Geology Reference
In-Depth Information
Figure 10.12 Measuring spacing of discontinuities. A tape is held in position
and the number of discontinuities and the spacing between each one are recorded
over a set traverse length (1.0m). These data comprise the Rock Quality Desig-
nation (RQD), a key component of the RMR (Table 10.2).
known horizontal distance. For convenience we will assume a sample distance of
1.0 m (although in practice it can be any sensible length equal to or greater than
this). Figure 10.12 shows the basic set-up using a horizontal tape measure. To
begin, simply record the total number of fractures that intersect the 1.0 m traverse
and the distance between them. From this, work out the RQD remembering that
an RQD of 100% would mean that the rock mass is equivalent intact rock (for
example, Figure 10.13). Continue working through Steps 1 - 5 in Table 10.2. At
the end you should have five individual ratings that added together equal 100 or
less. For most rock types encountered your RMR range should lie somewhere
between 30 and 80.
10.4.1.2 Step 2: estimating rock mass strength using the
Hoek-Brown criterion
In the 1980s, Hoek and Brown proposed a novel method for estimating rock
mass strength that makes use of the RMR classification and the general empirical
criterion that rock mass strength is defined by the relationship between the
principal stresses at failure. Once you have a value for the RMR then the next
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