Page 458 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

and improving the load carrying capabilities of tunnel support and reinforcement
systems.

15.3 Longwall coal mining

15.3.1 Basic geomechanics considerations
The elements of the longwall method of coal mining are illustrated in Figure 12.12.
As with longwall mining in hard rock, the primary objective of mining design is to
achieve pseudo-continuous deformation of the main upper strata overlying the seam.
The differences between the layouts and the mining and strata control techniques used
in the two cases arise essentially from the appreciably lower strengths of coal measures
rocks. In the United Kingdom, for example, the uniaxial compressive strengths of the
roof, seam and floor rocks are typically in the range 20–40 MPa. The pre-mining stress
field is difficult to measure in these weak, sedimentary rocks, but back analyses based
on observed excavation performance suggest that, in the UK coal fields, the stress
field is approximately hydrostatic with magnitude p = h, where is the weighted
average unit weight of the superincumbent strata and h is the mining depth.
Longwall mining is a highly productive method which aims to extract almost 100%
of selected blocks of the resource. In most applications of the method, a number of
parallel panels or longwalls are extracted side-by-side as illustrated in Figure 15.7.
The most common longwall method of mining coal is now a retreat method in which
the roadways are developed to the end of the panel which is then extracted on retreat
as illustrated in Figure 15.7. The pillars left between adjacent longwalls serve to pro-
tect the roadways from the excessive displacements that may be associated with the
mining of adjacent panels, to isolate a particular panel where unfavourable geologi-
cal structures or fire, water, or gas hazards exists, or to assist in controlling surface
subsidence. The important issue of the surface subsidence associated with longwall

Figure 15.7 Development and ex-
traction of adjacent longwalls us-
ing multiple roadways illustrating
the chain pillar loading cycle (after
Colwell et al., 1999).

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