Geology Reference
In-Depth Information
σ
1
σ
f
φ°
σ
3
τ
f
θ
/2
°
Test 2
Test 1
c
θ°
σ
3
σ
1
, Normal stress
σ
The nature, origin and even existence of cohesion
-
strength at zero
normal load
causes considerable debate and confusion. This is partly
because it can be either apparent (the result of dilation during a test and
varying with con
-
ning stress) or a real physical entity and due to
cementation, grain bonding or impersistence of discontinuities in the
rock mass. Quite often both factors contribute to the measured strength
in the same test, for example, if shearing intact rock. In arti
cially
prepared samples of remoulded soil there is no true cohesion and
apparent cohesion is a function of the density of packing of the soil
grains relative to the con
ning stress. A theory of critical state soil
mechanics has been developed for such soil that links shear strength to
deformation characteristics (Roscoe et al., 1958; Scho
eld, 2006).
Burland (2008) however notes the importance of geological history to
natural soils, with the development of bonding and fabric leading to true
cohesional, non-dilational and stress-independent strength. While
Burland was really discussing relatively young soils, it has been demon-
strated earlier (
Figure 5.1)
how, with time, true cohesion can become
very high and far outweigh the contribution of friction to shear strength.
Conversely, as rock is gradually weathered it is primarily the cohesional
strength that is lost
-
friction remains essentially constant.
After high shear displacement, cohesion is lost, and shearing continues at a
residual friction level. This is non-dilational friction but in nature can be
lower than the critical state
-
also non-dilational
-
because of change in
structure with, for example,
flattening and alignment of particles in a clay
or thedevelopment of highlypolished shear surfaces. Suchstrengths canbe
very low (sometimes of the order of 7 degrees for montmorillonite clay-
rich rocks) and very signi
cant, especially for landslides (see discussion of
Carsington Dam failure in
Chapter 7).
To test residual strength, torsional
