Geology Reference
In-Depth Information
Therefore, many attempts have been made to represent the overall
strength of the rock mass using simple Mohr-Coulomb parameters, fric-
tion and cohesion, based on overall rock quality, using classi
cations such
as those presented in Appendix C. For example, using the Rock Mass
Rating (RMR) of Bienawski (1989),
'
poor rock
'
would be assigned cohe-
sion 1
-
200 kPa with friction angle 15
-
25 degrees, and
'
good rock
'
would
be assigned cohesion 3
-
400 kPa with friction angle 35
-
45 degrees.
flexible and geologically realistic approach is to use
the Hoek-Brown criteria (Hoek &Brown, 1997; Brown, 2008), which
is linked to a Geological Strength Index (GSI) for rating overall rock
mass conditions such as
A rather more
and the roughness or otherwise of
discontinuities. The GSI chart is presented and discussed in Appendix
C. Given a GSI estimate, the uniaxial compressive strength for the rock
blocks and a constant, mi,
i
, which differs for different rock types and
has been derived empirically from review of numerous test data (Hoek
&Brown, 1980), one can calculate a full strength envelope for the rock
mass. A program, RocLab, is downloadable from
https://www.
rocscience.com
and allows values for cohesion and friction to be
calculated but it needs to be checked that these relate to the appropriate
stress level for the problem at hand. For example,
Figure 5.16
shows a
steep cut slope in weathered tuff. The question is whether it needs to be
cut back or otherwise reinforced or supported. The rock mass is severely
'
blockiness
'
