In situ rock stresses are directly associated with the stability of underground openings and thus the state of in situ rock stress is a crucial factor for the design of underground excavations. The consequences of the rock stresses are rock deformation and rock failure. Concerns have to be taken into account when the in situ stresses are in such magnitudes so that rock failure would occur after excavations. The modes of rock failure observed in fields will be first presented as the base for subsequent discussions in the article. Some rock mechanics principles aiming to improve the stability of underground openings are also presented. For instance, a wide opening can be excavated in a rock mass where the horizontal stresses are high enough. In a case of extremely high horizontal stresses, the stability of the roof could be improved by increasing the width of the opening. The basic design principles with regard to in-situ stresses will be illustrated through a few examples in the article. Finally, measures dealing with instability issues caused by in situ rock stresses are presented through a few case studies. Among others, the case studies include positioning of a mine shaft in a rockburst-prone condition, excavation of a shaft that was subjected to uneven lateral rock deformations, assessment of the causes for the rock failure in a deeply located underground workshop, and field observations of mine pillars in an abandoned open stope.
Underground excavation is affected by three factors from viewpoints of design and stability. They are the quality of the rock mass, the state of in situ stresses and the size and shape of the excavation, Figure 1. The quality of the rock mass essentially describes the "strength" of the excavation material - the rock mass. One has nothing to do with the material, as soon as the excavation location is determined. The state of in-situ stresses in the rock mass cannot be changed by human either. What one can do with these two factors is to carry out assessment and measurement in order to form a solid base for engineering design. Excavation method is the only factor, among the three, which one can interfere. The stability of an underground opening is not only determined by the quality of the rock mass, but also the state of in-situ stresses. The state of in-situ stresses refers to the magnitude and the orientation of the principal stresses in the rock mass prior to excavation. The stability of the underground opening can be improved by employing some principles related to in-situ stresses. For instance, a wide opening can be excavated in a rock mass where the horizontal stresses are large enough. In a case of extremely high horizontal stresses, the stability of the roof could be improved through increasing the width of the opening. The basic design principles with regard to in-situ stresses will be illustrated through examples in this paper.
