Geology Reference
In-Depth Information
Figure 5.4
Measured strength
envelope with
apparent cohesion
and friction, which
can be corrected to
a basic friction line
(non-dilational).
Textural
interlocking:
deformation and
damage
i
'apparent'
cohesion
Lower bound
mineral friction
φ
Normal stress
true area of contact
dense to a less dense state or as a rock joint lifts over a roughness
feature. If the raw strength data from a test are plotted against normal
stress, then the peak strength envelope may show an intercept on the
shear strength axis (apparent cohesion), albeit that the peak strength
envelope may be very irregular, depending upon the variability of the
samples tested. If corrections are made for the dilational work during
the test, in many cases the strength envelope will be frictional: the
strength envelope passes through the origin. At very high stresses, all
dilation will be constrained and the soil or rock asperities will be sheared
through without volume change. These concepts are illustrated sche-
matically in
Figure 5.4.
5.3.4 True cohesion
Rocks and natural soil may also exhibit true cohesion, due to cementa-
tion and chemical bonding of grains. For a rock joint, it is derived from
intact rock bridges that need to be sheared through. This additional
strength, evident as resistance to tension, is essentially independent of
normal stress and proportional to sample size. This is discussed further
below.
5.3.5 Geological factors
In
Chapter 1
(
Figure 1.5),
the concept of a rock cycle was introduced
whereby fresh rock deteriorates to soil through weathering and then
sedimented soil is transformed again into rock through burial,
compaction and cementation. Clearly, at each stage in this cycle the
geomaterials will have distinct properties and modes of behaviour.
