Integrated experimental and statistical evaluation of hydraulic and strength behavior in compacted high-plasticity fine-grained soils

This study investigates the hydro-mechanical behavior of twenty compacted high-plasticity fine-grained soils by integrating laboratory testing with water retention modeling and statistical analysis. Soil–water characteristic curves (SWCCs) were generated using the Perera and Zapata empirical formulations, and air-entry values (AEVs) were determined through a reproducible tangent-line procedure. Undrained shear strength behavior was evaluated using connected bilinear failure envelopes in p–q space, from which mechanical transition thresholds (Pbreak) were identified. The results show that the Perera model consistently predicts higher and more physically representative AEVs for plastic soils compared with the Zapata model. AEV was found to relate closely to Pbreak for many samples, indicating that hydraulic desaturation and mechanical strength transitions originate from similar microstructural mechanisms, although the two thresholds do not always coincide. Principal component analysis and K-means clustering revealed three distinct soil behavior groups governed by plasticity, compressibility, suction retention, and strength characteristics. Soils with high fines content and high plasticity formed a cluster with the largest AEV and Pbreak values, reflecting suction-controlled mechanical behavior. In contrast, denser and less plastic soils transitioned earlier to friction-dominated response with lower hydraulic and mechanical thresholds. The findings demonstrate that incorporating both hydraulic and mechanical transition parameters provides a unified perspective for interpreting the behavior of compacted unsaturated clays. This dual-threshold framework enhances understanding of hydro-mechanical coupling and offers a practical basis for classifying fine-grained soils in engineering design.