Geology Reference
In-Depth Information
Intact rock
Single discontinuity
Two discontinuities
Several
discontinuities
Jointed rock mass
Figure 10.11 The effects of scaling of geotechnical properties on rock mass
strength using a volcanic cone as an example (cf. Figure 10.10). In hand speci-
men the absolute strength of intact rock is dominant. But with increasing scale
an ever greater volume of rock is available to move as discrete blocks within
bounding discontinuities. In this way, the overall strength of the jointed rock
mass is always less than that of the intact rock comprising it.
It is important to realise that descriptive, field-based rock mass classification
schemes such as the RMR can be used to make predictions about the mechan-
ical properties of the rock mass if used in conjunction with relevant empirical
expressions. A particularly powerful combination involves combining the RMR
with a method known as the Hoek-Brown Criterion devised to assess rock mass
strength. To do this we need to follow a series of steps that progressively cal-
culate several key variables through the cumulative processes outlined below.
In this way it can be shown using RMR data that the rock mass strength of,
for example, basalt is up to 80% weaker than its intact compressive strength as
measured under laboratory conditions.
10.4.1.1 Step 1: data collection
The first step of the process involves data collection. Key data are the number
and density of discontinuities, and their spacing, as measured along a linear
traverse of the rock face. In addition we need to know something about the
physical dimensions and properties of cracks and fractures, for example widths,
whether they are closed or open, rough surfaces or weathered, and their moisture
content as a measure of groundwater activity. These parameters are summarised
in Table 10.2 along with a descriptive rating for each.
The first step is to work out the RQD. To do this you will need to measure the
number of (i) intersections and (ii) spacings between all discontinuities along a
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