Study On The Evolution Of Coastal Saline Soils Under The Effect Of Seawater Intrusion

Soil salinization is a type of disaster,and saline soil is a kind of special soil.Reducing and preventing soil salinization disaster is one of the hot issues in the field of geotechnical engineering disaster prevention and reduction.Coastal saline soil is a kind of superior soil with full profile salt formed under the action of seawater intrusion or high salinity groundwater.Its particularity is reflected in its characteristics of high salt content,strong moisture absorption and poor stability,which may aggravate the deterioration of ecological environment and reduce the safety of coastal engineering foundation.Its damage degree is generally controlled by the changes of ionic composition,physical and chemical properties and microstructure of mineral particles in coastal saline soil.Mineral composition characteristics of coastal saline soil are often closely related to the complex hydrogeochemical and soil physicochemical effects in the process of formation and evolution.Therefore,the study on the formation and evolution mechanism of chemical components of coastal saline soil will not only help to reduce and prevent the disaster of seawater intrusion and soil salinization in coastal areas,but also deepen the understanding of the micro mechanism such as the difference of mechanical properties（plasticity,swelling and shrinkage,shear strength）between coastal saline soil and other types of soil,The research results are of dual significance to promote the environmental restoration of ecologically fragile coastal areas and reduce the safety risk of coastal projects.The evolution process of coastal saline soil is affected by the dual effects of seawater intrusion and brackish water irrigation.In this process,there is both nearly horizontal underground salt and fresh water mixing process and vertical soil water and salt transport process,which is fundamentally different from the formation process of inland saline soil dominated by vertical water and salt transport process.Therefore,how to couple the mixing process of variable density salt and fresh water with the process of soil water and salt transport,and clarify the formation mechanism of two-way salinization is the key problem to be solved in the study of coastal saline soil evolution.Taking Xiaoling River alluvial proluvial fan in Jinzhou City,Liaoning Province as the research area,this paper comprehensively uses technical methods such as field hydrogeological survey,indoor soil column water and salt transport simulation experiment and numerical simulation,focusing on the response of soil salinization characteristics to seawater intrusion and the evolution mechanism of soil salinization under seawater intrusion and brackish water irrigation,And the dynamic response characteristics of coastal saline soil site.The main results are as follows:（1）The influence of hydrogeochemical action on the changes of groundwater and soil chemical components in the process of seawater intrusion is explored,and the response law of soil salinization characteristics to seawater intrusion is clarified,which lays a foundation for the simulation of coastal saline soil evolution process.By testing and analyzing the chemical composition of 106 groundwater samples and 248 soil samples,the temporal and spatial distribution characteristics of seawater intrusion and soil salinization in wet season,normal season and dry season are found out,and the evolution law of groundwater chemical composition in the process of seawater intrusion is clarified;On this basis,the temporal and spatial variation characteristics of soil salt and ion components are analyzed by geostatistics,and the response relationship between coastal soil salt and groundwater characteristics is given by correlation analysis and principal component analysis.（2）The simulation experiments of water and salt transport in coastal saline soil under water irrigation with different salinity were carried out,and the water salt coupling coefficient e（c）was proposed to quantitatively characterize the effect of irrigation water salinity on soil water movement.The experimental results show that increasing the salinity of irrigation water moderately will increase the wetting front distance,cumulative infiltration,infiltration rate and soil water content,and vice versa;According to the experimental results,the water salt coupling coefficient e（c）is used to improve the soil water movement equation.At the same time,the HYDRUS model simulating the water salt movement of the experimental soil column is used to verify the reliability and scientifically with the improved equation.The improved soil water movement equation can quantitatively describe the changes of soil water diffusivity and hydraulic conductivity under different brackish water irrigation conditions,It effectively solves the problem that the existing soil water movement equation can not reflect the influence of irrigation water salinity on soil water movement.（3）The coupling between the fluctuating density saturated water and salt transport process of seawater intrusion and the unsaturated water and salt transport process of soil salinization is realized for the first time.A quasi three-dimensional variable density saturated unsaturated water and salt transport coupling model is established,which reveals the evolution mechanism of coastal saline soil under the dual action of seawater intrusion and brackish water irrigation.In the SEAWAT-HYDRUS coupling model,the density effect and tidal effect of the mixing of salt and fresh water in the saturated zone are considered,and the saturated zone is simulated horizontally.The unsaturated zone in each zone is simulated through one-dimensional vertical soil column,and the saturated unsaturated zone is coupled through the vertical flux at the lower boundary of the soil column.The model is identified and verified with the field measured data.The reliability of the model is improved.（4）The influence of precipitation,groundwater exploitation and brackish water irrigation on the evolution of coastal saline soil is quantitatively analyzed by numerical simulation method.Using the coupling model after identification and verification,the evolution laws of seawater intrusion and soil salinization under the influence of diverse factors are simulated.The results show that groundwater exploitation and brackish water irrigation have the greatest impact on the distribution area and average salt content of coastal saline soil,especially under the joint action of seawater intrusion caused by groundwater exploitation and 5g/L and 3g/L brackish water irrigation,The area of coastal secondary saline soil increased by more than 10%,and the average salt content of saline soil increased by more than 15%.（5）On the basis of clarifying the evolution law of coastal saline soil in the study area,the influence of coastal saline soil on site dynamic response is discussed,and the site dynamic response characteristics of saline soil and non saline soil are compared and analyzed.In this paper,drilling sections of typical soil layers in non saline area and saline area are selected,and the corresponding soil layer calculation model is established.Taking El Centro wave as the input wave,the seismic response characteristics of soil layer are calculated by SOILQUAKE program.The results show that when the local vibration intensity is constant,the normalized response spectrum characteristic period T_gof saline soil is slightly reduced compared with that of non saline soil,while the plateau value β_Max increased slightly.