Rock Mechanics.pdf - Mining and Blasting

Figure 15.12 Rock fracture and displacement
pattern near the face of a
longwall coal mining panel: (a) section
transverse to the panel axis; (b)
longitudinal section (after Kelly et al.,
2002, adapted from Peng and Chiang,
1984).
LONGWALL COAL MINING
when the face has advanced such that the recently fractured zone lies above the
mined-out area.
Valuable new understandings of longwall caving and of longwall geomechanics
generally, have been developed in Australia in recent years through a major industrially
sponsored research program (Kelly et al., 1996, 1998, 2002). Studies were carried
out at a number of mines in differing mining environments using a multi-path approach
involving a range of tools including geological and geomechanical characterisation
techniques, microseismic monitoring, 2D and 3D numerical stress-displacement
modelling, a range of underground monitoring methods and 3D visualisation techniques.
Some of the findings from the case studies summarised by Kelly et al. (2002)
were:
Micro-seismic event locations showed that rock fracture or shear occurred generally
30–50 m, but as much as 80 m, ahead of the face in a cyclic, arcuate pattern in
plan view as illustrated in Figure 15.13 for the Gordonstone (now Kestrel) Mine.
Micro-seismic event locations showed that rock fracture or shear may occur up to
100 m above or below the face as illustrated in Figure 15.14 for the Gordonstone
Mine. These events may be influenced by the presence of previously mined seams.
Water content and pore pressure increases are likely to exert an influence on rock
failure mechanisms. At Gordonstone, pore pressure increases were detected up to
250 m ahead of and 55 m above the face and piezometer cables were sheared up
to 80 m ahead of and 100 m above the face as illustrated in Figure 15.14.
Micro-seismic events demonstrated the activation of slip on major structures up
to 300 m from the face and generally on the “solid” side of the longwall panel.
Longwall caving mechanics is complex being influenced by the local geology
and previous mining. Thick sandstone beds may exert a strong influence on the
initiation and propagation of caving.
Massive strata, usually of sandstone, immediately above the coal seam being extracted
can have a number of important influences. They can inhibit the regular, cyclic
445