Unloading Mechanical Behaviors of Clay Considering Initial Principal Stress Direction Effect: Experimental Study and Modified Constitutive Model

Excavation-induced unloading can alter soil response through both stress-path effects and material orientation. This study investigates remolded clay from Jinan using conventional consolidated undrained (CCU) and lateral unloading undrained (LUU) triaxial tests at orientation angles α = 0°, 30°, 45°, 60°, and 90° and effective consolidation pressures of 100 and 200 kPa. The measured responses were strain-hardening, with no objective yield point or distinct peak. Under CCU, the operational failure stress generally decreased from α = 0° to 60° and increased slightly at 90°, while the fitted initial tangent modulus decreased with α and the hyperbolic asymptotic deviator stress was highest at 45°. Under LUU, the terminal deviator stress showed pressure-dependent orientation effects, and the initial tangent modulus decreased with α. The apparent cohesion was lowest at 45°, whereas the apparent friction angle was lowest at 90°. An orientation-dependent Duncan–Chang extension was developed for the prescribed LUU path by parameterizing K, n, Rf, c, and φ as functions of α. An internal hold-out check at α = 60° gave parameter deviations of 2.1% or less, supporting interpolation within the investigated orientation range. Accordingly, the formulation is interpreted as an empirical fixed-orientation relation for the investigated LUU path.