Geology Reference
In-Depth Information
weight of water displaces and relieves some of the
downward force created by the weight of the sed-
iment. Second, under unsaturated conditions, a
negative pore pressure
or
suction force
tends to
hold the water within the pores and even draw
it up from the water table by capillary rise. Such
negative pore pressure increases the normal force
between sediment grains and increases their resis-
tance to movement. This capillary cohesion force
keeps sandcastles from collapsing. Falling raindrops
also create a force when they strike the ground.
Depending on their size and terminal velocity, they
may create a force strong enough to move sediment
grains.
4
Expansion forces
. Sediments, soils, and even solid
rock may expand and contract in response to changes
of temperature (heating and cooling, freezing and
thawing) or moisture content (wetting and dry-
ing), and sometimes in response to chemical changes
in minerals. Expansion tends to act equally in all
directions, and so any movement that occurs is
reversible. However, on slopes, the action of grav-
ity means that expansion in a downslope direction
greater than contraction in an upslope direc-
tion produces an overall downslope movement of
material.
5
Global fluid movements
. Wind carries water sedi-
ment in much the same way as water does - along
the 'bed' or in suspension. But, as air is far less dense
a fluid than water, for the same flow velocity it carries
sediment of smaller grain size.
6
Biological forces
. Animals and plants create forces
that influence sediment movement. Plant root
systems push material aside, and if this occurs
on a slope an overall downslope movement may
result. Burrowing animals mine soils and sedi-
ment, redistributing it across the land surface (see
Butler 1995). Where animals burrow into slopes,
a tendency for an overall downslope movement
occurs.
Shear stress, friction, cohesion, and
shear strength
A handful of key mechanisms explain much about
transport processes - force, stress, friction, and shear
strength. The case of soil resting on a slope demon-
strates these mechanisms. The force of gravity acts upon
the sediment, creating stresses. The normal stress (acting
perpendicular to the slope) tends to hold the sediment
in place. The
shear stress
acts in a downslope direction
and, if large enough, will move the soil downhill.
Three factors resist this downhill movement - friction,
cohesion, and shear strength.
Friction
resists sliding.
Many factors affect it, the most important being:
friction between the sediment and the underlying
rock
internal friction of grains within the sediment
(which depends upon their size, shape, arrangement,
resistance to crushing, and the number of contacts
per unit volume)
normal stress (the larger this is, the greater the degree
of friction)
smoothness of the plane of contact between the sed-
iment and the rock, which influences the angle of
friction.
A soil mass on a slope needs no externally applied force
for it to move. If the slope angle is steep enough, the
downslope component of the soil's weight will provide
sufficient downslope force to cause movement. When
the slope angle reaches a critical value, the soil will start
to slide. This critical angle is the static angle of slid-
ing friction,
j
m
, the tangent of which is equal to the
coefficient of static friction. The effective normal stress,
which allows for the pore water pressure in the soil,
also influences sliding. In dry material, the effective nor-
mal stress is the same as the normal stress, but in wet
but unsaturated soils, where pore water pressure is nega-
tive, the effective shear stress is less than the shear stress.
Cohesion
of the soil (the degree to which the individual
grains are held together) also affects sliding, cohesive sed-
iment resisting sliding more than non-cohesive sediment.
Finally,
shear strength
, which is the resistance of the soil
to shear stress, affects movement. Mohr-Coulomb's law
In summary, most movements of sediment require a
downslope force resulting from action of gravity, but
climatic, meteorological, and biotic factors may also
play an important role in moving materials.
