Geography 323 -- Geomorphology

Physical Properties of Rock, Soil and Water

Soil and Rock Mechanics

Resistance of soil and rock to

  1. tensile, compressive and shear stresses
  2. abrasion by solids moving over a rock surface (ice or rock fragments carried in wind, water, snow, ice or under the force of gravity; is controlled by the harness of constituent minerals and cementing agents
  3. transport by fluids, i.e. bed shear stress

Behavior of rock

  1. Elastic
  2. Plastic
  3. Brittle

Strength of rock masses

Controls on resistance of rock to stress: lithology and structure

Controls on rock mass stability

  1. Strength of intact rock: cohesion and friction between mineral crystals and grains within rock blocks
  2. Extent of weathering: from negligible discoloration and disaggregation to total disaggregation where the rock has weathered to sediment or soil
  3. Spacing of joint and fractures: no cohesion along open joints, so the denser is jointing the weaker is the rock mass; failure in nearly always long preexisting discontinuities
  4. Orientation of discontinuities (joints, bedding planes): those dipping out of a slope favour sliding of rock blocks
  5. Width and roughness of fractures: no cohesion across open joints and friction only at points of contact between crystals or grains
  6. Continuity of fractures: governs the likelihood that a fracture will form a failure surface; also circulation of water promotes deeper weathering
  7. Amount of infill: strength along a joint is that of the infill
  8. Water flow: excess cleft water pressure in a saturated joint applies a buoyant force on the overlying rock

Soil strength

shear strength (S)
resistance to shear force = f(normal force, friction, cohesion, pore pressure)

mechanical resistance

cohesion (c)

pore pressure (p)
portion of the normal stress supported by air and water in interstitial spaces

positive porewater pressure is a buoyant force, that is, is supports part of the weight of the soil and therefore wet sediment has very low shear strength

Coulomb equation

  1. dry soil
  2. wet soil
  3. saturated soil


Factor of Safety
shear strength/ shear stress

Atterberg limits

behavoir of fine sediment:

plastic limit
water content at the transition from solid to plastic behavior, measured when a wet thread of fine soil begins to crumble
liquid limit
water content at transition from plastic to solid behavior, measured when soil in a shallow dish flows to close a 12.5 mm groove after 25 drops from 1 cm
plasticity index
liquid limit - plastic limit, that is the range of water content over which sediment behaves

Sensitive clay

[ Course Outline | Next Topic ]