Study On Characteristics Of Preferential Flow In Three Typical Filling Slopes Of Heda Highway

As a common form of soil water movement,preferential flow is widely used in water and pollutants transport in field soil environment.In recent years,with the increasing number of development and construction projects,the pollution of groundwater and the damage to the environment are becoming more and more serious.Exploring the law of water movement under artificial natural disturbance is particularly important.The highway slope shows heterogeneity,which water and solute move in a way of nonequilibrium flow.The introduction of preferential flow to analyze the rapid transport mechanism of water and solute in highway slope has great significance to reveal the transport law of pollutants in the slope.In this study,the low subgrade slope near Kuandian charging station of Heda highway was taken as the research object.The characteristics of preferential flow in the slope were analyzed by dyeing tracer test,and the main factors affecting the occurrence of preferential flow were also discussed.Combined the indoor dyeing tracer test with soil column breakthrough test,and using image analysis,mathematical statistics analysis,O-ring statistical analysis,multi-index comprehensive evaluation,fractal dimension and time moment method to analyze the dyeing morphology characteristics of preferential flow,preferential flow paths spatial distribution pattern,fast transport characteristics of water and solute,preferential flow degree in 3 kinds of common filling type slopes.A systematic study has been done on the characteristics of preferential flow in Heda highway slopes.The results were summarized as follows:(1)There was a very obvious preferential flow phenomenon in low subgrade highway slope.Analysis on dyeing images of horizontal profile showed that the dyeing area ratio decreased with the increase of soil depth.Analysis on vertical profile images showed that the preferential flow mainly distributed in 10-40cm soil layer.Surface 10cm soil layer was matrix flow area and the distribution of solute transport was uniform.The change of the subgrade flow type expressed as "homogeneous matrix flow—macropore flow" based on SPW.With the increase of soil depth,the total number of preferential flow paths decreased and the number of preferential flow paths with same influence radius also decreased.In the same soil layer,the number of preferential flow paths with small influence radius is much larger than paths with large influence radius.Preferential flow paths with small influence radius were dominated by random distribution,and the larger ones were mainly composed of aggregated distribution.The larger the influence radius is,the more obvious the aggregation distribution is.The correlation between preferential flow paths with different influence radius is relatively independent.Soil bulk density,porosity and mechanical composition are the main influencing factors of preferential flow in slope.(2)There are significant differences in the morphological characteristics and the spatial distribution characteristics of preferential flow paths among three kinds of filling slopes.Vertical profile images showed that preferential flow differentiation degree was high in break stone soil and fine gravel soil slopes.Development degree of matrix flow area was high and preferential flow differentiation was low in fine sand soil slopes.Six preferential flow characteristic parameters can represent the difference of three filling soil slopes.The horizontal direction changes of preferential flow dyeing patterns was"multi-peak" types in break stone and fine gravel slopes,and "homogeneous" types in fine sand slopes.The vertical direction changes of preferential flow dyeing patterns were complex in break stone slopes,which showed "five section" types.Fine gravel and the fine sand slope were "three section" types.The trend of flow process was "homogeneous matrix flow-large pore flow" in break stone and fine gravel soil slopes,and"homogeneous matrix flow-finger flow-large pore flow" in fine sand.The total number of preferential flow paths was break stone>fine gravel>fine sand.The distribution pattern of preferential flow path was random in break stone and fine grave slopes,and aggregated in fine sand slopes.(3)The results of soil column breakthrogh test showed that the penetration of Cl-first appeared in break stone soil columns,followed by fine gravel soil columns and sand soil columns.There was a preferential flow phenomenon in each treatment soil column and the outflow rate was unstable.The breakthrough curves of 3 kinds filling soil columns were left lateral asymmetric curves with trailing.The two region model in the CXTFIT 2.1 software was used to simulate and measure the breakthrough curve with good fitting effects.The relevant parameters were also corrected.(4)According to extraction of preferential flow parameters,preferential flow indicator PFI was calculated by multi-index comprehensive evaluation method,which showed break stone(0.97)>fine gravel(0.46)>fine sand(0.05).The fractal dimension of preferential flow wetting front curve was calculated,which showed break stone(1.343)>fine gravel(1.240)>fine sand(1.206).According to the simulation results of soil column breakthrogh test,PFSP was calculated by time moment method,which showed break stone(314.07%)>fine gravel(151.97%)>fine sand(99.72%).The results obtained by the three methods were the same.It is also consistent with the previous qualitative analysis,which were break stone>fine gravel>fine sand.