A study of tunnel stability with special reference to the effect of the stress field environment
The thesis is a study of the stability, closure behaviour and rock fracture development associated with mine tunnels with particular reference to Coal Measures conditions. A detailed survey has been carried out of relevant theories and mathematical concepts which relate to tunnel stability and the effects of in situ stresses. Of special importance has been identifying appropriate mathematical theories which relate to the field of scholarship undertaken. The literature survey has found useful application especially in giving guidance on those areas needing further investigation. The author discusses mathematical theories in relation to the research undertaken. The major area of investigation has been the effect of different in situ stress fields on various aspects of tunnel design and geometrical configuration. Attention has focussed on available research methods which allow ease of investigation of the parameters governing mine tunnel stability. After careful consideration, the author selected physical modelling using small scale sand plaster models of different mine tunnel and geological conditions. Much research effort has been directed at establishing the properties of such physical modelling materials, mathematical scaling aspects and the type of test rig for carrying out the investigations. Time was spent on establishing the accuracy and suitability of the research method employed. A range of experiments were carried out whereby the horizontal and vertical components of the in situ stress field were varied. The tests were repeated using the common range of mine tunnel profiles which exist in UK coal mines, namely arch, circular, square and rectangular. The research enabled the fracture pattern to be observed in association with the different tunnel profiles tested under the various stress field conditions employed. Closure of the model tunnels was observed in relation to the increasing stress field. Discussion has focussed on how various combinations of horizontal to vertical components of in situ stress influence mine tunnel stability. The results are discussed in relation to the choice of support type. In particular, the merits of standing support types such as square sets, steel arches and concrete linings are discussed in relation to the results of the research. The thesis draws attention to the practical application of the research method to investigate various mining situations as encountered in UK coalfields and in the Datong coalfield in North China, of which the author has particular experiences regarding rock mechanics and mine tunnel stability.