Study Of Slope Deformation And Failure Induced By Gas Movement In Shallow Seabed

The ocean occupies 71%of the earth's area and contains extremely rich marine resources,such as offshore oil resources,which account for about 34%of the global total.However,there are many kinds of marine geological disasters,such as submarine pockmarks,submarine landslide,which will greatly affect the development and utilization of marine resource.At the same time,in recent years,much more human activities such as offshore drilling platforms,cross-sea bridges and other engineering are easy to disturb the shallow gas-bearing sediments on the seabed during the implementation process,thus the escape of high-pressure gas in the sediments would disturb the sediment and cause the destruction like pockmarks.It was generally believed that the slow movement or violent ejection of the gas will continue to carry sedimentary material away,lead to the reduction of the mechanical properties of the sedimentary layer,deformation and destruction.Since the presence of submarine gas could cause serious secondary disasters,it was necessary to study the forming mechanism of failure induced by submarine gas.In this paper,the laboratory physical model test was used to simulate the escape process of high-pressure gas(shallow gas,gas hydrate decomposition,etc.)by aeration,so as to study and analyze the disaster formation process and formation mechanism of the failure on the seabed.The main results are as follows:(1)The aeration test shows that the formation of pockmark can be divided into three stages,gas accumulation stage,crack development stage and crack expansion stage.The gas accumulation stage means that after the gas was filled into the soil of the model,the gas cannot break through the overlying clay layer,so it accumulated in the bottom sand layer,and even formed a cavity between the sand layer and the clay layer.When the gas accumulated continuously in the sand layer until its pressure reached the sealing pressure,which can break through the overlying clay layer,the cracks were generated and the gas escaped upward along the cracks and enters the development stage of the cracks.As the gas continued to escape along the crack,it caused strong disturbance to the soil around the crack,liquefaction occur in the soil,and the particles were constantly trapped away,so the crack expanded,forming a stable ventilation channel,and the pockmark was formed.(2)To explore the change of the surrounding soil particle size distribution before and after pockmark formation,the surrounding soil was divided into three regions and sampled,it was found that the grain size distribution of soil changes in central area and margin area,fine grain percentage has decreased,but the soil particle size distribution in the outside area almost no change,which means pockmark formation is related to the loss of soil particles.(3)The original soil particles in pockmark was carried into the water by the joint action of airflow and water flow,part of them would leave with the bottom flow,and the other part would be deposited on both sides of pockmark.To explore whether pockmark formation mechanism completely for granular soil erosion,measure the pockmark volume,the soil volume extracted by siphon and the soil volume deposited on both sides of the pockmark,the results show that pockmark volume was parabolic with gas pressure,at first pockmark volume and gas pressure were positively related,after reaching peak,pockmark volume are negatively with the gas pressure.It was found that there is a certain difference between the volume of the pockmark and the sum of the volume of the soil deposited on both sides of the pockmark and the volume of the soil extracted by the bottom flow.(4)To study the influence of the formation of pockmark on the mechanical properties of the surrounding soils,a miniature CPT was used to test the soils in and around the pockmark.It was found that the soil strength in the center area and the margin area of pockmark was significantly increased,while the soil strength in the outside area of pockmark was almost negligible.(5)To explore the mechanism of the soil strength surrounding the pockmark increased induced by aeration,the vibration consolidation test was established,and Geo-PIV calculation module was used to capture test image pixels change to analyze the movement of soil mass,the results showed that the soil surrounding the ventilation channels was compacted under the combination of air and water,similar to the consolidation by vibration.(6)To explore the influence of aeration on underwater slope,the physical simulation test of failure induced by aeration was carried out.The test results show that after the model is filled with gas for a period of time,the gas escapes from the slope and the slope produces deformation failure,which is manifested in pockmarks and small-scale collapse failure.At the same time,in order to explore the influence of gas on the internal pore pressure of the model,pore pressure gauges were preset in the model.The results show that the pore pressure of the soil in the model has undergone three stages after aeration:rapid rise stage,decline stage after rising to the peak value,stable stage after stopping gas charging,and pore pressure near the inflation point response fast and significant,and gradually decreases with the increase of distance.(7)To explore the formation mechanism of pockmark in the physical model test of underwater slope,the movement of soil particles in water was simplified as sediment model.By measuring the flow velocity and analyzing the particle flow theory in the test,it was found that when the particle size of soil was greater than about 0.4mm,the particle settlement velocity was greater than the flow velocity and cannot be carried by the flow,while the particle size was less than 0.4 mm,it can be carried away by the flow movement,thus forming a centralized flow migration channel.