material and structure.
3. Preparation of slope stability maps.

Various engineering methods, used to try to minimize the danger and damage of mass movements, include:

Slope dewatering where surface and subsurface drains are installed to remove excess water. This reduces weight and increases shear strength of the slope.

• Slope reduction, which may be accomplished by two common methods:

1. Cut-and-fill where material is removed from the upper part of the slope and used to fill at the bottom of the slope.
2. Benching where a series of steps are cut into the slope (an approached commonly used on steep slopes in conjunction with drains).
• Stabilizing structures, which may have mixed results in attempts to control mass wasting:

1. Retaining walls can provide support for base of slope.
2. Rock bolts are used to fasten potentially unstable rock masses to stable bedrock, and locally used in conjunction with drains and chain-link fencing.