Study On The Law Of Water Migration Under The Rainfall Infiltration Based On Flux-concentration Relation

Rainfall is often one of the main factors that cause the subsidence of subgrade of railways and highways and the instability of slopes.It is undoubtedly helpful to investigate the settlement of subgrade,the changing mechanism of bearing properties and the stability analysis of slope under rainfall conditions to make clear the migration law of water infiltration to soil.Based on the flux-concentration relation,the water content distribution model of the soil vertical profile and the wetting front push-down model at each stage of the whole rainfall process under two types of rainfall conditions(rainfall intensity R is greater than and less than the saturation permeability K_s)are derived.The numerical simulation method was used to verify the applicability and accuracy of the two types of water movement models for sandy loam,loam and silt,and the evolution law of the water content distribution and the wetting front push-down depth to the three types of soil is discussed under different rainfall intensity conditions.Aiming at the proposed two types of critical points,the reasonable range is analyzed and the trend of the critical points and point to ponding with the variation of rainfall intensity is discussed.Based on the existing laboratory experimental data,the rationality of the infiltration process in each stage under the condition that R is greater than K_s is further verified,and the accuracy of the water content distribution model under two types of rainfall intensity is checked.The main conclusions are as follows:(1)Three types of soil(sandy loam,loam,and silt)were selected,and numerical simulations were used to verify the water content distribution model derived under the two types of rainfall.The calculation results of three types of soil all showed high accuracy under R<K_s conditions.For the condition of R>K_s,the calculation accuracy of sandy loam,loam,and silt in successive stages of rainfall infiltration is decreasing,but the overall maximum error can be controlled within 2%,the model is suitable for a wide range of soils.(2)Under the condition that the rainfall intensity is less than the saturation permeability(R<K_s),the water content decreases slowly with the depth,and decreases suddenly near the wetting front.the soil water content profile is divided into a stable layer(after the first type of critical point)and a transition layer And dry soil layer.Under the condition that the rainfall intensity is greater than the saturation permeability(R>K_s),the water content of the soil profile sequentially shows the saturated layer(after the point to ponding),the transition layer and the dry soil layer from top to bottom.with the increase of rainfall intensity during the same rainfall duration,the distribution of water content in the soil profile is the same,and it is not sensitive to rainfall intensity.(3)Under the condition of R<K_s,according to the changing trend of the water content in the upper boundary,the whole process of rainfall infiltration is divided into two stages of non-compressive infiltration transition and non-compressive infiltration stabilization by using the defined first type of critical point.In the first stage,the push-down depth of the wetting front increases nonlinearly with the rainfall duration,while a linear increasing trend in the second stage.that is to say,there is a critical line,which divides the push-down process of the wetting front into non-uniform speed and uniform speed.The push-down depth each stage is obviously affected by the rainfall intensity.After reaching the critical line,with the increase of rainfall intensity,the wetting front advances at a uniform speed,the depth is deeper and the advancement speed is faster in the same rainfall duration.Under the condition of R>Ks,according to the point to ponding and the defined second type of critical point,the curve of infiltration rate can be divided into three stages,the non-compressive infiltration with unsaturation,the compressive infiltration with transition and saturated;the first and second stages of the wetting front push-down depth both increase nonlinearly with the rainfall duration,and the third stage shows a linear increasing trend.That is to say,the push-down process of wetting front is divided into three stages:fast,slow and uniform speed according to the critical line and t_s line;The push-down depth of the wetting front is only related to the saturation permeability,while it is not sensitive to rainfall intensity after reaching the ts line.(4)The rationality of the existence of two types of the critical point,the point to ponding and rainfall infiltration stages is proved by the combination of numerical simulation and existing experiment.With the increase of rainfall intensity,both the critical time and time to ponding show a nonlinear trend of shortening,and the first and second critical time shorten increment,time to ponding shorten increment show a trend of decreasing in turn.The thickness of the soil layer is the main influencing factor of the distinction between the second type of critical point and saturation point.Under existing experimental conditions,it is found that the saturation point of the soil layer greater than 80cm is equivalent to the second critical point,and does not change with the thickness of the soil layer.(5)Based on the existing experimental data of the soil water content profile under two types of rainfall intensity,the rationality of water content distribution model is further verified,and it is considered that the data monitored using sensors can't really reflect the soil water content profile when the wetting front enters the effective range of the monitoring point.