| As the main battlefield for the implementation of important economic development strategies such as "Western Development" and "The Belt and Road",the Loess Plateau region has developed rapidly in recent years,and the accompanying engineering problems and geological disasters have always hindered social and economic development.The key to solving the problem is to explore the special properties of loess.Among them,the collapsibility of loess is the most special.Scholars at home and abroad have made a lot of excellent results in exploring the collapsibility of loess.However,the collapsibility process of loess is accompanied by complex The causes of physical and chemical changes are very complex,and more in-depth exploration is still needed.In recent years,with the help of high-precision micro-observation technology,scholars have conducted in-depth research on the collapsibility of loess from a micro perspective,aiming to reveal the mechanism of loess collapsibility in essence,and make contributions to solving engineering problems and preventing and controlling geological disasters.This time,with the goal of studying the microstructure evolution of loess during collapsibility,the micro-soil sample hydraulic-mechanical loading equipment and micro-CT(μ-CT)equipment were used to realize the loess collapsibility test at the micro level,and the micro loess sample was completed at the same time.After multiple scans,scanned images of loess microstructures in the original state,loaded state,and collapsed state were obtained,and then based on the watershed image segmentation algorithm,an iterative segmentation method suitable for complex loess microstructures was improved,and loess particles were realized.Based on the segmentation and extraction,a three-dimensional quantitative model of loess particles was constructed.Based on the quantitative models under different conditions,the dynamic evolution of the loess microstructure during the collapsibility process is quantitatively characterized;the "ID-track" particle tracking technology is used to match the particle parameters,track the same loess particles in the three states,and quantitatively characterize the loess Movement characteristics of particles during collapsibility.It aims to pave the way for the study of loess collapsibility mechanism from a microscopic point of view and provide technical ideas for research.This research has achieved the following main research results:(1)Through the developed micro soil sample hydraulic-force loading equipment,the pressure consolidation of the micro soil sample and the immersion collapse of the micro soil sample are realized,and then the micro CT is used to separately load the same micro soil sample without loading(not immersed in water)and adding water.High-precision scanning is performed under 800 kPa load(not immersed in water)and 800 kPa load(immersed in water),and then a series of image processing is performed on the microscopic scanned image through the three-dimensional visualization system software(Avizo)to realize the segmentation and separation of loess particles and binding substances.Finally,three-dimensional particle models of loess in different states were constructed for subsequent quantitative characterization research.(2)Based on the watershed image segmentation algorithm,this research improved an iterative segmentation method suitable for loess microstructure,which can segment and extract most of the skeleton particles with a particle size greater than 25μm.The particle size distribution of the extracted particles is in good agreement with the analysis results of the laser particle size analyzer in the range of greater than 25μm,and the number of extracted particles is sufficient for the subsequent quantitative characterization of the microstructure and the motion analysis of the particles during the collapse process.The particle size distribution of loess particles in the original state,loaded state and collapsed state is compared and analyzed.The particle size distribution of loess particles below 30μm in the collapsed state is slightly lower than that of the other two states,which may be due to wetness.Some small particles in the sinking state are discarded in the process of segmentation and screening.(3)Based on the constructed three-dimensional particle model of loess,the microstructure characteristics of the same sample under different conditions are quantitatively characterized,and the sphericity and orientation of the particles are mainly studied.The morphology of loess particles can be divided into the following five categories according to the sphericity: flake(0.4-0.5),angular(0.5-0.6),sub-angular(0.6-0.7),sub-spherical(0.7-0.8)and spherical(0.8-0.9),of which about 80% of the loess particles are distributed in the range of 0.6-0.8 sphericity,that is,most of the loess particles appear sub-angular and sub-spherical,and the distribution law remains basically unchanged before and after collapsing.The orientation of loess particles is characterized by the particle direction angle φ.The φ angles of loess particles are distributed from 0 to 90°.About 45% of the particles have φ angles distributed between 70° and 90°,which are nearly horizontal or horizontal;about 40% of The φ angle of the particles is distributed at40°-70°,which is in an inclined state;about 15% of the particles are distributed at 0-40°,which is approximately upright,and the distribution law remains basically unchanged before and after collapsing.Most of the particles have a relatively large φ angle,which tends to be nearhorizontal.The presence of particles with a smaller φ angle causes the uneven arrangement of the loess particles and is easier to form a meta-stable overhead structure.(4)Based on the constructed three-dimensional loess particle model,the "ID-track" particle tracking technology is used to match the parameters of the loess particles in three different states before and after the collapse,so as to realize the dynamic tracking of the same particle in different states,and through tracking The changes in the spatial position of the obtained particles quantitatively characterize the movement characteristics of the loess particles in the process of collapsing.Under the 800 kPa load pressure(not immersed in water),the particle displacement changes more uniformly,and the particle model shows a layering law that gradually decreases the displacement from the top layer to the bottom layer;under the 800 kPa load pressure(immersed in water),the particle displacement is overall Increase,but the particle displacements at different positions in the particle model are different and there is no distribution pattern,indicating that different degrees of collapsibility has occurred in different positions of the sample;quantitatively characterizes the movement of loess particles in the radial and axial directions.Non-uniformity,collapsible state is more non-uniform in particle motion than loaded state. |
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