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Abstract In light of that the surrounding rock mass of diversion tunnel group of Jinping II hydropower station lies in the condition of high geostress，two different unloading tests of marble from the diversion tunnel were carried out. In order to analyze rock strength characteristics，deformation behavior and fracture mechanism under different stress paths，the conventional triaxial compression tests of marble were also performed. The results show that：(1) Even if the stress path and initial confining pressure are different，but as long as the real-time confining pressure is the same，the peak strength is equal. (2) The peak strength increases as the initial confining pressure increases. Comparing with triaxial compression test，it decreases by 19.5% of unloading confining pressure at pre-peak，and the peak strength appears when the real-time confining pressure accounts for about 97.2% of the initial confining pressure during the post-peak test. (3) The cohesion c decreases and the internal friction angle increases at pre-peak test，compared with the post-tests. The value of c decreases about 27.5% and increases about 22.6% at pre-peak test，but the value of c increases about 13.7% and decreases about 6.5% at pre-peak test. (4) From the unloading point to strain softening stage，the stress-strain curves have obvious strain platforms，which is the characteristics of the ideal plastic deformation. The strain platform increases with the confining pressure，which shows the feature of the marble from brittleness to plastic when the confining pressure increases. (5) The unloading confining pressure test at pre-peak shows that the marble expansion is limited. (6) The fracture mode is mainly the single shear fracture for the triaxial compression test and post-peak test；and the fracture mode of the pre-peak test is the splitting failure—X conjugate shear rupture—single shear fracture as the confining pressure increases. These conclusions reveal the loading and unloading mechanical properties of marble under high geostress and provide reliable theoretical reference for the numerical simulation about excavation and design for the stability analysis of deep diversion tunnels in West China.
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Received: 10 November 2011
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2012, Vol. 31 
