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Abstract A series of pre-peak unloading tests on sandstone from Chongqing Yuzui under the range of initial confining pressure between 10 MPa and 130 MPa have been carried out，in order to analyze rock deformation behavior，failure deformation，peak strength and residual strength criterion，the evolvement property of dilatancy parameter during the process of unloading. The conventional triaxial compression tests of sandstone have been also performed. The results show that：(1) The triaxial compression tests performed under different confining pressures discover that the stress drop of the stress-strain curves does not appear when confining pressure reaches above the value of 130 MPa；so it can be considered as the transition point of brittleness and ductility. (2) Under conventional compression，the dilatancy amount at the pre-peak of deviatoric stress is smaller compared with post-peak；while under unloading stress path，the sample exhibits greater dilatancy amount at pre-peak of deviatoric stress. (3) Under the same initial stress，the variation in stress required for rock unloading failure is comparably less than that of triaxial compressive process，which reveals that unloading stress path make sandstone sample more prone to failure. (4) Under the same confining pressure，the rupture angle of unloading failure is greater than that of loading failure. (5) Shear strength parameters have been evaluated by Coulomb laws；it is found that the cohesion c decreases，but the inner friction angle increases during unloading；under the two loading stress paths，the cohesion c decreases greatly and the inner friction angle has a little change at the residual deformation stage. (6) The restriction on the behavior of dilatancy by confining pressure is significant，when the confining pressure is higher，the peak dilatancy angle would be smaller，the dilatancy angle increases quickly to the peak after the beginning of unloading；the peaks of deviatoric stress and the dilatancy angle exhibit a time asynchrony；moreover，the former lags behind the latter；the dilatancy angle under unloading stress path is greater than that under conventional loading stress path and needs smaller plastic shear strain to arrive at the peak dilatancy angle，which shows that the unloading stress path makes the dilatancy property more significant. These conclusions have revealed the unloading mechanical properties of sandstone under high geostress and provided reliable theoretical reference for the numerical simulation about excavation and design for the stability analysis of deep-buried access tunnel in West China.
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Received: 26 October 2010
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2011, Vol. 30 
