The stress state of earth and rockfill dams is close to plane strain stress state or three-dimensional stress state

while the conventional triaxial test underestimates the mechanical properties of coarse granular material. The mechanical properties of coarse granular material under conventional triaxial stress state

plane strain stress state and true triaxial stress state( 0.25) were investigated by compression tests using large-scale true triaxial apparatus. The test results show that the relationship curves between the difference of major principal stress and minor principal stress and the strain in the direction of major principal stress become higher and steeper in sequence from conventional triaxial test

to plane strain test

to true triaxial test under the same minor principal stress. The volumetric strain increases with the increase of the spherical stress under a certain test loading condition

which shows a slow increase at the initial shearing stage and then increases linearly. The volumetric strain curves are close for different minor principal stresses. The strengths of plane strain stress state and true triaxial stress state are much greater than that of conventional triaxial stress state

and the percentage of strength increase under true triaxial stress state is larger than that of plane strain stress state. The initial elastic modulus increases linearly with the increase of the minor principal stress. In plane strain stress state

the coefficient of intermediate principal stress increases with the increase of the strain in the direction of major principal stress. The coefficient of intermediate principal stress increases slowly in the initial shearing stage and then increases linearly with the increase of the strain in the direction of major principle stress. From conventional triaxial stress state to plane strain stress state and then to true triaxial stress state

the strains in the direction of minor principal stress are all expansion deformations

and the expansion deformations increase successively under the same minor principal stress. The relationship curve between the ratio of deviatoric stress to spherical stress and deviatoric strain of small minor principal stress is located at the top for a certain test loading condition.