Analysis of characteristics of excess pore water pressure changes in the process of gasbag formation in marine sediments
Under the action of external factors,the deep gas in seafloor sediments moves upward,which is easy to accumulate at the interface of shallow coarse-fine sediments and form gasbags.When the gas in the gasbag escapes,it will induce a variety of submarine geological disasters and bring harm to hu-man engineering construction.Therefore,real-time monitoring of the formation-growth-dissipation process of gasbags is of great significance for effectively judging its development law and predicting the occurrence of follow-up potential disasters.In this study,the multi-point pore water pressure monito-ring technique was used to simulate and monitor the forming and dissipation of gasbags in saturated fine-grained sediments by mathematical simulation and physical simulation experiments.The variation characteristics and mechanism of typical excess pore water pressure of fracture formation,gasbag for-mation and gas release were discussed in detail.According to the results of mathematical simulation and physical simulation experiments,combined with theoretical analysis,the formation-growth-dissipa-tion process of gasbag was divided into three stages:transverse crack generation,gasbag longitudinal expansion and oblique or vertical crack generation,and its corresponding excess pore water pressure was concave.The formation of gasbag began with the transverse cracks at the interface of coarse and fine sediments,and excess pore water pressure attenuated rapidly.The gasbag showed the expansion of longitudinal cavity,and excess pore water pressure remained almost unchanged;oblique or vertical cracks appeared at the end of the evolution of gasbag,and excess pore water pressure accumulated and dissipated again.The variation curve of excess pore water pressure in gas release stage had obvious saw-tooth fluctuation characteristics,and it could be used to distinguish whether the gas was released intermittently or continuously.The accumulated extreme value of excess pore water pressure could be reduced during gas reinjection,which meant that a lower pressure could create cracks and reopen the gas channel,destroying the sediment and causing the gas to escape.
pore water pressure monitoring technologyexcess pore water pressuregasbagcracksgas migration
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