Mechanical properties of red clay under different stress paths and loading and unloading rates

During foundation pit excavation, soils are subjected to multiple coupled factors and present complex failure characteristics, making it difficult to capture their mechanical behavior through single-factor analysis. In this study, the natural red clay in Kunming area was used to carry out the undrained triaxial shear test under K0 consolidation conditions, and the variation of the stress-strain curve and pore pressure curve under different stress paths and loading/unloading rates were explored. The initial tangent modulus and strength limit variation of red clay were then analyzed. The test results show that the stress-strain curves of red clay under three stress paths were hyperbolic type, with strength ranking as axial compression > constant mean principal stress compression > lateral unloading. The influence of the loading/unloading rate on the stress-strain curve is divided into three stages: when ε_a≤ε₀, the loading/unloading rate has limit influence on the shear strength of the soil; when ε₀<ε_a≤ε₁, the faster the loading/unloading rate, the faster the corresponding shear stress increases; when ε_a>ε₁, the shear stress under different loading/unloading rates remains unchanged. Pore pressure evolution varies significantly with stress path, with rate effects being minor at the early stage of shearing but becoming more pronounced as shear progresses The Duncan-Zhang model was used to establish the stress-strain hyperbola prediction equation of K0 consolidated red clay under different paths, and the prediction results were compared with the experimental values yielding good agreement with experimental results. This study can provide basic information for understanding the deformation parameters and constitutive relationship of red clay under different stress paths and loading/unloading rates.