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Abstract Guandi underground powerhouse is typical deep cavern groups in hard rock region. The challenge is that the geostress in the powerhouse area is high and discontinuities are well developed at the same time. The major principal stress is 25–35 MPa，no regional fault or major weakness zone have been found，but shear zones and joints are well developed. Failure phenomenon of the surrounding rock mass during the excavation of the underground cavern groups has been reviewed thoroughly. Rock structures and failure modes of surrounding rock mass have been studied in full length. The influence of geostress and rock structure on the stability of surrounding rock mass have been investigated in detail. The results show that the rock masses are fresh and belong mainly to class II and have a relative high strength，and so a high strength-stress ratio. The surrounding rock masses has strong ability to resist stress-induced failure. The influence of rock structure on rock masses is relatively stronger than that of geostress. Local instabilities and large deformation of some locations are all controlled by adverse geological discontinuities. In general，the characteristics of the rock structures of machine hall，transformer chamber and surge chamber are similar. This results in the similarity of the deformation characteristics and failure modes of the surrounding rock masses. The three cavers have certain differences in rock structures. Rock structures have different influences on the surrounding rock masses in different caverns，and also the failure modes. Jointed hard rock may assume time-dependent deformation under high geostress and this is another adverse influence of rock structures on rock masses. So，in hard rock regions with high geostress where the strength-stress ratio is high and joints are well developed，the influence of rock structure on surrounding rock masses must be fully considered.
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Received: 25 September 2011
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2012, Vol. 31 
