(1. School of River and Ocean Engineering，Chongqing Jiaotong University，Chongqing 400074，China； 2. College of Civil Engineering，Chongqing University，Chongqing 400040，China)
Abstract：Based on the uniaxial compression，the strength index of rock under triaxial compression can be determined with Mohr criterion or Heok-Brown criterion，however，there has been not a proper method to determine the index of strain. According to the characteristics that the vary grads of microcrack density is a constant at rock¢s elastic limit point(the damage threshold value)，the macro- and meso-damage composite models have been set up，according to the correlation between the critical value of damages on rock¢s stress-strain curve and the damage threshold value，the critical value of damage at the elastic limit point under different confining pressures has been derived from the microcrack density on the damage threshold and the vary grads of microcrack density at the elastic limit point under uniaxial compression，and the rock peak strength may be given by the Mohr-Coulomb criterion or Heok criterion under different confining pressures，and then the index of corresponding strain has been gained. The calculative formula of determining the index of triaxial compression by the index of uniaxial compression has been derived. The rationality of the formula has been proved by calculation case and experiments.
