Geology Reference
In-Depth Information
not the same as the dilation angle, i°, which needs to be assessed at
eld
scale, although the mechanics are the same. To avoid confusion, the
laboratory-scale dilation angle measured during a test
is here
designated
field-scale dilation angle to be judged and
allowed for in design is i°, as de
ψ°
, whereas the
ned by Patton (1966).
Typically, because of the complex nature of shearing, with damage
being caused to some roughness asperities whist others are overridden,
the dilation angle,
cult to predict for an irregularly rough
sample, although numerous efforts have been made to do so with some
limited success (e.g. Kulatilake et al., 1995; Archambault et al., 1999).
In practice, rather than trying to predict dilation, which will be unique
to each sample, stress level and testing direction, it is a parameter that
needs to be measured carefully during direct shear tests so that correc-
tions can be made to derive a normalised basic friction angle for use in
design.
Figure 5.20
shows the result from the well-instrumented
ψ°
, is dif
rst
stage of a direct shear test on a rough interlocking joint through lime-
stone. The measured strength throughout the test includes the effect of
the upper block having to override the roughness as the joint dilates
and work is done against the con
ning pressure. The dilation curve in
Figure 5.20
super
cially appears fairly consistent, but if one calculates
the dilation angle over short horizontal increments, from the same data
set, it is seen to be much more variable and strongly re
ects the
peaks and troughs of the measured shear strength throughout the
test (compare
Figure 5.21
with
Figure 5.20).
These instantaneous dilation angles can be used to correct (normalise)
the shear strength incrementally throughout the test, using the following
equations:
τ
ψ
¼ðτ cosψ − σ sin ψÞcos ψ
σ
ψ
¼ðσ cos ψ þ τ sin ψÞcos ψ
where
σ
ψ
are the shear and normal stresses corrected for positive
dilation caused by sample roughness. The signs are reversed where
compression takes place. By making such corrections, the basic friction
angle can be determined for the natural joint surface. In practice,
experience shows that for a system measuring to an accuracy of
about
τ
ψ
and
±
0.005mm, analysis over horizontal displacement increments
of about 0.2mm generally gives accurate dilation angles, even for a
rough tensile fracture (Hencher, 1995). By comparison, if one were to
use the average dilation angle throughout the test, as implied in the
ISRM Suggested Method (ISRM, 1974), this would not allow the
variable shear strength to be understood and might lead to serious
errors in determining basic friction values.
Tests can be run multi-stage, in which the same sample is used for
tests at different con
ning stresses, which is very cost-effective, given
