Mechanism Of Action Of Interlayers In Reconstructed Soil Filled With Yellow River Sediments And Its Simulation

Coal is the primary energy source in China.Underground coal mining causes land subsidence,a large area of cultivated land is destroyed.Cultivated land is the basic resource for human survival,in the 21st century,the contradiction between human and land has been gradually intense.Filling reclamation technology is an effective means to restore large area of mining subsidence land to cultivated land.Filling reclamation of mining subsidence land with the Yellow River sediment can not only dredge the sediment deposited in the lower reaches of the Yellow River,but also achieve the comprehensive improvement of land and increase the number of cultivated lands,and it gives significance in social and ecological benefits.However,the Yellow River sediments belongs to the sand,and the sand content is nearly 80%.If the traditional“soil-sand”double-layer profile design is adopted with the covering soil thickness of mulch soil,the reconstructed soil profile leaks water and fertilizer severely,and cause the low quality of reclaimed farmland.As for some mining areas with limited soil resources,the application of the technology is limited.The Yellow River interlayer filling reclamation technology can effectively improve the quality of reclaimed farmland in the condition of limited soil resources.At present,the mechanism of the interlayer soil profile is rarely involved,especially the influence of the location,thickness and quantity of the clay layer in the sediment layer of the Yellow River on the water and nutrient transport law of the reconstructed soil profile,which has not received enough attention and in-depth research.Based on the knowledge of land reclamation and ecological reconstruction,soil science,soil physics and soil hydrodynamics,and experiments of the indoor soil column infiltration and evaporation experiments,reveals the Yellow River sediment sandwich type filling reclamation characteristics and law of soil water and solute transport has been clear about the sandwich type reconstructed soil with different interlayer position,thickness and quantity relationship between the ability of water and solute transport parameters,clarified the intercalation mechanism,optimize the sandwich method.Through the experiment of solute transport in soil column,the effect of different position,thickness and quantity of soil layer of the Yellow River sediment layer on the non-adsorption Cl^-and adsorption K⁺was analyzed.The influence of solute transport law reveals the mechanism of soil solute transport in the Yellow River sediment interlayer reconstruction.The model of soil water movement and solute transport is established by Hydrous,the model parameters are optimized according to the actual test,the optimal parameters are used to simulate and calculate the soil water and solute transport characteristics of the interlayer filling reclamation soil,and the optimal construction to conserve water and fertilizer is predicted.The main conclusions are as follows:（1）In order to solve problem in the uncertainty of the volume density of the Yellow River sediment filling reclamation,the optimization of the volume density of the Yellow River sediment filling reclamation in the mining subsidence land was carried out based on the water characteristics.It is suggested that in the practice of filling and reclaiming the mining subsidence land with the Yellow River sediment,the reasonable mechanical compaction should be carried out to control the bulk density to 1.5g/cm³,and the capillary porosity of the Yellow River sediment should reach the maximum,which can improve the water holding capacity of the Yellow River sediment to a certain extent.（2）The water infiltration characteristics of“soil sand”double layer soil profile can be improved by the Yellow River sediment interlayer soil profile.The reconstruction soil infiltration of the Yellow River sediment is an unsaturated water movement process,and a typical“finger flow”phenomenon will be formed in the“soil sand”double-layer profile construct,after filling the sandwich soil layer in the sediment layer of the Yellow River,the movement of the wetting front is restrained and the water holding capacity of the infiltration process is improved.The transport velocity of the wetting front in the reconstruction soil profile decreases with the increase of the interlayer thickness,and it decreases first and then increases with the increase of the position and number of the interlayers;the water holding capacity of the infiltration process increases with the increase of the interlayer thickness and number,and it increases first and then decreases with the increase of the position of interlayer,however,when the number of interlayers change from 2 to 3,the water holding capacity increases dimly.