Numerical modelling of ground temperature evolution and soil subsidence as a result of underground coal fire

Coal occurs in underground seams of variable thickness and depth and in numerous cases they are known to be on fire. These fires can result from human activity or occur naturally. Underground coal fires are problematic for many reasons, including mine safety, damage to infrastructure due to combustion-induced subsidence (e.g. Centralia, Pennsylvania, USA) and damage to the natural environment. Understanding and predicting the temperature evolution in the ground is a key aspect when trying to extinguish underground coal fire. A site in New South Wales, Australia, where an underground coal fire has been active for the past 60 years (at a depth of around 30 m) has been the subject of an experimental and numerical study. In this paper, by taking Burning Mountain as an example, the general formation and development of underground coal fires and their associated physical-chemical coupled processes have been analysed and described. Then, a reactive model for coal spontaneous combustion has been implemented in a non-linear finite element code capable of simulating thermalhydraulic- mechanical behaviours of geomaterials. By incorporating the reactive model with heat transport, a thermal-chemical (TC) simulation has been conducted on an artificial simple set-up. The preliminary results show that the TC model is capable of reproducing the propagation of coal fire front accompanying with reasonable temperature evolution. The next step of model development is to couple the TC model with gas mass transport in the fractured overlying rocks. Furthermore, mechanical deformation will be taken into account for predicting the subsidence experienced by the overburden soil after passage of the burning front and the resulting collapse .