Abstract: Soil failure criteria in theoretical research and engineering applications have a significant impact on research and design outcomes. Additionally, the discrepancy in experimental results between plane strain tests and conventional triaxial tests, attributed to the influence of intermediate principal stress, has been widely acknowledged. In this study, the unified strength theory, unified slip line field theory, and soil arch failure theory are employed to investigate the anti-slide pile spacing of unsaturated loess slopes. Based on experimental data, the intermediate principal stress parameter b of unsaturated loess is determined, and the anti-slide pile spacing expression for unsaturated loess slopes is derived using the soil arch theory. Furthermore, the relationship between anti-slide pile spacing variation, intermediate principal stress parameter b, and matrix suction of unsaturated loess slopes is explored through engineering case studies. The analysis indicates that considering the effect of intermediate principal stress can lead to an increase in the design spacing of anti-slide piles, thereby reducing engineering costs. This research provides valuable insights for optimizing anti-slide pile spacing in unsaturated loess regions.

Key words: unified strength theory, soil arch, unsaturated loess, slope, anti-slide pile