Abstract: With the assumption that the trace of the principal stress arching is a semicircle, the relationships of average vertical stress and interlaminar shear stress of soil differential element vs. counterforce of wall were established, respectively. Then, a static equilibrium equation based on the extended unsaturated soil shear strength formula was set up to solve the active earth pressure. The interlaminar shear action was considered in the process, to make up the shortage of force imbalance of traditional static equilibrium equation. Compared with the earth pressure formula of unsaturated soil deduced from classical approach, principal stress deflection is considered in this study, and the active earth pressure is nonlinear along the height which is closed to the true stress distribution. Compared with the conventional earth pressure calculations considering arching effects, factors on active earth pressure are analyzed in the new method. These factors include the properties of unsaturated soil and groundwater level. The result can provide theoretical basis for earth pressure study and reference for the design of retaining wall.

Key words: unsaturated soil, active earth pressure, soil arching effects, matric suction, shear coefficient between layers