Rock Mechanics

Coal fires lower the stability of a coal seam and the adjacent rock. This leads to a volume deficit caused by the burnt coal and by changing the rock characteristics in the vicinity and above the coal fire.

Severe hazards induced by coal fires are ground subsidence, high temperature and the emission of poisonous and green house gases. These hazards can be limited and partly avoided by setting up rock mechanical models and taking actions according to the results. Such models can predict high temperature zones, cracks and fissures, for former coal bearing areas now being empty.

The professorship of mining engineering of TU Bergakademie Freiberg is setting up general and site specific rock mechanical simulation models for the coal fire in Wuda. Using an elasto-plastic behaviour of the material involved and a FDM (Finite Difference Modelling) approach different aspects are investigated carefully.

Fig.: A rock mechanical simulation model: In this example the influence of a coal fire in a lower seam to the upper seam and the surrounding rock is simulated.

Important results of the rock mechanical models are:

• subsided areas due to coal fires,
• high temperature due to coal fires and the exhaust gases,
• fractures and fissures in the coal seam and the overlaying strata acting as flow channels for air and exhaust gases can be predicted.

Techniques applied to set up the models are:

• parameter variations based on a model of a real coal fire in Wuda, Inner Mongolia, China,
• geometry variations – e.g. evaluation of the influence of coal fire depths and the coal seam thicknesses,
• check of fire fighting technologies by investigating their effects with the help of rock mechanical models – e.g. one or two layers of sealing clay on the top of an active fire area,
• temperature variation models to determine heat transfer.