Capillary Barrier And Breakthrough Effects During Infiltration Process In Layered Unsaturated Unconsolidated Sediments

The cyclical changes of the river,wind and other geologic agents formed a multicycle,coarse alternating with fine bedding architecture.Under the condition of water infiltration,these layered unsaturated loose sediments show some unique hydrodynamic properties due to the abrupt change of the medium sunch as capillary barrier and the capillary penetrating effect that will affect the movement of unsaturated water and contaminants.In this paper,the natural moisture distribution and infiltration test of layered unsaturated loose sediments in Weihe River floodplain and Maowusu sandy land will be studied.On this basis,the infiltration simulation of the river sediments and the corresponding numerica l simulation were carried out and the capillary barrier and penetrating effect of water movement under the condition of line source infiltration were quantitatively evaluated,based of which,the method of numerical model is used to study the canal irrigation method and the layered structure impervious method.The main results are as follows:(1)In the layered sediments of coarse-grained medium covered with fine-grained medium,due to the sudden change of the medium,when the moisture infiltration,the small unsaturated hydraulic conductivity of coarse layer led to wet front movement to the interface,then the vertical movement of water velocity decreased sharply,which resulting in capillary barriers,so that the water will preferentially select the crossflow and storage in the fine-grained medium.(2)From the consideration of soil and water characteristic curve(SWCC),the left extreme point of the SWCC curve curvature of the normalized coarse grain layer becomes the quantitative evaluation key index of the capillary barrier breakthrough.The water content of the coarse layer began to increase significantly resulting in a rapid increase in the unsaturated hydraulic conductivity,and the emergence of a recognizable wet front begins to move in the coarse layer at a rapid path,which is the macroscopic phenomenon of capillary penetrating.(3)The type of media and the initial moisture content are important factors in controlling capillary barriers and penetrating effects.The initial moisture content is related to the coarse-grained medium in the stage of normalized SWCC.When the initial moisture content is less than the extreme left point of the curvature,the capillary barrier phenomenon occurs.(4)The design of the furrow in the layered sediments and the layered medium impermeability can be carried out by using the capillary barrier and the penetrating effect principle.In the design of furrow irrigation,the water of horizontal movement is much larger than the vertical movement in the layered soil,The distance between the two adjacent ditches is designed to be 20 m to meet the needs of field irrigation.In the design of layered media,capillary barrier seepage effect is significant under the line strength is less than 1m/d,and the evapotranspiration had little impact on the capillary barrier effect with 1 m/d flow rate.