A numerical modeling was carried out to investigate the stress changes of a layered rock mass when it is heated. The aim of this project was to understand rock fracturing characteristics associated with underground coal gasification at a coal mine. The model contains three coal seams which are sandwiched between a number of sandstone and mudstone layers. The finite element software ANSYS was used to for this modeling. The modeling had two steps: thermal transmission modeling and thermal stress calculation. The results show that the high temperature generated by underground coal burning induced thermal stress which exceeded the strength of the rock mass. The modeling also simulated the propagation of coal burning development in a coal seam and the rock mass responses to the burn front.

1 Introduction

The underground rock mass will be fractured when it is under high temperature environment because the thermal expansion of the rock mass is constrained and can?t take place freely. When the thermal stress exceeds the strength of the rock mass, micro cracks will be produced. Thermal cracking in rock mass played an important role in engineer practices, such as dealing with nuclear waste storage [1], rock drilling [2], exploitation of crude oil [3], geothermal exploitation [4] and underground coal gasification [5] and so on. The temperature of underground coal burning in UCG (underground coal gasification) is above 1000° [6, 7]. This high temperature will produce a high thermal gradient and stress field in the surrounding rock mass. When the thermal stress exceeds the strength of the rock mass, the rocks will fracture. If the fracture was significant, it will affect the gasification process in UCG. In order to keep the gasification process working normally, we need to know the thermal stress distribution of layered rock mass under high temperature environment. Because of the complexity of rock mass and UCG process, the integrity theoretical analysis is very difficult. However, the numerical simulation not only can simulate the complexity mechanics of rock mass, but also can predict engineering problems. Therefore, the numerical modeling of UCG was set up and the finite element software ANSYS was used for this modeling. The software ANSYS can solve high precision nonlinear and coupled field problems, such as thermal-mechanical process. In rock engineering, ANSYS can be used to analyze large structural displacements and problems related to stress-strain and plastic behavior [8]. Therefore, it can be used in the study of thermal stress fields in rock mass.

2 Assumption for modeling thermal stress of layered rock mass

The thickness of coal seams is much smaller than its width and length. In order to reduce the calculation time, this numerical model was regard as a 2D model and plane strain was calculated. The UCG was a coupled phenomenon involving thermal, hydrological, mechanical and chemical processes. However, in this study, the modeling was carried out only to investigate the stress changes of a layered rock mass when it is heated. The aim of this project was for the understanding of rock fracturing characteristics with thermal stress.

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