390With the depth to the panel being 500 m, and the maximal subsidence
being 0.72 m, the strain profile is given in the table and the diagram
below. Again, the strain profile is drawn using the sign convention of
tension positive.Strain -7.1 -8.1 -1.0 -8.1 -4.0 0.0 4.0 8.1 1.0 8.1 6.0 4.0 2.0 0.0
x10-4 x10-3 x10-4 x10-4 xio+o x10-4 x10-4 x10-3 x10-4 x10-4 x10-4 x10-4 xio+O
Distance (m) 0.0 10.0 55.0 95.0 120.0 145.0 155.0 190.0 210.0 245.0 260.0 295.0 335.0 555.0
1.25E-3 ~7.50E-4 ~~
5 00E-4 ~~
2 50E-4 ~~
._ C
O.OOE+O--
600.0
-2 50E4 distance from panel centre line, m
-5.00E-4-c
-1.25E-3-From the results shown above, we can see that - unless there is a
constraint on the width of the subsidence trough - extraction of the
deeper seam is preferable, because both the maximal subsidence and
maximal horizontal strains are significantly less than for the shallow
seam. Other factors, such as the magnitudes of differential subsidence
(i.e. tilt of the ground), will indicate the same conclusion.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 safely 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(CT~)PILWR = 1 o + 45e-'*la
where A is the plan area of the pillar in square metres.
(a) What is the uniaxial compressive strength of the bauxite as
measured 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?