Questions 20.7 -20. IO: design of underground excavations 485420.8 In an attempt to improve the profitability of the mine in Q20.7,
the possibility of reducing both the opening width and the pillar size
is to be investigated. Plot the curve of extraction ratio against opening
width, for openings in the range 0.5 m to 4.0 m, and hence determine the
optimal opening width and the corresponding extraction ratio.
If the extraction ratio thus identified is to be kept, what value of the
factor of safety is required if the opening width is to be changed to 3.5 m?
420.9 A proposal has been made to use an old underground limestone
quarry as a storage facility. A site visit to the quarry has revealed that it
was mined using the room and pillar method, with a regular rectangular
array of pillars. The clear spacing between the pillars is 6 m, the pillars
are each 7 m square, and the excavation is at a depth of 80 m.
Examination of the pillars shows that the limestone is horizontally
bedded with moderate spacing and gentle undulations. The bedding
planes themselves are smooth to touch with slightly weathered surfaces
and no visible aperture. Conditions inside the quarry are generally dry.
A point load test of the pillar rock conducted at the quarry estimated
its uniaxial compressive strength to be 100 MPa; whereas a laboratory
triaxial test found that the rock failed when the axial stress in the sample
was 110 MPa and the confining pressure was 4 MPa. The unit weight of
the limestone is 28 kN/m3.
Estimate the Rock Mass Rating (RMR) for the pillars (see the RMR
table in Appendix C) and hence determine the Hoek-Brown strength
parameters m and s for the rock mass, by using the equations
(""F loo>.
RMR-100
rn = mi exp (
28) and s = exp
Use these values together with the Hoek-Brown criterion,
- u1 - -?+Jrn:+s,
uc oc
to determine the maximal vertical stress the pillars can sustain (1) at
their faces and (2) at their centres. Assume that the ratio of horizontal to
average vertical stress is 0.075 at the centre of each pillar.
Use the tributary area theory to estimate the average vertical stress in
the pillar, and hence determine the factor of safety of the pillars.
- IO A gold-bearing quartz vein, 2 m thick and dipping at 90°, is
to be exploited by a small cut-and-fill stoping operation. The mining
is to take place at a depth of 800 m, and the average unit weight of
the granite country rock above this level is 29 kN/m3. The strike of
the vein is parallel to the intermediate principal stress, and the major
principal stress is horizontal with a magrutude of 37.0 MPa. The uniaxial
compressive strength of the vein material is 218 MPa, and the tensile
strength of the country rock is 24 MPa. Poisson's ratio and Young's
modulus for the quartz are 0.2 and 48 GPa, respectively. During mining,
each blast will extend a stope up-dip by about 2 m.
Assuming that no stress-induced failure is permissible, what is the
maximum height of a stope?