For soil samples of same soil mass

based on traditional consolidation undrainage triaxial compression tests and true triaxial undrainage plane strain tests under loading and unloading conditions

it is pointed out that soil sample possesses different deformation characteristics and failure characteristics when soil sample is imposed on different stress paths; and these different characteristics are reflected by different changing tendency of average principal stress and ratio q/p of soil sample. At last

according to Matsuoka-Nakai failure criterion and its corresponding new transferred expressing formula

its strength failure parameter k2 value is optimized; and test results show that the Matsuoka-Nakai failure criterion can give a reliable strength failure prediction for soil mass.