Abstract: The constitutive model of unsaturated soil under dynamic loading is helpful to solve related engineering problems. In the framework of plastic incremental flow theory, by combining Barcelona basic model (BBM) with the plastic hardening criterion proposed by one of the authors and considering the cyclic loading with the controlled matric suction, an elastoplastic two-surface model is established for describing unsaturated cohesive soil. Combined with the concept of memory centre, a bounding surface and a geometrically similar loading surface describe the nonlinear, cyclic plasticity and deformation accumulation characteristics of unsaturated soil when the stress path periodically changes the loading direction jointly. Dynamic triaxial shear tests were conducted to determine dynamic mechanical properties of unsaturated clay silt, which analyses the influences of matric suction, net cell confining pressure and dynamic stress amplitude on cyclic plasticity. Experimental results of the unsaturated soil under static and dynamic loading are compared with the correlative model predictions, which shows a good agreement. It indicates that the elastoplastic two-surface model can simulate the mechanical behaviour of unsaturated soil under cyclic loading with controlled matric suction.

Key words: unsaturated soil, cyclic loading, matric suction, plastic hardening criterion, elastoplastic constitutive model