The significant differences between hard rocks（more brittle） and soft rocks（more ductile） may suggest the use of different failure criteria. A strength criterion for soft rocks that includes intermediate principal stress was proposed. The new criterion includes two independent parameters: the uniaxial compressive strength（rci）, which can be obtained from common laboratory tests or indirectly estimated by alternative index tests in the laboratory or field; and f（joint）, which is used to characterize the rock mass quality and can be easily estimated. The authors compared the predictive capabilities of the new criterion with other criteria using the database of soft rocks under two conditions: with and without triaxial data. For the estimation of triaxial and true-triaxial strengths, the new criterion generally produced a better fit. The proposed criterion is practical for an approximate first estimation of rock mass strength, even without triaxial data, as it balances accuracy（lower prediction error） and simplicity（fewer independent parameters）.
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