现有洞室最优支护设计均基于Mohr-Coulomb（简称M-C）准则，不能真实反映岩石强度的中间主应力效应以及围岩的三向不等应力状态。基于广义SMP准则和稳定蠕变J3准则，考虑中间主应力对岩石强度的提高作用，建立了软岩洞室最优支护力与围岩允许最大位移的理论解答。洞室最优支护解答对M-C准则、外接圆Drucker-Prager（简称D-P）准则均具有很好的拓展性，且工程实例验证了其正确性。研究结果表明：3种强度准则结果间的差异实际反映的是对中间主应力效应的不同考虑，M-C准则和外接圆D-P准则对应的结果是两个极端情况，推荐选用广义SMP准则解答；岩石长期强度是洞室最优支护设计的关键参数，围岩凝聚力与内摩擦角的影响亦很显著，应合理测定岩石的强度参数并充分考虑变异性。

The existing optimal designs of tunnel support are all based on the Mohr-Coulomb (M-C) criterion, which cannot truly reflect the effect of the intermediate principal stress on rock strength and the three-dimensional unequal stress state of surrounding rock. Theoretical solutions for both the optimal support pressure of soft rock tunnel and the allowable maximum displacement of surrounding rock are derived based on the steady creep criterion expressed by the third invariant of deviatoric stress. The generalized spatially mobilized plane (SMP) criterion is employed to evaluate the strengthening effect of intermediate principal stress on rock strength. The obtained solutions in this study can be well generalized to the M-C criterion and the circumscribed Drucker-Prager (D-P) criterion, meanwhile they can be validated by an engineering instance. It is shown through parametric study that the differences among these three strength criteria actually reflect different considerations on the effect of the intermediate principal stress; the results of the M-C criterion and the circumscribed D-P criterion represent the two extremes, and then the results of the generalized SMP criterion is recommended; long-term rock strength is a key parameter for the optimal design of tunnel support, and the effects of surrounding rock cohesion and its internal friction angle are also significant, so that the rock strength parameters must be properly determined and their variabilities should be fully taken into account.