| The Loess Plateau is one of the birthplaces of Chinese culture,and is the region with the most severe soil erosion and geological disasters in China.It is also a key area for the construction of water conservancy project and energy base,ecological protection and restoration in the northwest.Root pores,wormholes,macropores,and joint fissures form the preferential seepage channels of loess,where precipitation,soil particles,and pollutants can quickly migrate to the deep soil through preferential flow.Therefore,conducting research on preferential flow of loess is of great significance for exploring the mechanism of loess seepage,and can provide scientific support for the implementation of ecological protection and highquality development strategies in the Yellow River Basin.The preferential flow of loess appears as a non-equilibrium flow pattern,which is difficult to be quantitatively characterized and predicted at the microscopic scale.With the introduction of material microstructure observation techniques such as photoelectric microscopy,X-ray diffraction(XRD),scanning electron microscopy(SEM),and computer tomography(CT),micro to nano scale loess microstructures and seepage characteristics can be observed.Taking the loess in Lantian County in the Southern Loess Plateau as research object,this thesis studies the characteristics of preferential flow of loess through the field dye tracing test and scanning electron microscope observation,explores the relationship between the preferential flow and the microscopic pore structure of loess.The main research results are as follows:(1)The tracing test dye was optimized,and the ionic dye(iodine-starch)is suitable for preferential flow tracing test in the Southern Loess Plateau(with high clay content).The results of tracing test indicate that: Compared to ionic dye,brilliant blue dye has limitations such as small dyeing range,limited dyeing depth,and lagging behind the movement of water,which cannot reflect the true water flow movement.While ionic dye has better dyeing effects and can well trace water seepage after color reaction.(2)The preferential flow tracing results of 3 test pits and 15 loess sections show that: The types of preferential flow along the depth direction are homogeneous matrix flow,heterogeneous matrix flow and fingering,macropore flow with high interaction,macropore flow with mixed interaction;the preferential flow phenomenon is obvious.Six preferential flow indexes,such as dyeing area ratio,matrix infiltration depth,preferential flow fraction,length index,peak index and coefficient of variation were selected to calculate the preferential flow evaluation indicator for each test pit.The results show that: More infiltration water in loess will lead to greater matrix infiltration depth,higher matrix flow rate,and lower preferential flow ratio;higher initial water content in loess will reduce the preferential flow rate.(3)Loess pores are divided into scaffold pores,mosaic pores and cement pores,the number and proportion of small pores(pore size between 2-8 μm)are the highest.Size parameters such as pore area,porosity,horizontal and vertical lengths,perimeter,major and minor axes,morphological parameters such as abundance,roundness,fractal dimension,and arrangement parameter such as anisotropy rate are selected,and classify the frequency and cumulative frequency.The results show that: Microscopic pore size of loess in the Southern Loess Plateau is small,with moderate morphological regularity and low complexity;microscopic pores are closely related to preferential flow.(4)Selecting microscopic pore parameters for significance testing,and applying principal component analysis to study parameters with significant difference,one principal component is extracted for each of the non-seepage and seepage zones.The results show that: There is a significant difference in the microscopic pore size between the non-seepage area and the seepage area.The pores in the seepage area are significantly larger than those in the non-seepage area.The preferential flow is mainly affected by the microscopic pore size,which controls the initiation of the preferential flow of loess.When the preferential flow starts,it mainly migrates towards relatively large pores.Based on dye tracing test and scanning electron microscope test,combined with statistical testing methods,this thesis studied the impact of loess microscopic pore structure on preferential flow in the Southern Loess Plateau,aiming to provide scientific basis for geological disaster prevention and groundwater pollution mechanism in the Loess Plateau,and provide a new way for microscopic loess hydraulics. |
