Geology Reference
In-Depth Information
relates shear strength to cohesion, gravity, and friction
(see below). When shear stress (a driving force) exceeds
shear strength (a resisting force), then slope failure occurs
and the soil moves. In rock, weathering (which may
increase cohesion), the presence of joints and bedding
planes (which may reduce the angle of friction), pore
water (which reduces effective normal stress and increases
cohesion), and vegetation (which increases the angle of
friction and may increase cohesion) affect shear strength.
Other factors influencing shear strength include extra
weight added to a slope as water or building materials,
earthquakes, and erosion or excavation of rock units.
stick together. It arises through capillary suction of water
in pores, compaction (which may cause small grains
to interlock), chemical bonds (mainly
Van der Waals
bonds
), plant root systems, and the presence of such
cements as carbonates, silica, and iron oxides. Soil par-
ticles affect the mass cohesion of a soil body by tending
to stick together and by generating friction between one
another, which is called the
internal friction
or
shearing
resistance
and is determined by particle size and shape,
and the degree to which particles touch each other. The
Mohr-Coulomb
equation defines the shear stress that
a body of soil on a slope can withstand before it moves:
τ
=
+
σ
φ
c
tan
s
GRAVITATIONAL PROCESSES
where
s
(tau-s) is the
shear strength
of the soil,
c
is
soil cohesion,
τ
(sigma) is the
normal stress
(at right-
angles to the slope), and
σ
Stress and strain in soils and sediments
φ
(phi) is the
angle of internal
friction
or
shearing resistance
. The angle
Earth materials are subject to stress and strain. A
stress
is any force that tends to move materials downslope.
Gravity is the main force, but swelling and shrinking,
expansion and contraction, ice-crystal growth, and the
activities of animals and plants also set up forces in a soil
body. The stress of a body of soil on a slope depends
largely upon the mass of the soil body,
m
, and the angle
of slope,
φ
is not
necessarily the slope angle but is the angle of inter-
nal friction within the slope mass and represents the
angle of contact between the particles making up the soil
or unconsolidated mass and the underlying surface. All
unconsolidated materials tend to fail at angles less than
the slope angle upon which they rest, loosely compacted
materials failing at lower angles than compacted mate-
rials. The pressure of water in the soil voids, that is, the
pore water pressure
,
θ
(theta):
Stress
=
m
sin
θ
ξ
(xi), modifies the shear strength:
τ
=
+
σ
−
ξ
φ
Strain
is the effect of stress upon a soil body. It may be
spread uniformly throughout the body, or it may focus
around joints where fracture may occur. It may affect
individual particles or the entire soil column.
Materials possess an inherent resistance against down-
slope movement. Friction is a force that acts against
gravity and resists movement. It depends on the rough-
ness of the plane between the soil and the underlying
material. Downslope movement of a soil body can occur
only when the applied stress is large enough to overcome
the maximum frictional resistance. Friction is expressed
as a coefficient,
c
(
) tan
s
This accounts for the common occurrence of slope fail-
ures after heavy rain, when pore water pressures are high
and effective normal stresses (
)low.On10and
11 January 1999, a large portion of the upper part of
Beachy Head, Sussex, England, collapsed (cf. p. 316).
The rockfall appears to have resulted from increased pore
pressures in the chalk following a wetter than normal year
in 1998 and rain falling on most days in the fortnight
before the fall.
The Mohr-Coulomb equation can be used to define
the shear strength of a unit of rock resting on a failure
plane and the susceptibility of that material to land-
sliding, providing the effects of fractures and joints are
included. Whenever the stress applied to a soil or rock
σ
−
ξ
µ
(mu), which is equal to the angle at
which sliding begins (called the
angle of plane slid-
ing friction
). In addition to friction, cohesion between
particles resists downslope movement. Cohesion mea-
sures the tendency of particles within the soil body to
