At present, large-scale triaxial tests are used to investigate the mechanical properties of the filling of earth-rockfill dams, which are generally under plane strain state. Because the intermediate principal stress is equal to the minor principal stress in triaxial stress state, the mechanical properties of the filling under plane strain state will be underestimated. To fully comprehend the potential of the fill material for scientific design and rational evaluation of earth-rockfill dams, it is necessary to conduct plane strain test of the filling material. Large-scale plane strain tests were performed on the filling material using a large-scale true triaxial apparatus. A series of plane strain isotropic consolidation and drainage shear tests were carried out on coarse-grained soils with four different initial dry densities. Test results indicate that the stress-strain relationship predominantly exhibits strain hardening, with a rising stress curve and shear shrinkage-induced volumetric changes. Under a given minor principal stress, the maximal difference between the major and minor principal stresses increases linearly with the initial dry density. Similarly, when the initial dry density remains constant, this difference also increases linearly with minor principal stress. During the initial shear stage, the slope of the stress ratio curve between deviatoric and spherical stresses rises with increasing initial dry density. As shear deformation progresses, the stress ratio curves for different initial dry densities converge. A monotonically increasing deviatoric stress with spherical stress denotes strain hardening, whereas a deviatoric stress that first rises and then falls indicates strain softening. The initial elastic modulus under the same minor principal stress rises linearly with the initial dry density. At a specific initial dry density, the initial modulus increases linearly with the minor principal stress. The intermediate principal stress coefficient grows with the strain in the major principal stress direction, forming a three-segment broken line curve. Initially, the growth is slow during shear stage, but it accelerates linearly with increasing shear deformation. A slightly downward-bent curve end correlates with strain hardening, while a slightly upward-bent end corresponds to strain softening.