Geology Reference
In-Depth Information
without resetting the sample in some cases. For some tests, complete runs of about 15mm shear displace-
ment were conducted and in one test the sample was tested at the highest stress level
first, which was then
reduced in stages incrementally. Samples were photographed, roughness measured and damage described
carefully. For reference, a series of tests were conducted on saw-cut samples, ground with grade 60
carborundum powder.
Results from the tests are presented in
Figure B5-2.2.
Tests on natural joint surfaces were corrected for
dilation incrementally. It can be seen that the saw-cut surfaces gave a friction angle of about 28 degrees,
which is about what might be expected.
The tests from natural joints fall into distinct groups. The data from joints coated with iron
oxides de
finds for many weathered
rocks (Hencher et al., 2011). The data for the chlorite-coated joints were much lower, however, and
unexpectedly so. At low stress levels especially, values were very low, below that of the saw-cut
joints, as can be seen from the inset
ne a friction angle of 38 degrees, which is the same as one
figure and about the same as the angle of dip of the planes
along which the failure took place (
20 degrees at the lowest stress levels). Field-scale roughness
was measured at 5 degrees using a 420 mm diameter plate and 9 degrees using a base plate of 80
mm. It was concluded that the failure was progressive, probably having been exacerbated by
blasting and previous rainfall and that the initial movements overcame the field-scale roughness.
The eventual failure was explained by the presence of persistent chlorite-coated joints with inher-
ently low frictional resistance (Brand et al., 1983).
ϕ
≅
Roughness at the
field scale will often be the controlling factor for the
stability of rough or wavy persistent joints and for engineering design
must be added to the basic friction,
b
, of the effectively planar rock
joint, as determined from corrected shear tests. Roughness is expressed
as an anticipated dilation angle, i°, which accounts for the likely
geometrical path for the sliding slab during failure (deviation from
mean dip). There are two main tasks for the geotechnical engineer in
analysing the roughness component:
ϕ
firstly, to determine the actual
geometry of the surface along the direction of likely sliding at all scales
(
Figure 5.24)
and secondly to judge which of those roughness
features along the failure path will survive during shear and force the
joint or joints to deviate from the mean dip angle. This is the most
dif
cult part of the shear strength assessment, not least because it is
impossible to establish the detailed roughness of surfaces that are
hidden in the rock mass. Considerable judgement is required and has
to be balanced against the risk involved. Hack (1998) gives a good
review of the options, and the dif
culties in exercising engineering
judgement are discussed in an insightful way by Baecher & Christian
(2003).
In practice, the best way of characterising roughness is by measure-
ment on a grid pattern in the way originally described by Fecker &
Rengers (1971), adopted in the ISRM Suggested Methods (1978)
and described in Richards & Cowland (1982), although spatial
variability may be an important issue;
rst-order
roughness represented by major wave features may vary considerably
the important
