Mass Wasting

Factors Influencing Mass Wasting:

• Force of gravity vs resisting force
• Shear strength: depends on material strength, cohesion, internal friction
• Angle of repose: measure of the steepness of a pile of the material - usually 30-40°

Slopes (like most of Geology) are in dynamic equilibrium

Slope Angle: how can a slope become oversteepened?

• Stream or wave under-cutting
• Road cuts and building site excavation
• Any mechanism which removes material from the toe or base of a slope, effectively increasing the slope of the remaining material

Weathering and Climate: remember the role of parent rock type

• Arid vs tropical climates

Water Content: in rock or soil

• Weight of the water increases weight of slope (displaces air in pores)
• May decrease friction between grains

Vegetation:

• Absorbing water and transpiring it decreases water saturation
• Root systems help stabilize slope
• Remember the truth about bent trees and the role of creep

Overloading: human activity of building, dumping, filling on top of slope

Subsurface geology: angle of dip of subsurface sedimentary layers

• Certainly not the only factor but commonly an important one
• Don't forget the orientation of joints as well as bedding

Triggering mechanisms: failure can commonly be temporally related to input of energy

• Earthquakes, volcanic eruptions, sudden cloudbursts, snow melt

Failure occurs when the slope passes the elastic limit and deforms by rupture (brittle) or deformation (plastic)

Types, Rates of Movement and Material:

• Falls, Slides, Flows
• Fast vs slow
• Rock, soil, debris (mixture)

Flow:

• Creep: cm/year, surface faster than depth
  • Leaning fences
  • Freeze-thaw cycle/movement
  • Consider talus or a rock glacier
• Debris-, Earth- or Mud-flow: m/yr to km/hour
  • Usually water saturated regolith

Slide: (slump, rockslide)

• Slump: concave fracture failure
  • Trees stay rooted and will tip backwards
  • Slump and flow often occur together
  • "cats-paw lakes"
• Rockslide or rock avalanche: up to 100 km/hour
  • Gros Ventre slide, Madison Canyon slide creating Slide or Quake Lake

How to ameliorate the effects of mass movement:

• Drainage: reduce pore pressure of water by drilling and pumping or horizontal drain pipes
• Reduction of slope angle: support base or more commonly remove material from the top of the slope
• Benching: produce a stair-step pattern - failures that still occur will be much smaller (individually) and easier to clean up
• Rock bolts: expensive but effective in some circumstances
• Retaining walls: commonly found along highways in mountainous terrain