Applying an Appropriate Cap in an Elastoplastic Model with Open Bounding and Yield Surfaces at High Pressures

Background: In some projects such as dam construction, soil experiences changes to some an extent as a result of high pressures. The majority of constitutive models used in geotechnical engineering research specifically for sand are presented without cap which they fail to show the actual plastic behavior of soils at high levels of stresses. In our research, a powerful elastoplastic model from the family of bounding surface plasticity which has an open conical yield surface as well as bounding surface is selected. This model is able to show the plastic behavior of sand in deviatoric loadings very well. However, when confining stress increases, it cannot simulate the plastic behavior of sand specimens especially at high pressures. Here, by introducing an appropriate algorithm for cap, model formulation is developed and modified. It is accomplished by introducing a secondary plasticity mechanism and a yield surface for cap which closes deviatoric yield and bounding surfaces along with the hydrostatic axis. Finally, validity of the model predictions is evaluated with some experimental data available in literature and capabilities of the modified constitutive model with cap are shown at high levels of the stress. These experimental data are provided for Toyoura and Nevada sand; and it is observed that prediction of the constitutive model modified with cap is significantly more satisfactory at high pressures.