Questions 12,1-12,10:
rock mass classification
412.1 A mudstone rock mass at a depth of 200 m contains three fracture
sets. One set comprises bedding planes; these are highly weathered,
slightly rough surfaces, and are continuous with an orientation of 180/10.
Another set is jointing; these joints are slightly weathered, slightly rough,
and have an orientation of 185/75. The third set is also jointing; again, the
joints are slightly weathered and slightly rough, and have an orientation
of 090/80. The strength of the intact rock has been assessed as 55 MPa,
and values for the RQD and mean fracture spacing are reported as 60%
and 0.4 m, respectively.
Use the RMR system to classify this rock mass, and assess the stability
of a 10 m wide excavation being driven from east to west.
412.2 A 7-m-diameter tunnel is to be driven through a sequence of
shale and basalt rock at a maximum depth of 61 m. The shales dip
towards the east, and the basalts form sub-vertical dykes. The bedding
dips between 15" and 20°, the joints dip between 70" and 90". The joints
in the shale are rough, and most of them are thin and healed with calcite,
but overall the rock is described as 'blocky and seamy'. The groundwater
level is about^50 m above the invert of the tunnel. The average uniaxial
compressive strength of the shale is 53 MPa, of the basalt it is 71 MPa.
The vertical stress is about 1.0 MPa, and the horizontal stress is about 3.4
MPa. The snaking nature of the tunnel's route means that at some place
along its length it will head in all directions between 090" and 180".
Use the RMR system to predict how the rock will behave in the
excavation.
412.3 Use the Q system to assess the stability of the rock mass as
described in Q12.1.
412.4 Use the Q system to assess the stability of the rock mass de-
scribed in Q12.2.
412.5 Using your assessments of RMR and Q for questions Q12.1,
Q12.2, Q12.3 and Q12.4, investigate the relation between the Q and RMR