ABSTRACT
Many strength criteria have been proposed since the well-known Mohr-Coulomb strength theory was established in 1900. However, due to the complexity of rock, no criterion can predict the strength of all types of rock. Among these criteria, the MohrCoulomb criterion is the most popular, but it ignores the intermediate principal stress. Many evidences have verified that the strength of intact rock is dependent upon the intermediate principal stress. Murrell (1963) compared the results from two different series of triaxial tests conducted on Carrara marble: triaxial compression tests and triaxial extension tests, and noted that the rock strength under σ2 = σ1conditions (extension) was higher than under σ2 = σ3 conditions (compression). Hojem and Cook (1968) carried out tests using a polyaxial cell. The results showed that σ2 had a first order effect on rock strength. Mogi (1967; 1971b) tested rock samples from a variety of lithologies and found more significant strength differences than had been reported in (Hojem and Cook 1968). Lade (1933) summarized the test results of some rock types, which showed the intermediate principal stress had significant effect on the strength of rocks. Haimson and Chang (2000), Chang and Haimson (2000) completed an extensive series of tests on Westerly granite and the results indicated that for the same least horizontal stress, strength may increase by as much as 50% or more over that determined in a conventional triaxial test, depending on the intermediate principal stress magnitude. Michelis (1985;
1987), Takahashi and Koide (1989) also verified that the strength of intact rock was dependent upon the intermediate principal stress.
