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Volumetric Change
c) Research between the 1960s and 1970s :
Hargraves (1963a) conducted a series of tests on the Bulli seam coal at
Metropolitan Colliery, NSW. Using a simplified shrinkage apparatus as shown
in the Figure, he was able to determine the sorption coefficient of expansion
of the Bulli coal to be approximately 1.75 x 10-3cm/cm at 1
MPa. The test was conducted using 25.4 mm (1 in) cubes subjected
to CO2 gas at various pressures.
Gunther (1968) investigated swelling of different ranks of coal using both methane and carbon dioxide, and calculated the swelling coefficients for different types of coal, ranging from low rank, high volatile coals to anthracite. He observed greater swelling of coal in CO2 than in CH4. The reported swelling coefficients ranged from 1.90 x10-8 to 4.76 x 10-8 MPa-1.
Czaplinski (1971) examined the relationship between the kinetics of sorption and the swelling of coal. Tests were conducted parallel and perpendicular to beddings. He showed that at low pressures, the sorption of carbon dioxide was faster than the development of swelling strain, but at higher pressures (> 4 MPa) the two occur simultaneously.
Czaplinski's (1971) studies, stated that "the delay in coal dilation at the initial low pressure levels, causes the gas to enter the coal macro-pores, causing a minimum of volume change. Increased swelling of coal takes place when the gas, at higher pressures, is forced into the micro-pores."
Ettinger (1977) carried out extensive studies on the swelling of different ranks of coal from deposits in Russia, Ukraine and other former Soviet republics. The Table below depicts data on coals from the Donetsk basin and other regions of the Ukraine. The stresses due to swelling cause the release of free energy in coal which, according to Ettinger; contributes to the onset of outburst.
Coal deposit |
Yield of volatile substances vg, % |
Swelling of coal, % |
Swelling stress P0, MPa |
Relative free energy of stress state , J/kg |
Komsomlets pit, |
30.6 |
0.14 |
21.5 |
0.5* 102 |
Karl Marx pit, |
18.2 |
0.17 |
25.5 |
0.88* 102 |
Kondratevka pit, Derezovka seam, |
11.4 |
0.24 |
42.3 |
1.5* 102 |
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