Abstract
This study presents a unified constitutive model capable of simulating monotonic behaviour of clay and sand, incorporating a non-associated flow rule and critical state concept. The bounding surface approach has been used to anticipate a smooth transition from the elastic phase to the plastic phase of the soil. A novel dilatancy relationship is introduced to represent the volumetric behaviour of both sand and clay in a unified way. The model is implemented using the implicit Euler method. To capture anisotropic soil behaviour, the model is extended within a multilaminate framework comprising 13 elastic-plastic planes. The overall response is derived by integrating the behaviour of these individual planes, each governed by unconventional constitutive equations. The model realistically reproduces strain softening and induced anisotropy through a non-classical plasticity approach. Simulations of six soil samples under monotonic drained and undrained loading show good agreement with experimental results, demonstrating the model's effectiveness.