Abstract: Reasonable determination of active earth pressure is essential for safe and economical design of spatially axisymmetric shafts. Based on the strength equation involving two stress state variables for unsaturated soils and the spatially axisymmetric unified strength theory, a unified strength equation for spatially axisymmetric unsaturated soils was introduced through analogy extension method. Combined with the simplified slip line method, the slip line solution for active earth pressure on shafts in unsaturated soils was derived to capture the intermediate principal stress effect, specifically for uniform and linear suction distributions. Finally, it was validated through a comparative assessment with existing literature, discussing the influence of unsaturated characteristics and intermediate principal stress. The results reveal that the concise formulation of the unified strength equation for spatially axisymmetric unsaturated soils effectively addresses different intermediate principal stress effects, highlighting its theoretical importance. The slip line solution for active earth pressure on shafts simultaneously accounts for both intermediate principal stress effect and unsaturated characteristics, showing good agreement with results for shafts in saturated soils and retaining walls in unsaturated soils reported in the literature. Moreover, it is applicable to uniform, linear and nonlinear suction distributions, and it has broad engineering application prospects. The active earth pressure on the shaft decreases significantly as the matric suction, the intermediate principal stress coefficient, and the unified strength theory parameter increase, especially for uniform suction. Careful selection of the intermediate principal stress coefficient is crucial to prevent large deviations in active earth pressure. The different influencing extents of intermediate principal stress corresponds to different strength criteria for unsaturated soils.

Key words: circular shafts, active earth pressure, unsaturated soils, slip line method, intermediate principal stress