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Mass Wasting

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The downhill movement of soil and loose unconsolidated sediments is due to the downslope component of the force of gravity and is resisted by the force of friction. The downslope driving forces (shear forces) and frictional resistive forces are in balance at the angle of repose which is the maximum slope angle that unconsolidated materials can maintain. At angles steeper than the angle of repose friction is not sufficient to counter the shear force and mass wasting occurs. At angles less than the angle of repose the shear forces cannot overcome friction and sediments may accumulate to form steeper slopes.

Water plays an important role in mass wasting. Dry sediments have no cohesion. Damp sediments are cohesive as water coats the sedimentary grains and holds them together with its surface tension (surface tension is the result of the dipolar nature of water). The angle of repose is greater in damp sediment than in dry (think of a sand castle). In water-saturated sediments all the pore spaces are filled with water. Water pressure in the pore spaces partly counters the weight of grain on grain thereby decreasing friction and the angle of repose, and possibly causing mass wasting.

Saturated sediments are more likely to undergo mass wasting because pore pressure decreases the frictional force.

in a saturated sediment the weight of all the water above produces a pore pressure that tends to push the grains apart
the effective normal stress is the lithostatic stress (from the weight of the sediments in grain on grain contact) minus the pore pressure

The Factor of Safety is the ratio of resisting force to driving force on a slope.

FS  =   frictional resisting force
            gravitational driving force

(see angle of repose page for explanation)

Slope failure can occur as a result of:

I) Processes that increase shear stress (driving forces)
     a) loading the slope (increasing its weight) by
         -  increasing the water content
          - building on it
     b) earthquake shaking
     c) undercutting (oversteepening) by
          - stream erosion
          - road or building excavation
     d) subsurface collapse
          - sinkhole
          - mine collapse

II) Processes that decrease the shear strength (frictional resisting forces)
     a) increased pore pressure (water pressure in void spaces)
          - increasing load of water and saturation
          - compaction (from vibration)
     b) fissuring - fracturing (frost wedging)
     c) dissolution of cements in sedimentary rocks
     d) removal of vegetation and roots that hold the soil
          - by fire
          - by clearing for building

The risk of slope failure can be reduced by:

I) drainage control
- surface channels to prevent infiltration on slope
- covering slope with impermeable barrier (cement)
- subsurface drainage to remove groundwater & reduce pore pressure

II) grading
- cut and fill to reduce slope

III) slope supports
- concrete retaining walls
- gabions (stone-filled wire baskets)
- piles (concrete, steel, or wooden beams driven into the ground)

Landslide Correction
- drainage, drainage, drainage!
at the head of the slide and in the water-bearing zone

Warning Systems for landslides and lahars require monitoring via:
- human inspection
- tilt meters
- geophones to measure vibrations
- groundwater monitoring wells

The two basic classes of mass wasting are flows and slides:

• In flows, the material behaves as a fluid. It deforms internally
  Soil creep, earthflows, and mudflows are examples.
• In slides, the material behaves as a rigid solid that detaches along one or more basal surface(s).
  Slumps and landslides are examples.