Study On The Dynamic Response Of Gas-bearing Sandy Seabed Induced By Wave Loadings

Shallow gas is widespread in the continental shelf areas of the East China Sea and the South China Sea.Under the wave load,the pore pressure of the gas-bearing sediments increases with moderate oscillations,the effective stress of the seabed soil decreases,and even the wave load leads to engineering disasters such as liquefaction and structural instability.The impact of shallow gas has to be considered in the design and construction of marine structures.Previous studies on the pore pressure response of seabed mainly focused on saturated sediments,while limited studies on the pore pressure response of the gas-bearing sediments under wave load were reported.In this work,to investigate the dynamic response characteristics and reveal the mechanism of sandy gassy seabed under wave loads,both theoretical analysis and large-sacle water flume experiments are carried out.Based on Biot's porous elastic theory and linear wave theory,the influences of gas content on the characteristics of excess pore pressure in gas-bearing seabed under wave load are studied,and the development trend of excess pore pressure along depth under different gas content is calculated.Based on the large-scale fluid-soil-structure interaction flume experiments,the response of the gas-bearing seabed soil under wave load is studied and the dynamic evolution of excess pore pressure of the sandy gas-bearing sediments is revealed.The excess pore pressure of shallow soil first oscillates periodically,and then expands to the deeper layer and reaches steady state;the influence of wave loading history on the pore pressure response of sandy gas-bearing sediments is discussed;the amplitude of the excess pore pressure of sandy gas-bearing sediments increases with the increase of wave loading cycles under the same wave load intensity;during the process of wave loads,the bubbles in the sandy gas-bearing sediments continuously escape,forming a volcanic cone-shaped channel on the bed surface.When the wave load terminates,the excess pore pressure of the seabed soil dissipates,the drainage consolidation develops from bottom to top,and-obvious settlement of the sand bed surface is observed;the process of dissipation of excess pore pressure of gas-bearing sediments soil is accompanied by compaction of soil,which leads to large deformation and settlement of seabed bed surface.The history of previous wave loading densifies the soil seabed.The effect of wave parameters on the pore pressure response of sandy gassy seabed is studied by comparing the water flume test with the numerical results.The effects of soil gas content on the pore pressure response are explored,and the theoretical prediction is validated.It is found that the higher the gas content is,the slower the dissipation rate of excess pore pressure becomes;the greater the gas content of the soil,the smaller the amplitude of excess pore pressure.With the increase of gas content,permeability coefficient decreases,saturation decreases and phase lag becomes more signficant on the seabed surface.