Chapter 14

 

MASS WASTING

 

Mass wasting (mass movement)- downslope movement of material (soil/rock) as result of gravity.

Occurs on slopes when gravitational force exceeds resisting force.

     

Factors influencing mass wasting

When the gravitational force exceeds the slope resisting force, slope failure occurs.

Resisting forces are: 1) slope material strength and cohesion, 2) amount of internal friction between grains, 3) any external support of slope (fig. 14.2).  Together = shear strength.

 

Angle of repose- steepest angle slope can maintain without collapsing (~25-40˚ for unconsolidated material) (shear strength=force of gravity)

 

Factors are:

      1) Slope Angle- steeper the slope, the less stable; steepened by undercutting (waves, stream,         man, etc.) (fig. 14.3; 14.4; 14.5).

2) Weathering- rock weathered to unconsolidated material; climate controls amount of weathering.

      3) Water Content- increased water content adds weight, decreases friction between grains, and     increases pore pressure, which greatly reduces strength.

      4) Vegetation- the more vegetation, the more stable the slope; roots stabilize slope and absorbs     water from rainstorms.

      5) Overloading- excess weight on slope decreases stability; causes increased pore pressure.

      6) Geology and Slope Stability (bedding, joints, cleavage)- slope unstable if planes of weakness dip in same direction as slope (fig. 14.7).

 

Triggering mechanisms- some force that disturbs slope equilibrium: earthquake, volcanic eruption, explosion, heavy rainstorm, and excessive water from snowmelt.

 

Types of Mass Wasting

Classified based on 3 criteria: (1) rate of movement (fast [visible], slow), (2) type of movement (fall, slide, flow), and (3) type of material (rock, soil, debris) (Table 14.2).

 

Falls: rockfall- rock free falling or bouncing down cliff) (figs. 14.9, 14.10); result of failure of         plane of weakness in rock; talus- pile of rock at bottom of slope (fig. 14.9).

 

      Slides: movement of material along one or more surfaces of slip.

            -slump (rotational slide)- movement of weak material along curved surfaces(figs. 14.12, 13).

            -rock slide- rocks move downslope along ~planar surfaces; occur when planes of weakness dip parallel to slope (figs. 14.14 to 14.15).

 

      Flows- material flows as a viscous fluid or plastic movement.

-mudflow- rapidly flowing mixture of mud and water (>50% silt/clay and >30% water) (fig. 14.17).

-debris flow- like mudflows but with larger particles and less water.

-earthflow- tongue-shaped mass of wet soil; slump on upper slope and flow on lower slope (fig. 14.19).

-solifluction- slow flow of water saturated soil over some impermeable material, usually permafrost; impermeable material keeps water from draining (fig. 14.22).

-creep- very slow, continuous downslope movement of soil or debris (fig. 14.24).

-complex movement- combination of different movement types (fig. 14.25).

 

      How recognize effects of mass wasting?

      Before building in an area, geologic study should be done to determine if area is at risk to   significant mass wasting; identify areas of potential slope failure, look for: scarps,

open fissures, moved or tilted objects, hummocky surface, sudden changes in vegetation, determine physical properties (shear strength, permeability, etc.) of soil and bedrock;

based on this information, can construct a slope stability map (fig. 14.27).

 

How minimize danger of mass wasting?

Surface and subsurface drainage of water (fig. 14.28), plant vegetation, reduce slope of hillside (cut and fill fig. 14.29, benching fig. 14.30), properly constructed retaining

wall (fig. 14.31), rock bolts (fig. 14.32).