Behavior Evolution And Constitutive Characterization Of Lianyungang Marine Soft Clay During Fresh-water Infiltration Process

In the past 20 years,with the formulation and implementation of coastal development strategy of China,more and more infrastructures have been built in the coastal regions,where the phenomenon of fresh-water infiltration was commonly identified.The fresh-water infiltration extremely improves the sensitivity of soft clay's structure,which will trigger a series of engineering disasters.For example,the instability of slopes in coastal region of Norway and Colombia inducing by the high sensitivity of soils was widely identified.Up to now,the sensitization mechanism of soft clay is not clear.To answer this question,the soft soil sedimentary region in Lianyungang was selected as a natural testing site in this study,and then a series of in-site geological surveys,laboratory tests,numerical simulations and constitutive theory research were carried out respectively to study the following aspects:(1)Identify and verify the sensitization mechanism of soft clay inducing by the fresh-water infiltration;(2)Establish a compression/rebounding model of soft clay considering the freshwater infiltration;(3)Propose the quantitative characterization method for the prediction of soft clay's sensitivity during the infiltration;(4)Propose the constitutive model of natural sedimentary soft clay considering the fresh-water infiltration effect.The main conclusions of the full text are listed as follows:(1)The freshwater infiltration is an open system.In this system,the pore-water salinity leaching together with the internal erosion of colloidal particles(with a particle size between 500 and 1000nm)are both identified.This infiltration(the salinity leaching and colloidal erosion)cannot change the water content and plastic limit of soft clay,but reduce the liquid limit and plasticity index,which in turn leads to an increase of liquidity index.Because the negative correlation between liquidity index and the shear strength of remolded soil is identified,therefore the freshwater infiltration can reduce the shear strength of remolded soil.As the shear strength of natural soft soil is unchanged during freshwater infiltration and thence the soil's sensitivity is improved.(2)The fresh-water infiltration is affected and controlled by the interaction between pore-water chemistry and clay minerals.In the early stage of fresh-water infiltration,the pore-water salinity inside the soils gradually decreases,which increases the thickness of DDL of colloidal particles,and the clay minerals are in the dispersed state.Therefore,the colloidal particles are easily released,which eventually expands the internal seepage channels inside soils.In the late stage of fresh-water infiltration,as the pore-water salinity continues to decrease,a large amount of colloidal particles wasreleased,which clogs the seepage channels inside the soils in reverse.When the pore-water salinity in the soils tends to zero,with the continuous fresh-water infiltration,only the colloidal erosion inside the soil is identified.(3)Although the fresh-water infiltration(salinity leaching with the colloidal erosion)makes the structure of soils more sensitive,but it cannot change the compression and rebonding behavior of soft clay during the elastic loading and elastic unloading stages.Fresh-water infiltration can induce the larger compression of soft clay in the small stress area during the elastoplastic loading stage.However,with the stress increases(ie,the large stress area in the elasto-plastic loading stage),the sensitization induction effect disappears.Based on above characteristics,a compression/rebounding model of soft clay considering the sensitization effect is established.(4)The COMSOL Multiphysics software was used to simulate the fresh-water infiltration.The simulation results reveal that the pore-water salinity leaching together with the internal erosion of colloidal particles are both identified during the short-term infiltration of fresh-water,whereas only the internal erosion of colloidal particles during the long-term infiltration is identified.The relationships between internal erosions and the basic properties of soft clay are established,and the quantitative characterization methods are proposed respectively for the predictions of sensitivity and other physical-mechanical parameters of soil(such as the liquid limit,plastic limit,plasticity index,liquidity index,undrained shear strength of undisturbed soil and remolded soil).(5)A constitutive model of natural soft clay considering the fresh-water infiltration effectis proposed.The comparisons between calculation and testing result verify the validity and accuracy of this proposed model.At the same time,this model is used in the analysis of specific engineering case to verify its practicability.Finally,a map of the sensitivity of marine soft clay in Lianyungang region was drawed in this study to better serve for the pre-project planning.