484 Questions 20.7 -20.7 0: design of underground excavations
estimates of the surface subsidence profile and horizontal strain distri-
bution transverse to the direction of mining.
Assess the suitability of the extraction geometries for each seam
(based on estimates of surface subsidence and horizontal strain), using
the following data taken from the SEH.
The SEH gives values of the ratio of maximal subsidence to extraction
thickness of about 0.75 and 0.72 for the upper and lower seams, respect-
ively. The proportion of subsidence that occurs at a given position along
the transverse profile for both seams is given by the table below, taken
from the SEH.
Subsidence
proportion 0.95 0.90 0.80 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Ratio of
distance from
panel centre
to depth 0.08 0.1 0.17 0.2 0.25 0.29 0.32 0.38 0.42 0.62 1.11
Similarly, the strain proportion that occurs at a given position along
the transverse profile for both seams is given by the table below, also
taken from the SEH.Strain
proportion
Ratio of
distance from
panel centre
to depth 0.00 0.02 0.11 0.19 0.24 0.29 0.31 0.38 0.42 0.49 0.52 0.59 0.67 1.11-0.70 -0.80 -1.00 -0.80 -0.40 0.00 0.40 0.80 1.00 0.80 0.60 0.40 0.20 0.00In addition, the multipliers for the maximum contractile and extensile
strains for these panel width to panel depth ratios are both equal to
about 0.7.420.7 A bauxite room and pillar mine is to be opened at a depth of 100
m in a rock mass whose unit weight is approximately 27 kN/m3. The
design of the mine calls for a factor of safety of 2 against compressive
failure of the pillars, and the substantial body of data regarding the
design of pillars in this bauxite shows that their compressive strength
may be calculated from the expression
= 10 + 45e-0.'8A
where A is the plan area of the pillar in square metres.
(a) What is the uniaxial compressive strength of the bauxite as meas-
ured in the laboratory, and what is the super-REV compressive strength
of the pillars?
(b) For operational reasons, the rooms must have a minimum width of
5.0 m. What is the greatest extraction ratio that can be achieved with this
size of opening?
(c) If the roof of a room is reinforced with rock bolts, then the
maximum achievable opening width is about 8 m. Is it cost effective to
do this?