Degree Name

Doctor of Philosophy


Department of Civil and Mining Engineering


One of the most hazardous forms of dynamic phenomena associated with underground coal mining has been the occurrence of coal and gas outbursts. Outbursts have been a problem to mining for over a century and have become increasingly more common in recent years. This is because of the increase in both the depth of workings and rate of mining excavations. High seam gas content and ground stress are two major factors which contribute to outburst occurrence. Also, outbursts are influenced by various coal properties such as coal rank, porosity, permeability, and gas type. However, the exact triggering mechanism of outburst is not yet fully understood.

Recently, there has been a growing emphasis on the effect of coal strength on outbursts, particularly in relation to the type of the gas contained in coal Research has shown that gases such as carbon dioxide sorbed onto coal surfaces could have some influence on the strength of coal. This thesis is therefore concerned with the laboratory investigations of the effect of gas pressures on the strength of coal. The previous work on this subject has been reviewed in conjunction with the various parameters mentioned above. Emphasis has been placed on the development of suitable laboratory techniques to examine the effect of gas pressures on various coal properties. The apparatus designed and constructed includes

1) a gas pressure chamber fitted with loading jaws for testing the effect of gas pressure on the tensile strength of coal by the Brazilian method. The equipment was further modified to allow a study of the effects of gas pressure and pressure gradient on the bearing capacity of coal at pressures of up to 3,000 KPa.

2) a drill rig capable of drilling coal under various confining gas pressures. Two types of coals were used in the study program, these being coking coal from both the Bulli seam, West Cliff Colliery and Great Northern coal obtained from the Myuna Colliery. Tests were made in air at normal conditions and in CH4 and C02 gases at various gas pressures up to 3,000 KPa.

The following conclusions were drawn from the test results:

a) Gas pressure had no significant effect on the tensile strength and bearing capacity of coal.

b) Gas pressure gradient has a reducing influence on the bearing capacity of coal. A reduction of 25% in the bearing capacity values was obtained at 700Kpalmm gas pressure gradient. This was in reasonable agreement with the theoretically predicted values of about 18%.

c) Drilling under gas pressure confinement influenced the drilling speed and the particle size distribution. An increase of 7.4% in drilling speed was obtained when drilling in CH4 gas at 1,500KPa.



Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.