Classifications
of mass movement
The types of processes can be classified in a
number of different ways:
- Speed of
movement
- Water
content
- Type of
movement: flows, slides, slumps
- Material
The
mechanism of mass movement
Rock particles on slopes are held on the slope by
friction in a state of dynamic equilibrium. Their steady state (not moving)
represents a balance between the internal (within/ between the particles known
as internal or shear strength) and external forces (known as external/shear
stress). When shear strength = shear stress = no movement. If one is greater
than the other = movement.
Water as
a factor in changing the dynamic equilibrium of the slope:
Water is a very important factor in influencing
slope stability. Particles in the soil stick together if it rains, the rain
infiltrates via the pores and lubricates the weathered material therefore
reduces friction and makes the weathered material easier to move down the
slopes. Water may also increase external stress because it adds weight to the
slope (because of an increase in pore pressure).
What
keeps slopes in place?
Vegetation (Binds soil thus stabilising slopes),
friction (will vary with the weight of the particle and slope angle, and can be
overcome with the help of water), cohesive force (An act to bind the particles
of the slope and prevalent is water-less clay) and pivoting (occurs in debris
layers that contain material embedded in the slope).
What
factors lead to increasing stress and decreasing shear resistance?
Water, weathering and the type of material can
reduce resistance. Stress can be increased by steepening of a slope,
undercutting of a slope, addition of a mass of regolith, dumping of mining
waste, sliding from higher up the slope, vibrational shock and earthquakes.
Factors
that contribute to increased shear stress:
·
Removal of lateral support through undercutting
or slope steepening – Erosion by rivers, glaciers, wave action, faulting,
previous rock falls or slides.
·
Removal of underlying support –Undercutting by
rivers, waves, sub-surface solution, loss of strength by extrusion of
underlying sediments.
·
Loading of slope – Weight of water, vegetation,
and accumulation of debris.
·
Lateral pressure – Water in cracks, freezing in
cracks, swelling and pressure release.
·
Transient stresses – Earthquakes and movement of
trees in the wind.
Factors
that contribute to reduced shear strength:
·
Weathering effects – Disintegration of granular
rocks, hydration of clay materials, dissolution of cementing minerals in rock
or soil.
·
Changes in pore water pressure – Saturation or
softening of material
·
Changes in structure – Creation of fissures in
shales and clays, remoulding of sand and sensitive clay.
·
Organic effects – Burrowing of animals and
decaying tree roots.
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