Engineering Geo-mechanics System for Coal and Gas Outbursts

Coal and gas outbursts (CGO) are a dynamic geo-disaster happening during mining and tunnel excavation in coal mines. Considered as a mechanical system, they are investigated by a systematic approach to their structural characteristics and mechanism of evolution, for which a localisation theory for CGOs was proposed. There are three size scales for the structure of a CGO system: meso-scale, the engineering scale, and a regional geology scale. In meso-structures of the system, tectonic coal was defined as a two-phase material (gas and coal). It was classified into six types with different geological geneses and outburst properties, as follows: (ductile or brittle) + (gas and/or strength) + (tectonic coal). The type of tectonic coal may indicate its tendency to outburst. On the engineering scale, the mining outburst system could be divided into two different structures: outbursts in a coal seam and outside a coal seam. They are the chief structures of any outburst system. A two-body structure is formed by the hard wall rock and the soft coal seam within the chief structure of the system, which has a strong burst effect. On a macro-regional geology level, the primitive geological process and tectonism would control the properties and distribution of tectonic coal. It can also influence the boundary conditions of the system. The mechanical evolution of the outburst system, that is the mechanical mechanism behind a CGO, is a destabilisation and failure process of a coal seam that is prone to outbursting. The outburst process can be distinguished by two consecutive stages, the initiation of the outburst and the development thereof. The tectonic coal would deform significantly in a localised zone by shearing stress under action from in situ stress and different boundaries: this starts the outburst. This leads immediately to large elastic energy release from the hard part of the two-body structure, which corresponds to the surrounding rock, causing a burst in the softer coal seam. The entire system is then destroyed. Therefore, CGOs are engineering geo-mechanics systems. It is vital to investigate CGOs systematically and distinguish their structural scales and evolutionary stages before giving an explanation of their complexity: it is also the basis for taking effective measures to prevent, or predict, CGOs.