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.