Constitutive modeling of cyclically loaded clays with entrapped gas bubbles under undrained and unexhausted conditions

Fine-grained gassy marine sediments are widely distributed around the world, and the cyclic behavior of this type of sediments has a great influence on the foundations of offshore structures. However, in previous investigations, constitutive models for repeated loading have been mainly focused on saturated soils, while existing constitutive relationships for gassy clays have been developed primarily for monotonic loading. In this study, a constitutive model is proposed for cyclically loaded clays with entrapped gas bubbles under undrained and unexhausted conditions. This soil model extends the bounding surface model of Huang et al. for fine-grained saturated soils (Huang et al. in Comput Geotech 38:341-353, 2011) and considers the gas effects via including soil volume change due to the compression of gas and a pore pressure-dependent plastic modulus. By comparing against test data, it is shown that the proposed model can reasonably represent the response of fine-grained gassy soils during undrained cyclic loading. In particular, the model can well replicate the beneficial and detrimental effects of gas depending on pore pressure levels. Under low pore pressure, the presence of gas suppresses pore pressure accumulation and promotes cyclic shakedown, while under high pore pressure gas bubbles accelerate the accumulation of pore pressure and promote cyclic ratcheting. The influence of soil structure on the cyclic responses of gassy clays is investigated, which shows that the presence of soil structure can promote the accumulations of excess pore pressure of gassy clays and turn cyclic shakedown into ratcheting.