Research On Micro And Meso Void Structure And Preferential Flow Characteristics Of Loess Based On CT

Loess is a kind of aeolian sediment in arid and semi-arid regions,which has a loose and porous,developed columnar joint and weakly cemented metastable structure.It exhibits special physical mechanics and hydraulic properties such as collapsibility and water sensitivity,which leads to the increasing difficulty of engineering construction,the frequent occurrence of geological hazards,and the restriction of the rapid economic development and ecological environmental protection of the Yellow River Basin,especially the Loess Plateau.With the wide application of experimental techniques such as photoelectric microscope,microcomputer image,mercury intrusion method,scanning electron microscope,X-ray diffraction(XRD),X-ray computed tomography(X-ray CT)scanning,etc.,the research including quantitative detection and advanced characterization on the void structure of loess has made important progress and become the hot spot of loess mechanics research.Structures such as macropores and fissures provide dominant seepage channels for rainfall and irrigation infiltration in the loess.Hence,the multi-dimensional and multi-scale CT detection of the macropores and fissure structures(pore size,pore shape,spatial distribution,connectivity,water conductivity,etc.)of the loess and paleosol,pore classification and recognition,digital modeling,and research on the preferential flow effect of voids are of great significance for exploring the macroscopic seepage characteristics of the loess and the mechanism of pore compression deformation.It is a new subject urgently needed to be studied in the hydraulics of the loess.Under the support of the National Natural Science Foundation's key project "Study on the mechanical mechanism of loess slope catastrophe at multi-scale(Micro-meso-macro)under the environmental influence"(No.: 41630634),this thesis takes the undisturbed loess in the Southern Tableland of Jingyang County,Shaanxi Province as the research object.Based on literature review,field investigation and section selection,samples of undisturbed loess and paleosol were collected.First,nondestructive CT testing technology was used to digitally model and visually analyze the three-dimensional(3D)void structure of loess at the micro and meso(10.61um)scale to extract characteristic parameters of the loess structure;then dynamic contrast CT scan of the loess samples under humidified conditions was conducte0519 d to explore the two and three-dimensional distribution of water and the change of the void structure under dry and wet conditions;finally,the percolation theory is used to perform the mesoscale research on the characteristics of the preferential flow pathway of loess.The study established a macropore network model of the loess,systematically studied the flow characteristics of preferential flow seepage,and is expected to provide a scientific basis for the study of the characteristics of the preferential flow of the loess and the mechanism of landslide disaster promotion.The main innovations achieved by the thesis are as follows:(1)By using industrial CT,non-destructive scanning experiments of Malan loess(14 groups),paleosol(6 groups)and Lishi loess(3 groups)were conducted to obtain a series of continuous slice images.The processes of image filtering and enhancement,image thresholding segmentation,and three-dimensional visualization reconstruction were conducted on the AVIZO software system to establish a three-dimensional quantitative spatial structure model of loess.The above process procedures solved the technical problem of three-dimensional classification and quantitative characterization of the micro-scale structure of prefrential flow storage pores and water conducting channels.(2)From the macropore structure model of loess,the spatial geometric characteristic parameters such as two-dimensional equivalent diameter,shape factor,aspect ratio in XY,XZ and YZ directions and three-dimensional pore dip angle were extracted respectively,which proves that loess is a kind of porous geological structure with obviously heterogeneousity and spatial anisotropy.The pores with water conductivity show strong vertical connectivity,which determines the vertical permeability is greater than the horizontal direction;According to the shape factor of the voids,the two-dimensional loess pores were divided into columnar pores,elliptical pores and circular pores.The three-dimensional pores are divided into bifurcated pores,cylindrical pores,ellipsoidal pores,spherical pores and micro-fissures;3D equal diameter,porosity and effective porosity,tortuosity and fractal dimension are the key factors that determine the loess seepage characteristics.(3)In order to solve the technical problem of preferential flow motion observation of micro and meso scopic voids,we independently developed the “Preferential Flow Dynamic Observation Device and Method for CT Scanning”.When using the device,place the loess in a visualized container,and continuously inject potassium iodide(KI)contrast agent with a concentration of 60g/L into the seepage chamber through the liquid adding device.CT can clearly and non-destructively scan the pore changes inside the loess sample and the seepage path filled with the contrast agent.Through spatial correction of 3D dynamic image data,the flow field change law in the seepage process was comparatively analyzed,the maximum wet front depth of the contrast agent during the seepage process were measured,the distribution area and volume on the 2D section and 3D space were calculated,etc.The above measurements can effectively reveal the movement of preferential flow in loess.(4)The results of seepage dynamic monitoring show that:(1)the water infiltration in Loess preferentially flows along the macropores and also makes the original isolated pores become connected channels with the increase of water content,forming a larger water percolation group;(2)The water infiltration process presents a "preferential flow-piston flow-mixed flow(preferential-piston flow)-piston flow" seepage evolution mode;(3)the water seepage can transform the microscopic pore structure,which determines the variability of interstitial loess pore structure under seepage and external pressure.(5)The order of the percolation threshold of various types of loess is Malan loess(0.562%)< paleosol(0.611%)< Lishi loess(3.202%),which indicates that Malan loess has the best permeability.On the micro scale of pore structure,paleosol has stronger permeability than Lishi loess.However,in macroscopically,the permeability coefficient of Paleosol is often smaller than Lishi loess,reflecting the difference in endowment of macroscopic and microscopic view of paleosol seepage.(6)The results show that among all the throats there are 17.51% being non flow throats,26.35% being the low permeability,36.15% being the stable seepage ones,and 21.99% being preferential flow channels.(7)By grouping statistics of pore radius in different Reynolds numbers,it is found that when Re <2300,the minimum pore throat radius in the laminar flow range is 6.95?m,and the average pore throat radius is 53.29?m;when Reynolds numbers range in 2300?4000,that is,2300?Re <4000,the pore throat has a minimum radius of 83.74?m and an average of 207.31?m,at the same time,the flow state of pore throats of this size is no longer laminar and begins to transition to turbulent flow;when Re?4000,the minimum radius of pore throat is of 128.50?m and an average of 250.45?m,the flow pattern of this part of the pore throat becomes turbulent completely.