Page 201 - Handbook of Gold Exploration and Evaluation

176 Handbook of gold exploration and evaluation

consisting of pediment surfaces, alluvial fans and playas is formed. A most
important climatic change relates to the overprinting of parts of a deeply
weathered regolith formed under humid warm to tropical conditions by later
features related to weathering under semi-arid to arid climatic conditions.

3.5.1 Stability of slopes
Slope characteristics are natural responses to the interaction of ever-changing
patterns of denudation systems inherited from past erosional and depositional
cycles and are inherently complex and unstable. Variables affecting those
characteristics are bedrock geology, climate, soil, vegetation and angle of slope.
The stability of the regolith is a function of climate and the ability of the
weathered rock material to withstand the stresses imposed upon it.
The conventional view of straight evenly graded hillsides over which the
intensity of energy is equal across each horizontal plane is too simplistic.
Hillside slopes are invariably marked by convexities, concavities and other
irregularities, which direct the movement of materials along the lines of least
resistance. Colluvial trains break up into separate flow paths and sorting
relationships are governed by slope lengths, the manner in which individual
mass flows split or converge, and the differential fluidity that follows an uneven
distribution of seepage and run-off. The rate of movement is variable and
sometimes abrupt depending upon the angle of slope, the intensity of precipita-
tion and the nature of the slope materials. Spontaneous downward movement
occurs when the internal strength of the regolith declines to a critical point at
which the force of gravity cannot be resisted. As modified by presently acting
processes, two opposing forces determine existing levels of slope stability, shear
strength and shear stress.
`Shear strength', as the resistance of a material to shear, is a function of
internal friction and cohesion (the electrostatic attraction among very small
particles). `Shear stress', as the applied force tending to cause failure, is a
function of particle friction (which depends upon the friction angle) and
gravitation forces. Soils of different particle size and mineral composition have
different strength capabilities and respond differently to the application of stress.
Just prior to failure shear stress is equal to shear strength, which then represents
the maximum strength of the material. Failure is imminent only when shear
stress exceeds shear strength.
For mixed soils, subject to changing pore water pressure, shear strength is
generally represented by Coulomb's `failure law', which may be expressed as:
S ˆ
f ˆ C ‡ tan s 3.6

where S ˆ shear strength,
f ˆ shear stress at failure, C ˆ cohesion, ˆ
pressure normal to the shear plane, and s ˆ static angle of plane sliding
friction.