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Abstract As one of the geological hazards，rockburst is often generated in underground engineering under excavation unloading conditions in high geostress areas. According to actual stress state of surrounding rock during excavation，conventional triaxial and unloading confining pressure tests have been conducted for brittle granite under pre- and post-peak strength using different controlling methods and unloading rates. The rock failure process has been studied；the accumulation-release of energy during unloading，characteristics of deformation，failure and accumulation-dissipation-release and rockburst criterion based on energy theory are also discussed. The test results show that granite is in brittle failure both in pre- and post-peak unloading confining tests；and the brittle characteristic in pre-peak test is more stronger than that in post-peak test. The maximum strain energy stored in rock before failure is controlled by confining pressure and unloading rate both in loading and unloading confining pressure processes using different controlling methods. A new discriminant index is defined that the ratio of energy storage to limited energy storage U/U0 for the same stress in the viewpoint of energy and engineering practice. The index can reflect the change of energy，degree of energy accumulation and rockburst in underground engineering under excavation reasonablely；and the grade and position of rockburst can be determined from numerical results.
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Received: 31 December 2008
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