Large-Scale In Situ Tests for Shear Strength and Creep Behavior of Moraine Soil at the Dadu River Bridge in Luding, China

Abstract

The shear (τp), creep (τc), and long-term shear (τ∞) strengths of moraine soil are mechanical properties of high concern during the engineering construction and management process and were obtained by conducting large-scale direct shear and shear creep tests on the soil layer at the Dadu river bridge in Luding, China. The size of the test samples was about 1.0 × 1.0 × 0.50 m (length × width × height). The shear strength and creep behavior under different normal (σ) and shear (τ) stresses were analyzed, and the long-term shear strength was obtained from stress-displacement isochronal curves during the creep process. The results indicate that the shear strength of moraine soil relates to the content and distribution of large particles. The shear strength parameter of moraine soil is 0.765 (φ = 37°) and the cohesion parameter (c) is 64.2 kPa. The moment of the maximum creep displacement rate at the primary creep stage depends on the shear stress. The step-like increasing and jumping of shear creep displacement are evident during the middle-process of the secondary creep stage (e.g., 20–60 h when τ is 411 kPa under σ of 300 kPa), due to large particles rotating or overturning under high shear stresses. The empirical Burgers model predicts the creep displacement well. The ratio (τ∞/τp) of long-term to shear strength ranges from 0.53 to 0.92, indicating that the creep effect has decays in the shear strength of moraine soil. The ratio (τ∞/τc) of long-term to creep strength ranges from 0.65 to 0.85, indicating that moraine soil could easily transfer from viscous creep deformation to shear failure when the loading stress is higher than long-term strength. The results provide a valuable reference for shear strength analysis and long-term strength assessment of moraine soil regions.