Rock Mechanics.pdf - Mining and Blasting

LONGWALL AND CAVING MINING METHODS
15.3.6 Rib and chain pillar design
The design of rib and/or chain pillars has been traditionally based on precedent
practice, often represented by “rules of thumb”. Modern design methods may use
empirical, analytical or computational methods combined with performance monitoring
(e.g. Badr et al., 2002, Cassie et al., 1999, Colwell et al., 1999). As indicated
previously and illustrated in Figure 15.7, chain pillars are subjected to complex stress
paths and undergo a number of loading stages during their lives. The five main stages
of the chain pillar loading cycle in the case illustrated in Figure 15.7 are:
development loading, usually calculated using tributary area concepts;
front abutment loading when the first longwall face is adjacent to the pillar;
main gate or side abutment loading when the load has stabilised after the passage
of the first face;
tailgate loading when the second longwall face is adjacent to the pillar. It is in this
stage that the pillar is likely to experience its maximum vertical loading; and
double goaf loading when the pillar is isolated between two mined-out panels.
Because of this loading history, the design of chain pillars is not as straight-forward
as that of the pillars formed in room-and-pillar mining as discussed in Chapter 13.
There may also be design objectives in addition to the criterion of pillar stability.
For example, in developing their design methodology, Colwell et al. (1999) sought
to optimise chain pillar size so as to:
maintain the serviceability of main and tail gates so that both safety and longwall
productivity are unaffected;
minimise roadway drivage requirements in a way that does not impact adversely
on the continuity of extraction of successive longwall panels; and
maximise coal recovery.
Rib and chain pillars may be from 20 m to more than 100 m wide depending on the
mining depth, the in situ stresses, the mining geometry, the mass strength of the coal,
the geological structure and the geotechnical properties of the under- and over-lying
strata. Data collected by Cassie et al. (1999) for UK longwall coal mines showed
that the rib pillars (without cross-cuts) typically used had width to height ratios of
up to 40 and could be classified as wide pillars. In a study of 19 Australian longwall
mining sites at depths of 130 to 475 m, Colwell et al. (1999) recorded chain pillar
widths of 26–55 m and lengths of 40–125 m. The roadways were 4.8–5.2 m wide,
2.5–3.6 m high and the pillar width to height ratios ranged from 10 to 20. Pillars of
these geometries would not be expected to “fail” in the terms discussed in Chapter
13, although spalling at the ribs might be anticipated. Other considerations such as
gate road serviceability then become of major concern.
Although the pillars may not be expected to “fail” in the Australian cases outlined
above, pillar failure or yield may be expected to occur if smaller pillars are used
under higher stress conditions. For example, Badr et al. (2002) refer to a case in
which chain pillars at a depth of 700 m were 8 m wide, 26 m long and 3 m high. Small
pillars may be considered under USA regulations which require a minimum of three
entries in longwall gate roads. Simple calculations based on tributary area loading
for the development stage, and the pillar strength formulae presented in section 13.3,
give extremely low factors of safety for which the probability of survival is negligibly
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