Cyclic Dynamic Characteristics of Gassy Clay: Experiments, Bounding Surface Model and Simulation

Fine-grained gas marine sediments are widely distributed all over the world. Due to the existence of large gas bubbles, the structure of the soil have been altered which have significant influence on the instability of offshore structures and the occurrence of submarine landslides. However, the existing constitutive models mainly focus on the response of saturated soil under cyclic loading and the response of gassy soil under monotonic loading. To understand the cyclic characteristics of gassy soils, a bounding surface elastoplastic constitutive model is proposed based on the cyclic triaxial test results and critical state soil mechanics. The proposed model has the following three characteristics: (1) The elastic stiffness of soil is modified by intergranular strain theory to make it closer to the stiffness shown in the test, which can also reflect the sudden change of stiffness when the loading direction changes. (2) The variable shape bounding surface equation is used to describe the stress path under different combinations of initial pore pressure and gas content. At the same time, the relationship between the shape parameters with the initial pore pressure and gas content is established to facilitate the calibration of the parameters. (3) Gassy soil is considered as a three-phase composite material. In the process of undrained cyclic shear, a positive volumetric strain increment of saturated soil matrix is introduced to simulate the bubble flooding effect. The cyclic constitutive model quantitatively reproduces experimental results ofmeasured effective stress pathforgassyclay.

Keywords: Gassy clay, Bounding surface model, Undrained shear behavior, Three-phasecompositematerial.