Civil Engineering Reference
In-Depth Information
4.6 Rock durability and compressive
strength
The strength parameters that are usually of most
significance to the analysis of slope stability are
the cohesion and friction angle on the sliding
surface, as discussed previously in this chapter.
However, both the durability and compress-
ive strength of the rock may be of importance
depending on the geological and stress condi-
tions at the site, as discussed later. The dur-
ability and compressive strength test procedures
are index tests best used to classify and compare
one rock with another; if necessary, the index
measurements can be calibrated by more precise
laboratory tests.
deformation measurements, from extensometers
placed before the commencement of excavation,
in the Mingtan power cavern in Taiwan. It was
found that a zone of blast damage extended for a
distance of approximately 2 m around all large
excavations. The back-calculated strength and
deformation properties of the damaged rock mass
give an equivalent disturbance factor
D
0.7.
From these references it is clear that a large
number of factors can influence the degree of
disturbance in the rock mass surrounding an
excavation, and that it may never be possible to
quantify these factors precisely. However, based
on experience and on the analysis of the details
contained in these papers, Hoek
et al
. (2002) have
drawn up a set of guidelines for estimating the
factor
D
(Table 4.6).
The influence of this disturbance factor can be
large. This is illustrated by a typical example in
which
σ
ci
=
4.6.1 Slake durability
Widely occurring rock materials are prone to
degradation when exposed to weathering pro-
cesses such as wetting and drying, and freezing
and thawing cycles. Rock types that are partic-
ularly susceptible to degradation are shale and
mudstone, which usually have a high clay content.
The degradation can take the form of swelling,
and the time over which weakening and disin-
tegration can occur after exposure may range
from minutes to years. The effect of degrada-
tion on slope stability can range from surficial
sloughing and gradual retreat of the face to slope
failures resulting from the loss of strength with
time (Wu
et al
., 1981). In sedimentary form-
ations comprising alternating beds of resistant
sandstone and relatively degradable shale, the
weathering process can develop overhangs in the
sandstone and produce a rock fall hazard due to
sudden failure of the sandstone (see Figure 1.4(e)).
A simple index test of the tendency of rock
to weather and degrade is the slake durability
test (ISRM, 1981a) (Figure 4.25). It is import-
ant that undisturbed samples are used that have
not been excessively broken in the sampling pro-
cedure, or allowed to freeze. The test procedure
comprises placing the sample in the wire mesh
drum, drying it in an oven at 105
◦
for 2-6 hours,
and then weighing the dry sample. The drum is
then partially submerged in water and rotated
45.
For an undisturbed
in situ
rock mass surrounding
a tunnel at a depth of 100 m, with a disturb-
ance factor
D
=
50 MPa,
m
i
=
10 and GSI
=
=
0, the equivalent friction angle
is
φ
47.16
◦
=
while the cohesive strength is
c
=
0.58 MPa. A rock mass with the same basic
parameters but in highly disturbed slope of 100 m
height, with a disturbance factor of
D
=
1, has
an equivalent friction angle of
φ
=
27.61
◦
and a
cohesive strength of
c
=
0.35 MPa.
Note that these are guidelines only, and the
reader would be well advised to apply the values
given with caution, and to compare the calculated
results with those obtained by back analysis as
shown in Figure 4.21. It is considered that the
Hoek-Brown calculations can be used to provide
a realistic starting point for any design and, if the
observed or measured performance of the excav-
ation turns out to be better than predicted, the
disturbance factors can be adjusted downwards.
These methods have all been implemented in
a Windows program called “RocLab” that can
be downloaded (free) from www.rocscience.com.
This program includes tables and charts for estim-
ating the uniaxial compressive strength of the
intact rock elements
(σ
ci
)
, the material constant
m
i
, the Geological Strength Index
(
GSI
)
and
Disturbance Factor (D).
