ABSTRACT
There are three types of in-situ stresses on the basis of numerical relation between horizontal stress and vertical stress. The in-situ stresses are as following:
σ σ σ σH H h Vt > >
σ σ σHV H Vσ htype > >
σ σ σ σV V H ht > >
1 INTRODUCTION
In-situ stress is natural stress in the formation of the earth, which is the main force that leads to deformation and destruction. When designing a mine, to gain a better knowledge of in-situ stress for specific project area is vital for a reasonable display of the whole mine (Cai 2002). The in-situ stress varies with the change of depth. It shows big differences for in-situ stress in shallow and deep part. GaiYi and his colleagues founded the concept of critical depth. For the part above the critical depth, the horizontal stress is larger than vertical stress. And the part under the critical depth, the horizontal stress is smaller than vertical stress. However, the critical depth is various in different countries. In South Africa, it is 500 m; in the USA, it is 1000 m; in Greenland, it is 200 m (Chen 1990). And for China, there is no standard statistics to give the scale of critical depth. However, in some mines such as Huafeng coal mine and Panxi coal mine, the survey results have shown the situation that the horizontal stress is larger than vertical stress (Wei et al. 2013, Kang et al. 2007).
