SOILS ON SLOPES

Large-scale geologic structures provide the initial framework upon which landscape development proceeds. Finer details of landscapes (i.e. individual landforms) are usually determined by DIFFERENTIAL EROSION.. E.g. domed strata provide rock layers dipping away radially from a central high point; differential erosion produces inward-facing scarps (S), outward-facing dipslopes (D) and radial strike valleys (SV).

Strength and Stress
In the context of geomorphology, strength refers to the ability to resist being moved by erosional processes, which normally operate in a downslope direction. The force exerted by erosional processes (including gravity) is a SHEAR STRESS directed downslope and causing a mass of rock or soil to shear over the underlying material.
Controls On Soil Characteristics
The characteristics of soil depend on: parent material; climate; vegetation; slope.
1. Parent material: influences;
a. the rate of soil development (rate of weathering)
b. soil composition e.g. shales produce a lot of clay; sandstone produces sandy soil
c. physical properties of soil e.g. permeability/drainage (number, size and connectivity of pore spaces); shrink-swell potential (amount of expansive clay); cohesive strength (clay content - clayey soils are "sticky" - this aids cohesion).

2. Climate: influences type and rate of weathering, amount of water moving through and over the soil; type of vegetation.

3. Vegetation: influences organic content of soil, strength of soil (roots increase cohesion)

4. Slope: Steeper slopes -> accelerated erosion, if rate of erosion > rate of soil development -> thin or no soil. Soils on steeper slopes also have lower water contents (lower infiltration) -> less weathering, less vegetation. Soils in low-lying areas have higher water contents, more weathering, thicker soils, more vegetation.

Rock And Soil Resistance To Shear Stresses
Depends on:
1. Frictional properties of material
2. Normal load (weight pushing rock/soil mass into slope)
3. Cohesion

Friction
Depends on hardness of shear surfaces; roughness of shear surfaces and number+area of points of contact between shear surfaces. Symbolized by angle of shearing resistance (f), which determines the coefficient of plane sliding friction tanf. (these values can be found in laboratory tests). Higher normal stress increases frictional resistance therefore for purely frictional materials (e.g. dry sand),
        shear resistance =dn tanf (where dn = normal stress).

Cohesion
refers to "binding together" of material e.g. chemical cementation in sedimentary rocks; surface water tension in pore spaces (i.e. sticky clay); binding root systems of plants. Therefore, TOTAL shear resistance (or strength)
                S = C + dn tanf - the Coulomb Equation.

Shear stress. Refers to the force "pushing" the material down the slope. This depends on:
i) the weight of the material and
ii) the downslope component of gravity (see below).
Force = mass x acceleration (due to gravity).

The arrow length is proportional to the magnitude of the force. Increased slope decreases normal stress and increases shearing stress.

Effect of water on soil strength. Water affects cohesion, friction and normal stress.
A. Moist soil: water films create a negative PORE WATER PRESSURE i.e. a "suction" which increases cohesion by "drawing" particles together.
B. Saturated soil: POSITIVE PORE WATER PRESSURES can develop, which "push" particles apart (i.e. water in pore spaces becomes "pressurized") - this ACTS AGAINST the normal stress, effectively reducing it. Since particles are pushed apart cohesion and friction are also reduced.
To account for water pressures, the Coulomb Equation is modified thus:
                S = C' + (dn - Pw) Tan f'
where Pw = Pore water pressure
C' = Cohesion, including Pw effect
Tan f' = Internal friction, including Pw
Note: Negative Pw will increase both cohesion and friction, increasing strength. Postive Pw will decrease both cohesion and friction, decreasing strength.

The evaluation of slope stability is essentially based on a comparison of shear strength and shear stress i.e.
Safety factor F = shear strength/shear stress

If F > 1 = stable slope
F = 1 = critical threshold
F < 1 = failure

These calculations can be made, but only for a given set of conditions. Often, however, it is a change in "normal" conditions that causes failure.

Changes That Decrease Stability
1. Increased water content: Increased water content can change the strength and stress ratio in a number of ways, which are often concurrent.
a) increased pore water pressure - in effect this is related to the height of the water table. It is not surprising that most sudden slope failures occur during or after particularly prolonged or heavy rains. This situation could be particularly critical when a weak layer exists in the soil (e.g. clay layer; volcanic ash) and the water table rises above the layer, saturating it.
b) changes to the soil's physical properties - applies to clay-rich soils. Clays, when dry, can be very firm and stable. However, because clays absorb water they can become much weaker when wet; at first being capable of slow internal deformation and then, being capable of flowing like a viscous liquid. Prolonged saturation of clay layers (again due to elevated water tables) can therefore cause a progressive reduction in shear strength.
c) water loading on slopes - another property of increased water content is that it increases the weight of the soil, thus raised water tables can increase the shear stress and at the same time decrease the shear strength.
Example; Snow Pack Melting. Considering the above points, snow pack melting can be particularly effective at generating slope failures - the melting snow can saturate the soil and the remaining snow pack can add a great weight to the soil.

2. Removal of vegetation - e.g. logging; there are many documented cases of slope failures resulting from the removal of trees from steep slopes. The loss of root networks reduces the cohesion of soil, while decreased evapotranspiration raises water levels. Often slope failures occur several years after logging, when root systems decay away.

3. Increased slopes - usually constructed; increases stress, while decreasing strength (gravity acts less into the slope and more down the slope) - can also be caused by:

4. Undercutting slopes - e.g. river erosion, road cuts, wave action - removes support from base of slope and causes steepening of slope.

5. Loading of slope - e.g. construction, watering lawn..

6. Liquefaction - occurs during earthquakes, the shaking causes rearrangement of particles (mainly sand and silt), increasing packing and decreasing porosity; thus the water content can change from unsaturated to saturated (without adding water).

Example Questions:

1. State the modified Coulomb Equation for shear strength of soil on a slope (i.e. including the effects of pore water pressure); define each term and explain why saturation of soil increases the likelihood of slope failure.

2. With the aid of diagrams explain shearing stress and normal stress acting on soil on a slope. Show how the magnitude of these two forces depends on the slope angle.

3. Describe five changes that decrease stability of slopes.