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Abstract In order to study the failure characteristics of the deep surrounding rocks in unloading path，in-situ true triaxial unloading tests under high pressure were carried out in the field deep experimental adit. The tests have features such as large scale(50 cm ×50 cm×100 cm)，high stress( = 11.2 MPa)，unloading fracture of . This true triaxial test can represent the real stress states of surrounding rocks of deep tunnel and the complete stress-strain curve of marbles in tunnel excavation stress path；and the triaxial strength of rock mass can be obtained. Based on the in-situ true triaxial test of marbles under high pressure in unloading path，Hoek-Brown(H-B) empirical strength criterion is adopted to study the true triaxial strength parameters of marbles in unloading path. The research results are as follows：(1) The values of true triaxial strength parameters in unloading path which are evaluated by using H-B empirical strength criterion may be relatively lower than those in in-situ true triaxial unloading test. The evaluated empirical parameter s is 0.003 951 7， is 3.414. However，the inverse computation parameters according to the test results s = 0.095 53， is 12.208. (2) Based on the evaluated results of rock strength by using H-B empirical strength criterion，Mohr-Coulomb criterion is used to evaluate the values of strength parameters. The true triaxial strength parameters of marbles in unloading path，tan? is 1.39，c is 6.61 MPa，while the true triaxial strength parameters of undisturbed marbles in unloading path，tan? is 1.60，c is 6.73 MPa. The former can stand for unloading damage rock mass.
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Received: 18 April 2012
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2012, Vol. 31 
