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Study On Sand Liquefaction Under Freezing Conditions In Cold Region

Posted on:2019-11-11Degree:MasterType:Thesis
Country:ChinaCandidate:L XieFull Text:PDF
GTID:2370330545452776Subject:Geological engineering
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China possesses widespread frozen soil areas,and seasonal frozen soil areas accounts for about 70% of its national territory areas.For water conservancy,transportation and other engineering construction in the frozen soil areas,construction risks caused by mechanical property changes of soil body should be taken into consideration,such as frost heaving,differential settlement,sand liquefaction,etc.Presently,there are relatively few studies on liquefaction of sand soil under the earthquake action in cold region,and it is not clear about whether the existence of permafrost is beneficial or unbeneficial to the formation of sand liquefaction.The main stream of Nenjiang River in the southwest of Heilongjiang Province is located in the cold region of high latitudes,the soil of which is in freezing state for almost half time of a year.Meanwhile,this region is close to the potential seismic region of Songyuan.As seismic activities are more frequent in recent years,it may easily come with7.0-magnitude quake in the future.On account of the mentioned above,this paper takes the foundation soil of embankment engineering located near the main stream of Nenjiang River as the object of studies,and is supported by “Study on Soil LiquefactionPrevention of Embankment Engineering in High Earthquake-Intensity Area of Nenjiang River”.By means of a series of research methods like in-situ test,numerical simulation as well as data collection and sorting,the author conducts a study on sand liquefaction under freezing conditions in cold region.,and thereby obtains a preliminary conclusion.Basically,the following work has been accomplished.(?.)Theoretical AnalysisThe pore water pressure and shear strength will change with the decrease of temperature in the process of soil freezing,the two factors of which also play an important role in causing sand liquefaction.On one hand,the shear strength of frozen soil is negatively correlated with temperature,namely the lower the temperature,the greater the shear strength.And the larger the shear strength,the less likely the soil is to be liquefied.Therefore,frozen soil actually plays a role in inhibiting the liquefaction of sand.On the other hand,when the soil freezes,the pore water pressure drops.What's more,soil freezing in vibration condition is beneficial to slow the increase of pore water pressure.Hence,frozen soil can mitigate the liquefaction of sand soil by reducing pore water pressure.(?.)Survey and CalculationA hydrogeology survey was conducted within 1500 square kilometers of research area,and drilling,shear wave velocity as well as standard penetration tests were carried out towards the foundation soil of the Old Longkou sluice gate in the research area.The main influencing factors of sand liquefaction in the research area are discussed via survey data.By means of distance discrimination method,the depth of underground water,the depth of sand buried,standard penetration test blow count,and the epicentral distance are selected as discrimination parameters to set up a discrimination model for soil liquefaction of embankment engineering in the Nenjiang River area.Besides,a flow field chart of groundwater level and a zoning map of sandliquefaction are drawn.Based on the surveys and studies,key information is obtained.The underground water level is about three to five meters,and the water flows from the southeast to the northwest.The engineering site of Laolongkou outlet belongs to the ? type.The type of foundation soil mainly belongs to fine grained soil and fine sand,which indicates large possibility of sand liquefaction.Besides,shallow groundwater level is also a main factor causing the liquefaction of sand in the research area.(?.)Numerical SimulationBy using FLAC3 D software,the foundation soil model of Laolongkou sluice gate as a key hydraulic structure was established.Under the simulated conditions of saturation and unsaturation,a test towards frozen soil of 0 meter,0.5 meter,1 meter and 1.5 meters thick was performed.After bedrock acceleration time history of the area was input,the influences towards pore water pressure of buried soil of 2 meters,4 meters and 6 meters deep were clear.Through the relationship between pore water pressure and effective stress,it is easily concluded that the frozen soil has an effect on the liquefaction of sand soil.Basic simulation results are impressive.Under the condition of saturation,as the thickness of frozen soil increases,the increase of pore water pressure decreases gradually,and the effective stress is gradually close to zero.At the same time,the pressure of the deep pore water is higher than the shallow one,and the effective stress is much closer to zero.The existence of permafrost is beneficial to the mitigation of sand soil liquefaction,and the deep soil layer is easier to liquefy than the shallow part.The result under unsaturated condition is in the same.Yet,the liquefaction hazard of sand soil under unsaturated condition is less than that of sandy soil under saturated condition.
Keywords/Search Tags:sand soil liquefaction, frozen soil, the main stream of Nenjiang River, engineering properties, pore water pressure
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