Page 448 - Rock Mechanics For Underground Mining

15 Longwall and caving mining

methods

15.1 Classification of longwall and caving mining methods

The essential geomechanical features of longwall and caving methods of mining were
outlined in Chapter 12. Longwall and caving methods are distinguished from other
mining methods by the fact that the near-field rock undergoes large displacements
so that mined voids become self-filling. In caving methods, the far-field rock may
also undergo large displacements. In increasing order of magnitude of country rock
displacement, and in decreasing order of strain energy stored in the near-field rock, the
basic mining methods to be considered in the present category are longwall mining
in hard rock, longwall coal mining, sublevel caving and block caving.
In this chapter, the geomechanics issues involved in each of these methods of
mining will be discussed in turn. If a relevant issue has been dealt with elsewhere, the
discussion will not be repeated, but a cross reference will be given. One of the major
geomechanics concerns in longwall and caving methods, mining-induced surface
subsidence, will not be considered here, but will be the subject of Chapter 16.

15.2 Longwall mining in hard rock

15.2.1 Basic geomechanics considerations
As was noted in section 12.4.6, longwall methods are used to mine narrow, flat-
dipping, metalliferous orebodies of large areal extent. The near-field rock is usually
strong, and mining often takes place at considerable depth where in situ stresses are
high. The deep-level mines of South Africa provide the classic example of these
conditions. The key elements of longwall mining in hard rock are illustrated in
Figure 12.11. In the form of the method illustrated in Figure 12.11, it is likely that
the down-dip spans of the longwalls would be limited by the use of regularly spaced
strike pillars. Vieira et al. (2001) describe one such example in which the strike pillars
are 40 m wide with a centre-to-centre spacing of 280 m. There are also a number of
variants of the method in which dip pillars are used for regional support (see section
15.2.2) and to bracket faults and dykes.
The basic geomechanical objective of the mining and support systems used in
this case is to preserve the pseudo-continuous behaviour of the near-field rock. This
pseudo-continuous behaviour may be disturbed by two influences. First, natural or
mining-induced discontinuities in the rock mass as illustrated in Figure 15.1 may
isolate rock blocks that become free to fall from the hangingwall. This condition
is exacerbated by the tendency for tensile boundary stresses to be induced in the
near-field rock above the mined-out void. Structurally controlled instabilities and
instabilities associated with mining induced fracturing in stope backs have long been
experienced in the deep-level gold mines and the shallower platinum mines of South
Africa (e.g. Heunis, 1980, Roberts and Urcan, 2002, Ryder and Jager, 2002, Stone,
430