Proposal of a new double hardening elasto-plastic constitutive model of soil skeleton based on integration of associated and non-associated flow rules

A sophisticated elasto-plastic constitutive model of soil skeleton was newly proposed. The proposed model is built on the basis of the SYS Cam-clay model in which soil skeleton structure concept is included. A new loading surface was introduced into the SYS Cam-clay model based on associated flow rule, and a Drucker-Prager model based on non-associated flow rule was mounted on the model. A new framework of double hardening elasto-plasticity was developed for integration of two different types of elasto-plastic models. Although there are some existing constitutive models called "double hardening model" other than the model proposed in this paper, these are the ones produced by simply connecting yield surfaces or by separately utilizing loading surfaces for isotropic and shear components. The proposed model is clearly distinguished from the existing double hardening models in a sense that two types of elasto-plastic models which should exist independently can work dependently showing their combined loading condition. In this paper, basic formulation and essentials of the proposed model were firstly shown. In addition, basic responses of typical sand and clay described by the proposed model were demonstrated compared with experimental results. Especially, some hollow cylinder torsional tests for sand specimen controlled by switching drained condition during shear were focused on, and it was demonstrated that the soil behaviors seen in those tests can be described not by Cam-clay type models but by the proposed model.