| Due to the influence of geological movement,biology and human activities,the natural soil will present layered structure.As an important soil and water conservation engineering measure in the Loess Plateau area,a layer of sediment with a grain size of lower coarser and upper finer will be formed in the dam every time during the process of blocking flood and sediment.Therefore,the soil profile of the dam has a heterogeneous multi-layered structure.The heterogeneous multi-layered soil structure of a silting dam will affect the vertical distribution and movement of infiltration water in the soil profile.It is of great theoretical and practical significance to elucidate the infiltration process and the infiltration mechanism of heterogeneous multilayer soil in the land of silted dam for studying the vertical recharge mechanism of groundwater and developing agriculture in the land.In this study,field investigation was carried out on dam No.3 of Wangmaogou Embankment,a tributary embankment embankment,located in the first sub-district of the loess hilly-gully region,and heterogeneous multi-layered undisturbed soil column was collected.On the basis of measuring the basic physical and chemical properties of soil sample and soil moisture characteristic parameters,The infiltration process and infiltration mechanism of the undisturbed multi-layered soil in dam under different layers and configurations are investigated through the laboratory test of constant head water infiltration of heterogeneous multi-layered soil column and the control test of packed disturbed homogeneous soil column infiltration.The one-dimensional vertical infiltration process of heterogeneous multi-layered soil under different experimental conditions was simulated by the Hydraus-1D software,and the applicability of existing multiple infiltration models to the unaltered heterogeneous multi-layered soil infiltration process was verified by the measured data of constant head water infiltration test.The main research achievements of this study are as follows:(1)The water infiltration characteristics of heterogeneous multi-layered undisturbed soil in the dam land of the silting dam are expounded.The results show that with the increase of the number of heterogeneous layers,the cumulative infiltration time of wetting front to the same interface will be delayed.The overall downward velocity of wetting front from large to small is in the order of homogeneous soil column>heterogeneous single-layered undisturbed soil column>heterogeneous two-layered undisturbed soil column>heterogeneous three-layered undisturbed soil column.Compared with the homogeneous soil column,there is little difference in the wetting front migration process at the initial stage of infiltration,but with the passage of infiltration time,the wetting front migration speed decreases more significantly.In the stage of rapid increase of soil water content,the soil water content at each soil depth of the undisturbed soil column and the undisturbed soil column showed a rapid and linear increase trend.The soil water content at each soil depth of heterogeneous two-layer undisturbed soil column and heterogeneous three-layer undisturbed soil column showed a trend of rapid increase at first and gradually decreasing increase per unit time.The change rate of soil water content at the same depth of the soil profile and the cumulative infiltration time required for soil water content to reach saturation water content are in the order of homogeneous disturbed soil column>heterogeneous single-layered undisturbed soil column>heterogeneous two-layered undisturbed soil column>heterogeneous three-layered undisturbed soil column.The infiltration process of heterogeneous multi-layer undisturbed soil column test can be divided into three stages:transient infiltration,gradual infiltration and stable infiltration.When the infiltration time is the same,the cumulative infiltration amount of each test is from large to small in the order of heterogeneous single-layer undisturbed soil column test>heterogeneous double-layer undisturbed soil column test>heterogeneous three-layer undisturbed soil column test.The cumulative infiltration amount of homogeneous soil column is greater than that of heterogeneous two-layer undisturbed soil column with the same height.(2)The vertical one-dimensional infiltration of constant water head in heterogeneous multi-layered undisturbed soil column and homogeneous disturbed soil column was simulated using Hydrus-1D software.Based on the LM method optimization algorithm,the numerical inversion of soil water characteristic parameters was carried out using Hydrus-1D software.The simulation results show that there are great differences between the simulated curve of soil water content change at different depths of the three heterogeneous multi-layered soil columns obtained by Hydrus-1D software and the measured curve,and the simulation results of vertical one-dimensional infiltration of constant water head in homogeneous disturbed soil columns are relatively good.The R2 values simulated by the two initial parameter methods reached 0.92 and 0.93,respectively.Therefore,when the initial parameters obtained by the two methods are applied to Hydrus-1D,without considering the heterogeneous multi-layered structure of the soil in the dam land,the uneven vertical distribution of soil texture and porosity in each layer,and the influence of the silting dense layer at the top of each layer on infiltration,It will not be possible to simulate the actual infiltration process of heterogeneous multi-layered soil in the dam site.According to the numerical inversion results,except for the large difference between residual water content θr and the relevant parameter α of air intake suction,when the parameter obtained by fitting soil water characteristic curve with method 1 RETC is the initial parameter,Using Hydrus-1D software,the θr values and α values of different layers of non-homogeneous three-layer undisturbed soil column in the dam are greater than the inversion results when the neural network prediction parameters are used as the initial parameters in method 2.(3)The applicability of four existing infiltration models in the simulation of heterogeneous multi-layer soil infiltration in the dam land of the silting dam is verified,and the internal reasons are analyzed.The research results show that due to the shortcomings of each infiltration model and the heterogeneous multi-layered structure of the soil of the dam,the fitting effect of Philip model,Kostiakov model and Horton model on the change curve of infiltration rate in the vertical one-dimensional infiltration process of the single,second and third layer constant water head in the dam is generally poor.The mean R2 values are 0.83,0.88 and 0.71,respectively,so they are not suitable for the one-dimensional vertical infiltration simulation of constant water head in heterogeneous multi-stratified soil of the dam.The general empirical model has a large deviation in fitting the gradual infiltration stage of heterogeneous multi-stratified soil of the dam.Compared with the other two models,the R2 fitting curves of Kostiakov model and general empirical model are 0.95 and 0.97,respectively,and the results are relatively good. |
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