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Abstract The uniqueness of strength parameters is discussed in detail，and three aspects of this multi-value problem are discussed as following：(1) the in-situ displacement results are simultaneously concerned with elastic and strength parameters；(2) the stress-strain relationship shows multi-value under plastic softening condition；and (3) strength parameters are mutually independent，and their different combinations can make certain element of the rock with the same plastic deformation. Aiming at these problems，the new approach is presented to solve the uniqueness of strength parameters in back-analysis by assuming that the rock is an ideal elastoplastic medium；and field measurement procedures to separate elastic and plastic displacement，and the inner action mechanism in optimization method are all considered，whereas the concept of non-conformity of plastic deformation is proposed. Moreover，the feasibility of this new method is verified. Finally，the research on reliability of strength parameters in the back-analysis is developed by numerical experiments. The results show that the accuracy and stability are all favorable，and the problem of initial value sensitivity is also solved.
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Received: 27 April 2009
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