（3）The Yellow River Sediment interlayer soil profile can improve the evaporation characteristics of“soil-sand”double-layer soil profile,and shorten the duration of the first stage of evaporation,and reduce its relative evaporation rate in the second stage.In the process of evaporation,the water content of the Yellow River sediment layer in the filling layer decreases firstly,while the moisture content of the subsoil interlayer keeps high all the time,with the increase of the quantity in interlayer,the coefficient of variation of the water content distribution of the soil profile increases obviously after evaporation.The volume water content of filling layer θ_f1 with different sandwich structure design is increased obviously,compared with the other two factors,the change of interlayer position has no significant effect on the value ofθ_f1.The water retention coefficient C_WRof each interlayer treatment has also been significantly improved,and the interlayer thickness has the most obvious effect on the improvement of water retention of reconstructed soil profile.However,in the vast majority of mining subsidence land reclamation practice,the amount of soil available for use is limited,so the interlayer quantity can be properly adjusted to make up for the lack of interlayer thickness.（4）Compared with the“soil-sand”double-layer soil profile,the Yellow River sediment interlayer soil construction can improve the penetration time of solute,the retardation and the fertility,and reduce the average pore velocity in the reconstructed soil.The breakthrough time of K⁺is longer than that of Cl^-,and the tailing phenomena of the breakthrough curve is obvious under the adsorption of soil matrix.Convection is the main transport mode in the process of Cl^-transport,and K⁺transport is affected by chemical nonequilibrium in some cases.The location,thickness and quantity of interlayer were significantly correlated with the solute transport parameters（P<0.05）.Regression analysis shows that the average pore velocity（V）,dispersion coefficient（D）and retardation factor（R）is obviously linear to the interlayer thickness,which can be described by a certain linear expression,but the linear relationship with the interlayer position is not obvious.The binomial relationship with the interlayer quantity is full,when the interlayer thickness is certain,it is not the case that the more the interlayer quantity is,the better,but an optimal quantity of layers.（5）The model of soil water movement and solute transport was established by using Hydrous.According to the indoor experiments of infiltration,evaporation and solute transport,the model parameters of soil water and solute were inversed and calibrated.The results show that the optimized model parameters can simulate the process of soil water and solute transport of the Yellow River sediment reconstruction.The model simulation makes up for the shortage of physical simulation because of its strong adaptability and short simulation period.In order to optimize the soil profile construct according to the available soil quantity in the mining subsidence area along the Yellow River,an economic and fast simulation method is proposed.（6）This paper simulates the situation in a total thickness of the interlayer reconstruction soil is 60 cm,in which the soil water movement characteristics and solute transport characteristics under the interaction of interlayer thickness location and quantity are considered.The effects of different treatments on soil water and solute transport are analyzed,and the optimal profile of the interlayer reconstruction soil to conserve water and fertilizer is predicted.The non-adsorption ion and water transfer law are consistent,which is easy to leach with the water,therefore,in the actual irrigation and fertilization process,In the infiltration process,the concentration of K⁺changes diminutively,and in the case of irrigation amount of 15.3L,there is no difference in each treatment.K⁺is mainly distributed in the soil layer of more than 30 cm.Based on the water holding capacity of the two stages of soil water infiltration and evaporation,the water and fertilizer holding capacity of the reconstructed soil of the Yellow River sediment is quantitatively evaluated.The interlayer thickness is the main influencing factor,its position has the least influence on the water holding coefficient,and the combination of the interlayer position and the quantity of interlayer has the strongest interaction effect.Finally,when the interlayer thickness is 30 cm,located in 20 cm,and the quantity of the interlayers is two,the effect of water and fertilizer conservation is the best.In this paper,based on a lot of actual measurement and test are discussed from the point of view of soil water and solute transport in the Yellow River sediment filling reclamation soil intercalation structure mechanism,and combined with numerical simulation,proposed the coal mining subsidence land reclamation in the Yellow River sediment sandwich type optimization design method of soil profile reconstruction,has important theoretical and practical significance.However,the exploration of its mechanism is limited to indoor physical test and model simulation calculation at present.In the future,it is necessary to comprehensively analyze the transfer law of soil moisture and nutrients in“the field plant-soil-atmosphere”,so as to further study on the mechanism of the Yellow River sediment interlayer filling reclamation of mining subsidence land.