Geology Reference
In-Depth Information
Iteration 5 RMS error = 2.8%
0.0
8.0
16.0
24.0
32.0
40.0
48.0
m.
Depth
0.5
1.5
H1
2.5
3.7
5.0
6.4
S
N
7.9
8.0
Inverse model resistivity section OHM.m
16.0
32.0
64.0
120
256
512
1024
Unit electrode spacing = 2.0 m.
Figure 5.1.15. Resistivity plot for Shirebrook Colliery Tip, Nottinghamshire, UK, showing localized zone of
intensely resistive material (anomaly H1), up to 3.7 m below ground level. Blue represents high conductivity, red
shows low conductivity or high resistivity, for the resistivity imaging sections. This was subsequently confirmed by
drilling and represents a localized heating front. Source: IMC Geophysics Ltd.
A proton precession magnetometer was used to measure the magnetic variability of the ground. Higher magnetic
values correlated well with the red shale at the southern end of the site, because of the higher concentrations of iron
oxides from baking. Lower magnetic field strengths were found at the northern end of the tip where burning did not
occur. Repeated ground investigations of these types using of borehole temperature monitoring, airborne thermal
imagery, and geophysical surveys could potentially be useful in monitoring a burning front in a colliery spoil tip, if
these are deployed on a routine basis.
Conclusions
T
he oxidation of coal, when exposed to air during underground or surface mining, can lead to spontaneous
combustion. The combustion depends on several interrelated factors that include the coal rank, the surface area of
the coal exposed to air, moisture content, temperature, pyrite (sulfide) content of the coal, and ignition source. In
addition to coal seams exposed to air in underground workings and opencast sites, colliery spoil heaps are also
subject to spontaneous combustion. Such spoil heaps, especially older ones, contain coal and carbonaceous shale
that may ignite.
One of the factors contributing to spontaneous combustion in abandoned shallow mines is subsidence. For
example, the collapse or partial collapse of pillars may generate fissures that extend to the ground surface, and
thus permit air to enter the mine workings. Similarly, crown holes at the surface allow air to penetrate into the
workings. This happened at an abandoned colliery in the Witbank coalfield, South Africa. There, pillar robbing
prior to closure was responsible for the failure of the pillars, followed by the development of fractures extending to
the surface, as well as the formation of crown holes. Spontaneous combustion has continued from when the time
the mine was closed until today. Unfortunately, various attempts to extinguish the subsurface fires have proved
unsuccessful.
Coal deposits in opencast mines in India and Colombia are susceptible to spontaneous combustion. This
generates hazardous working conditions for the mine workers and causes sterilization of the coal reserves.
Unfortunately, coal fires in pillars meant to retain spoils weakens the pillars, permitting waste material to
collapse into the mine workings, causing loss of production, and anecdotal reports that claim on occasion, the
loss of lives.
Some colliery-waste sites in the England have undergone spontaneous combustion for several decades. Hot spots
recorded at two colliery tips near Barnsley, England, were mitigated by removing the upper top soil and then
compacting with a vibrating roller. Special investigation techniques may enable combustion mechanisms and the
migration of burning fronts to be determined. These investigations at Shirebrook Colliery Tip in Nottinghamshire
consisted of temperature monitoring in boreholes, geophysical surveys, and thermal imagery which enabled the
burning front to be located. Airborne infrared surveys when repeated on a routine basis may potentially allow the
migration of the burning front in colliery tips to be monitored.
