Page 78 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 61

relates shear strength to cohesion, gravity, and friction stick together. It arises through capillary suction of water
(see below). When shear stress (a driving force) exceeds in pores, compaction (which may cause small grains
shear strength (a resisting force), then slope failure occurs to interlock), chemical bonds (mainly Van der Waals
and the soil moves. In rock, weathering (which may bonds), plant root systems, and the presence of such
increase cohesion), the presence of joints and bedding cements as carbonates, silica, and iron oxides. Soil par-
planes (which may reduce the angle of friction), pore ticles affect the mass cohesion of a soil body by tending
water (which reduces effective normal stress and increases to stick together and by generating friction between one
cohesion), and vegetation (which increases the angle of another, which is called the internal friction or shearing
friction and may increase cohesion) affect shear strength. resistance and is determined by particle size and shape,
Other factors influencing shear strength include extra and the degree to which particles touch each other. The
weight added to a slope as water or building materials, Mohr–Coulomb equation defines the shear stress that
earthquakes, and erosion or excavation of rock units. a body of soil on a slope can withstand before it moves:

τ s = c + σ tan φ
GRAVITATIONAL PROCESSES
where τ s (tau-s) is the shear strength of the soil, c is
Stress and strain in soils and sediments soil cohesion, σ (sigma) is the normal stress (at right-
angles to the slope), and φ (phi) is the angle of internal
Earth materials are subject to stress and strain. A stress friction or shearing resistance. The angle φ is not
is any force that tends to move materials downslope. necessarily the slope angle but is the angle of inter-
Gravity is the main force, but swelling and shrinking, nal friction within the slope mass and represents the
expansion and contraction, ice-crystal growth, and the angle of contact between the particles making up the soil
activities of animals and plants also set up forces in a soil or unconsolidated mass and the underlying surface. All
body. The stress of a body of soil on a slope depends unconsolidated materials tend to fail at angles less than
largely upon the mass of the soil body, m, and the angle the slope angle upon which they rest, loosely compacted
of slope, θ (theta): materials failing at lower angles than compacted mate-
rials. The pressure of water in the soil voids, that is, the
Stress = m sin θ pore water pressure, ξ (xi), modifies the shear strength:

Strain is the effect of stress upon a soil body. It may be τ s = c + (σ − ξ) tan φ
spread uniformly throughout the body, or it may focus
around joints where fracture may occur. It may affect This accounts for the common occurrence of slope fail-
individual particles or the entire soil column. ures after heavy rain, when pore water pressures are high
Materials possess an inherent resistance against down- and effective normal stresses (σ − ξ)low.On10and
slope movement. Friction is a force that acts against 11 January 1999, a large portion of the upper part of
gravity and resists movement. It depends on the rough- Beachy Head, Sussex, England, collapsed (cf. p. 316).
ness of the plane between the soil and the underlying The rockfall appears to have resulted from increased pore
material. Downslope movement of a soil body can occur pressures in the chalk following a wetter than normal year
only when the applied stress is large enough to overcome in 1998 and rain falling on most days in the fortnight
the maximum frictional resistance. Friction is expressed before the fall.
as a coefficient, µ (mu), which is equal to the angle at The Mohr–Coulomb equation can be used to define
which sliding begins (called the angle of plane slid- the shear strength of a unit of rock resting on a failure
ing friction). In addition to friction, cohesion between plane and the susceptibility of that material to land-
particles resists downslope movement. Cohesion mea- sliding, providing the effects of fractures and joints are
sures the tendency of particles within the soil body to included. Whenever the stress applied to a soil or rock