387These values can be seen to be identical to those obtained by iteration.
Clearly, the second of the two designs is preferable on the basis of
pillar stability. Adoption of this geometry also reduces the roofspan and
hence will mitigate instability in the excavation roof.420.6 A new mining operation is planned which can extract from
one of two coal seams. The first seam is at a depth of 250 m and has
a thickness of 3 m, and the second seam is at a depth of 500 m with
a thickness of 1 m. A preliminary design for the longwall mining
panels calls for the upper seam to be mined with a panel width of
200 m, and the lower seam to have a panel width of 400 m (see the
sketch below for typical geometry, in which the mining direction is
into the page).Centreline Panel edgeIt has been suggested that the Subsidence Engineers’ Handbook
(SEH) (National Coal Board, Mining Department, 1975)* can be used
to provide estimates of the surface subsidence profile and horizontal
strain distribution 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 ex-
traction thickness of about 0.75 and 0.72 for the upper and lower
seams, respectively. 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 dedh 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.National Coal Board, Mining Department (1975) Subsidence Engineer’s Handbook. Na-
tional Coal Board, Mining Department. London, 2nd ed. (revised).