The intermediate principal shear stress influence coefficient was discussed and the effect on active earth pressure and its parameters were investigated.

Biaxial compressive test apparatus,test techniques and test results of rock materials are reviewed in the paper,and pointed out shortcomings such as most current failure criteria ignore the affection of intermediate principal stress,samples too small to assess the scale effects and the friction between the loading platen and sample which affects the failure mode of sample.

The peak value of strength increases with the intermediate principal stress and the maximum of increase is 65.

Accordingly,the formula of active earth pressure considering the effect of the intermediate principal stress under plane strain condition is proposed.

The method of principal shear stress tracing line is presented first in this paper.

This formula can give some results obtained by previous formulas,and a series of new results which can consider the intermediate principal stress effect.

Two new concepts about the principal shear stress couples and their actions are proposed through the analysis of Mohr-Coulomb criterion and the twin shear criterion.

Effect of interlayer on shear stress between layers of PCC-AC composite pavement;