Fine Observation Of Wave-induced Pore Water Pressure And Liquefaction Evaluation Method Of Seabed Silt In Yellow River Estuary

Liquefaction of seabed sediments is a common geological phenomenon in coastal areas. Soil will lose carrying capacity after liquefaction, and then cause oil platforms dump, subsea pipelines fault, submarine cable short circuit and so on. There are several main factors that affect liquefaction: types and properties of sediments, wave power intensity and duration of action. Generally, liquefaction can only occur in fine sand and silt; only waves are large enough can cause Dynamic response in seafloor sediments. Silt is widely distributed on the surface of Chengdao sea area, and the dynamic action of waves is strong. Therefore, this sea area is an easy-liquefaction area. Besides, this area is the main part of Sinopec Shengli Oilfield. As a consequence, wave-induced pore water pressure and liquefaction evaluation study of this sea area is extremely important.The change of pore water pressure in sediments, which is induced by waves, is the root cause for liquefaction. In order to study the wave-induced pore pressure response and change in silt, we laid a probe in study area to monitor pore pressure changes. At the same time, water depth, wave and tide changes are also being measured. Analyzed the changing tend and affections of pore pressure and excess pore pressure by data processing. Summarized the law of pore pressure and excess pore pressure changes. The effect of wave can be divided into instantaneous effect and accumulate effect. The accumulate effect is not obvious, instantaneous effect plays a leading role, when wave height is low; the accumulate effect is obvious and play a leading role when wave height is high. Besides, “Hysteresis Effect” exists in wave-induced submarine pore pressure changes. When water depth is about 10 m, the relative changing time of pore pressure, which caused by waves, is delayed for 1-2h.Calculated the pore pressure in study area with formulas, further analyzed changing reason of pore pressure and affection of excess pore pressure, discussed the influencing factors of oscillate excess pore pressure and accumulate excess pore pressure. The accumulation of pore pressure is mainly affected by soil permeability and wave height. When permeability is certain, instantaneous effect play a leading role and accumulate effect is not obvious if wave height is low; accumulate effect play a leading role and instantaneous effect is not obvious if wave height is high. When wave height is certain, instantaneous effect play a leading role and accumulate effect is not obvious if permeability is high; accumulate effect play a leading role and instantaneous effect is not obvious if permeability is low.Calculated the liquefaction depth using different wave recurrence period data with both shear criterion and excess pore pressure criterion method. Make clear of the parameters required for the calculation. When wave height is low, there would not be any liquefaction. There is a certain value that liquefaction occurs when wave height is higher than this value and liquefaction depth deepens with wave height increasing. The water depth where maximum depth of liquefaction occurs is 10-12 m. Finally, we calculated liquefaction depth of the whole study area. This area can be divided into 2 main liquefaction zone. The calculation results of liquefaction and historical disturbance depth are close, shows that calculation results is accurate.