Abstract：For the plane strain problem of engineering construction in loess areas, the true triaxial apparatus of Xi′an University of Technology is used, and the plane strain tests with the major principal stress directed along the vertical fracture of loess samples withdifferent moisture contents are conducted under different confining pressures in order to study the correlations of intact loess strength, deformation characteristics and the intermediate principal stress. The study shows that under the condition of low confining pressure loess samples with the low moisture content tend to fail in the brittle manner and the stress-strain relationship curve goes softening with the damage of intact structure. However, with the confining pressure increases, the compressive damage dominates and the stress-strain curve goes hardening as the new structure develops. The damage of intact structure during shearing is accompanied by dilatant deformation. The compressive damage along wih the development of new structure during shearing is accompanied by contractive deformation. The investigation also reveals that with the development of vertical loading deformation, the principal stress in the direction of plain strain becomes from the minor principle stress to the intermediate one, and the coefficient of intermediate principal stress ceases to drop and starts to increase gradually, ranged from 0.15 to 0.35. The failure strength of loess approximately linearizes with the moisture content under plane strain condition. With the moisture content increases, the cohesion increases obviously, while the internal angle of friction decreases slightly.
