Abstract

The stress–strain behavior of soft clay soil is dependent on the applied strain rate and such time dependency behavior should be considered in geotechnical engineering projects such as the bearing capacity analysis for a foundation. Most previous studies focus on elastic-viscoplastic behavior of clay soil under an undrained stress path condition. The developed constitutive models cannot simulate the strong time-dependent stress-strain relation of soft clay under the drained stress path condition. In the long-term bearing capacity analysis for a shallow foundation, the neglect of the time-dependent stress-strain relation of soft clay may result in inaccurate result.
In this study, the strain-rate-dependent soil behavior is modeled using the Drucker-Prager/Cap model with the consideration of soil creep behavior. Both consolidation creep and shear creep mechanisms are considered in the modeling. A simplified approach is proposed to derive creep parameters from stress relaxation test results. The constitutive modeling is validated against experimental measurements on strain-rate-dependent behavior of Regina clay soil under the drained stress path condition. Finite element modeling on the long-term bearing capacity of a shallow foundation on soft clay is also performed. The result shows that the considering of viscoplastic behavior of clay soils does not affect the ultimate bearing capacity. However, it yields the lower bound of developed shear stress of a shallow foundation at a given vertical displacement, which is to be conservative from the engineering point of view.