Abstract: Based on the unified solution of shear strength in terms of two stress state variables for unsaturated soils, the unified solution of Coulomb’s active earth pressure for unsaturated soils is proposed which considers the effect of the intermediate principal stress rationally. Moreover, the comparability of this unified solution is analyzed. Influential characteristic of each factor is discussed by comparing with the unified solution of Rankine active earth pressure. The result shows that: the effects of intermediate principal stress and matric suction are significant; the Coulomb’s active earth pressure decreases until zero with the increase of the unified strength theory parameter and matric suction, which indicates that the consideration of the intermediate principal stress effect can make materials give full play to their self-bearing capacities and strength potential, bringing obvious economic benefits to the project; the Coulomb’s active earth pressure increases with the increase of the grading angle of retaining wall and slope angle of backfill, but decreases with the increase of the effective internal friction angle and matric suction angle; meanwhile, the influence of external friction angle is less significant. The proposed solution has very broad applications to the determination of soil pressure. The unified solution of Rankine active earth pressure is its a special case. It provides references for design of slope and foundation pit.

Key words: unsaturated soils, unified solution of shear strength, Coulomb’s active earth pressure, matric suction