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Abstract In order to explore the mechanism of rockburst induced by unloading，the process of instability failure under unloading is exhibited by rock failure process analysis software(RFPA2D). Taking the example of basic mechanical parameters of the granite，firstly，the two-dimensional numerical test of uniaxial loading failure for the square numerical specimen is simulated；and then，for the same specimen，the surrounding rock unloading test under the condition of bidirectional load is simulated. The numerical test results show that：(1) There is a large differences on the rock failure mechanisms and failure modes between the uniaxial loading test and unloading instability test. (2) The progressive microcracks propagation is dominant in failure process of uniaxial loading test；and the microcracks concentrate on the local fracture plane. Before the main fractures induced，Acoustic emission(AE) phenomenon which is the omen of many microruptures appears；and it is the typical rockburst mode of instability induced by the progressive failure. (3) The brittle characteristic during the rock unloading failure in the biaxial test under confining pressure is more evident. The unloading failure is mainly released by sudden，high energy density. At the same time，it shows intense dilatation failure characteristics along the unloading direction；and the failure mode is large-area shear fracture combined with local tension fracture. On the contrary to the uniaxial loading，there is nearly no omen of any microfracture before the main fracture；and it is the typical failure mode of transient rockburst.
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Received: 25 June 2010
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2011, Vol. 30 
