Geology Reference
In-Depth Information
Figure 5.1.5. Steam emitted from a crown hole above workings in the No. 2 coal seam, caused by a burning-coal
seam, Witbank coalfield, South Africa. Reproduced with permission of Taylor & Francis. From Bell and Donnelly
2006; photo by Bell 1976.
Slope Failure in the Singrauli Coalfield, India
Coal fires are common throughout India in opencast coal mines, underground mines, and on spoil heaps (Ghosh,
1989; Gupta and Prakash, 1998; Mansor et al., 1994; Prakash et al., 1997; Prakash and Gupta, 1998; Saxena et al.,
1991). Coal fires in India have had a dramatic effect on the landscape and have negatively influenced the lives of
people who work and live near the burning-coal seams.
In the Singrauli coalfield in India, coal seams were extracted during spontaneous combustion (Bell and Donnelly,
2002, 2006). Gas and dust was observed to sink to the lower part of the open pit, reducing visibility with blue
-
grey
smoke, creating unpleasant and potentially dangerous conditions for the mine workers (Figure 5.1.6).
The spontaneous combustion of coal in opencast mines transforms it into an unconsolidated residual ash. This
frequently causes local instability and slope failure, when the residual ash accumulates in moderate thickness.
However, since the zone of burning is usually restricted to the outer layers of the coal, penetrating to about 1m
behind the face, small rock falls and topples tend to be a more frequent occurrence than large slope failures.
Many of the large opencast mines in India are worked by draglines. Individual rock faces may reach at least 2 km in
length. The reach of the draglines, around 45m, and the extraction of the coal, along the face known as a high wall,
creates a working face up to 100m high in some mines (Bell and Donnelly, 2006). Where the coal seams dip into
the high wall, instability problems are less likely. The dragline operations leave behind remnant coal pillars which
vary from 2 to 10m at their base and up to approximately 18
-
20 m high. These pillars support the spoil tip
emplaced by the draglines.
The coal pillars are broken by a network of discontinuities. These include cleats (joints in the coal) bedding planes
and faults caused by mining operations. Smoldering begins along the discontinuity surfaces about 6 hours to
around 15 days following their exposure by large-scale removal of the overburben. Once burning begins, it was
described anecdotally by the mine workers to advance at about 1 m/day. Failure of the burnt residual ash may cause
collapse of the mine spoils onto the floor of the mine. This may be exacerbated by groundwater flow from the
monsoon rains that collects within the spoil tip (Figure 5.1.7). Examples of failures for a one year period in one of
the opencast mines are given in Table 5.1.1.
Evening inspections reveal that the coal fires tend to concentrate on and migrate along discontinuities in the coal.
This generates small but frequent block failures and topples. The toppled debris accumulates at the toe of mining
benches as a burning debris cone, sometimes accompanied by explosions, probably associated with the release of
gas as it expanded and escaped from the burning coal (Figure 5.1.8).
