Abstract: Gassy soils are widely distributed in offshore sediments around Yangtze River and Pearl River in China. The pore gas usually exists in the form of discrete bubbles and is sensitive to the variation of the temperature and the pressure, which in turn leads to the changes of soil properties such as compressibility and permeability. Thus, to investigate the influence of thermal expansion of occluded gas phase and creep deformation of soil structures on the consolidation process of gassy soil in the offshore region, a study of thermos-hydro-mechanical coupling (THM) behavior of gassy soil is conducted based on the viscoelastic model. In the governing equations, the change of gas phase volume subjected to thermal and mechanical loadings is introduced to take the thermal expansion into consideration. The fractional derivative Merchant model is also adopted to describe the process of rock creep. In addition, these problems are solved by precise integration method (PIM), which proves to be efficient and several orders more precise than conventional numerical methods, combining with integral transform. For verification, the results obtained by this newly proposed method are compared with the analytical solution for THM problems of saturated soils and numerical prediction by FEM for consolidation problems. Typical examples with different saturations and viscosity coefficients are performed to investigate the effects of viscoelasticity and thermal expansion of gas phase on the time-dependent behavior of gassy soils. It is found that the viscosity has a significant impact on the thermal consolidation process. The creep characteristic shows a greater influence on deformation than the excess pore pressure. When the viscosity coefficient is greater than 1×1012 MPa·s, the excess pore pressure is similar to that of elastic soil foundation, and meantime the deformation process is obviously delayed. The occluded gas phase increases the peak values of excess pore pressure and surface deformation, which may reduce the stability of buried pipelines with high temperature. For Gibson soils, using the means of shear modulus to predict the excess pore pressure may result in non-ignorable deviations but the predicted surface deformation is with a deviation of less than 5%.

Key words: gassy soils, viscoelasticity, thermo-hydro-mechanical coupling, time-dependent behaviour, precise integration method