Geoscience Reference
In-Depth Information
Any force (gravity is by far the most important) that
tends to move material downslope is termed a stress
(Figure 12.1). If a building is built on the slope, the
weight of the building adds more stress to the soil.
Additional weights on a slope are termed 'stress
increments'. Stress increments can consist of rain, soil
moved from upslope, buildings, or any other mass.
While gravitational stress is most obvious, there are
other stresses that can exist in the regolith. Molecular
stress arises with the movement of soil particles or
even individual molecules. It is associated with such
phenomena as swelling and shrinking of
colloids
(clay
soil particles) on wetting and drying, thermal expan-
sion and contraction, and the growth of ice crystals.
Biological stress refers to the stress exerted on a soil
body by the growth of plant roots or the activities of
animals.
amount of friction. While friction is dependent on the
degree of roughness of a surface, it is independent of
the area of contact between a body of regolith and the
underlying substrata. Small areas of soil material tend
to fail at the same angles as larger areas. Figure 12.1
illustrates the application of stress to an object on a
slope. There is a resisting force against movement due
to friction. Sliding will commence when the applied
stress exceeds maximum frictional resistance.
Friction is expressed as a coefficient µ and this
coefficient is equal to the tangent of the slope angle at
which sliding just begins:
)
-1
tan
= (
sin
) (
cos
(12.2)
where
= angle of plane sliding friction
= coefficient of friction
The equations of movement of objects can be
expressed in terms of critical frictional resistance
(
R
f
crit
) and critical applied force (
F
crit
). Failure com-
mences when the critical applied force exceeds the
critical frictional resistance as follows:
ω
sin
α
F
crit
>
R
f
crit
ω
α
where
R
f
crit
~
tan
(12.3)
Fig. 12.1
This equation implies that as the weight of an object
approaches zero the force required to move it down-
slope approaches zero. For non-rigid objects (uncon-
solidated material) this is not so, because as the weight
of the regolith approaches zero, there is still an addi-
tional force resisting downslope movement. This force
is termed cohesion, defined as the bonding that exists
between particles making up the soil body. The above
relationship, now including cohesion, can be expressed
as follows:
Effect of gravity on an object with mass
on a slope with
angle
.
The effect of stress upon a soil or a regolith is called
strain
. Strain is not directly equated with stress but
incorporates the additional factor of slope angle. Strain
may not uniformly occur in the soil body, but may be
restricted to joints where fracturing will eventuate.
Strain can affect inter-particle movements, or act on
the overall soil column. In combination, these strains
result from what is labelled 'net shear stresses'.
F
c
rit
>
R
f crit
(12.4)
Frict
ion, cohesion and coherence
where
R
f crit
~
tan
+
c
One of the forces of resistance against the movement
of material downslope is friction. Friction is the force
that tends to resist the sliding of one object over, or
against, another. For regolith material sitting on a firm
base, friction is due to irregularities present at the
contact between these two materials no matter how
smooth this contact may appear to be. The irregulari-
ties tend to interlock with each other and prevent
movement. Generally, the greater the weight of an
object pressing down upon the contact, the greater the
c
= cohesion
Strictly, the term cohesion refers to chemical or
physical forces between clay particles. The term
coher-
ence
is used to describe the binding of soil particles of
all sizes, as a group or mass, due to capillary cohesion
by water at the interface between individual grains, by
chemical bonds or by
cementation
. In cementation,
the chemical bonds are considered primary and, thus,
are very strong. Common cementing agents include
