DEFORMATION AND SEEPAGE CHARACTERISTICS OF MARINE GASSY SOIL DURING HIGH-STRESS LOADING AND UNLOADING

Gassy soils have special inner structures (e.g., massive gas bubbles dispersed in the pore water), and this leads to low shear strength, high compressibility, strong expansivity, and complex consolidation process. The presence of gassy soils beneath the seafloor has great potential to trigger various kinds of disasters during ocean engineering construction. Mechanical properties of marine gassy soils are inherently controlled by the effective stress for consolidation and the inner pressure of pore fluids. However, when subjected to high effective stresses for consolidation, experimental data are currently insufficient, and how marine gassy soils response remains elusive. In this study, intact marine sediments were cored from an offshore region rich in shallow gas near the city of Zhoushan, and the sediments, after indoor testing for basic physical and chemical indexes (e.g., particle size distribution, mineral composition, liquid and plastic limit water ratios), were remolded to prepare gassy soil specimens by using the zeolite method. Cyclic loading and unloading were performed on the gassy soil specimens under laterally confined condition, and the maximum vertical stress was 3200 kPa. Basic physics behind changes of the indexes due to salt removing were unveiled, and effects of the gas content on deformation behaviors as well as seepage characteristics during cyclic loading and unloading were analyzed. It is shown that coefficient of compressibility of marine gassy soils decreases during loading, and the compression index together with the expansion index increases with increasing gas content. Pore gas and water are produced mainly during the virgin loading, and more than one half of the cumulative volumes of produced gas and water are squeezed out of gassy soils at the beginning of the virgin loading. The hydraulic conductivity of water saturated soils monotonously decreases when the soils subjected loading. However, the hydraulic conductivity of marine gassy soils firstly decreases, then changes to increase, and finally returns to decrease during the virgin loading.