Geoscience Reference
In-Depth Information
carbonates, silica, alumina, iron oxide, and organic
compounds. Cementation can also occur because of
compaction, especially if material comes under
pressure from above. The compaction process gives
stability to materials on slopes. With certain grain
shapes, especially with clays, it is possible to realign
grains so that they interlock effectively, increasing
coherence and the resisting force to movement.
Chemical bonds consist mainly of oppositely charged
electrical fields that develop on the surfaces of large
molecules, especially clay minerals. These attracting
charges are called
Van der Waals' bonds
, which for clay
minerals remain active even when the clay particles or
colloids are moved relative to each other. For clays, this
gives rise to plasticity, which will be described later.
Figure 12.2 presents the type of bonds that can exist
depending upon particle size and the relative strength of
bonding. Van der Waals' bonds are restricted to material
less than 0.03 mm or 30 microns in diameter. As the
material gets smaller, the bonding strength increases
considerably until it reaches values of 1 kg cm
-2
for sizes
less than 0.001 microns. As grain size increases, capillary
cohesion (mainly due to water) becomes dominant.
Capillary cohesion is also very important in bonding clay
minerals, creating forces three orders of magnitude
stronger than Van der Waals' bonds. The only way that
coherence of material coarser than 1 cm can be achieved
is by compaction and cementation.
The effect of capillary cohesion can be demon-
strated easily. Damp sand can be shaped into almost
vertical walls without any sign of failure. As the sand
dries out and the cohesive tension of water at sand
grain interfaces is removed, the sand pile begins to
crumple, eventually reaching the angle of repose for
loose sand, which is only 33°. Cohesion in damp sand
is produced totally by the effect of water tension
between sand grains, and gives wet sand a remarkable
degree of stability. Note, however, that the sand cannot
be too wet, otherwise liquefaction occurs. In other
words, if more water (or any other material for that
matter) is added to the sand, such that the pore spaces
in the material are filled and the capillary thickness
of water at the grain interfaces is exceeded, then
the cohesion of the material approaches zero and the
material begins to behave as a fluid. This process will
be discussed in more detail later.
SHEAR STRENGTH OF SOILS:
MOHR-COULOMB EQUATION
The way that soil particles behave as a group or mass
(coherence) depends not only upon the inner cohesion
of the soil particles but also upon the friction generated
between individual soil grains. The latter characteristic
is internal friction or shearing resistance. How much
shear stress a soil or regolith can withstand is given
by the following equation, termed the
Mohr-Coulomb
equation
:
10
4
s
=
c
+
tan
(12.5)
10
3
where
s
= the shear strength of the soil
c
= soil cohesion
10
2
= the normal stress (at right angles
to the slope)
Cementation with
compaction
10
1
= the angle of internal friction or
shearing resistance
Cementation by
deposition
10
0
10
-1
The Mohr-Coulomb equation is represented diagram-
matically in Figure 12.3. Note that this equation is in
a form similar to Equation 12.4 except for two
differences. Firstly, in the Mohr-Coulomb equation
the critical force for movement is determined by the
stress normal to the ground surface, rather than by
the weight piled on a slope.
Secondly, the angle of the slope has been replaced
by the angle of shearing resistance, which represents
the angle of contact between particles making up
10
-2
10
-3
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Particle diameter (cm)
The strength of bonding on particles of different sizes
(Finlayson & Statham, ©1980 with permission
Butterworths, Sydney).
Fig. 12.2
