| The Songnen Plain is a crucial region in China for grain storage and urban development.In recent years,climate change has caused significant alterations in the region’s water and thermal resources,resulting in more frequent and intensified droughts and floods,leading to increasing salinization of the soil in the area.To restore and improve the regional ecological environment and ensure national food security,the Jilin Western River-Lake Connectivity Project was launched comprehensively in 2013.However,during processes such as precipitation,snowmelt,river-lake replenishment,and continuous evaporation,drying,freezing,and thawing,complex situations such as repeated healing and cracking of soil cracks on the slopes of water conveyance projects have emerged.These conditions provide favorable conditions for the infiltration of surface water into the soil during water storage,conveyance,snow melting,or rainy seasons.Additionally,the soil in this area exhibits high dispersibility characteristics,and the cracks provide channels for water infiltration.Under the combined effects of water infiltration and soil dispersibility,significant erosion damage has occurred on the main channels and slopes of water conveyance projects,severely affecting the stability and durability of the channel slopes.This paper focuses on the study of dispersive cohesive soils in the Qian’an area of the hinterland of the Songnen Plain.Through controlled indoor experiments manipulating single variables,the characteristics of soil cracks under varying parameters of cohesive soil samples and environmental conditions are observed.Based on quantitative analysis of crack parameters,the paper explores the influence of various factors on the development and expansion of cracks,as well as their underlying mechanisms.Combining the climatic characteristics of the study area,experiments on the spatial evolution of soil cracks during dry-wet cycles and dry-wet-freezing-thawing cycles are conducted.The development characteristics of cracks are inferred based on the parameters of soil properties at different sample locations,and validated by CT test results.Finally,a numerical model of crack development is established using the results of three-dimensional crack experiments through the discrete element method PFC3 D software.This model is used to analyze the characteristics of cracks at different stages of cycling and different positions in the soil layers,and discuss the influence of soil properties on the process of crack development.The main research contents and findings are as follows:(1)Through field investigations and laboratory experiments,significant phenomena such as surface salt accumulation and the development of dry cracks in the surface soil layer of the study area have been confirmed.The area is characterized by extensive erosion damage caused by water erosion,including gully erosion,rill erosion,and erosion-induced collapse.The particle size distribution of the soil in the study area is dominated by fine particles,indicating predominantly low liquid limit clay(CL).The primary indigenous mineral is quartz,while the predominant clay mineral is illite-smectite mixed layer.The soil samples exhibit a relatively high salt content,rendering the soil weakly alkaline and classifying it as carbonate saline soil.Moreover,the soil samples demonstrate pronounced dispersibility,sensitivity to moisture,and strong adsorption characteristics.(2)Soil samples were prepared by controlling parameters such as sample surface area,thickness,and compaction degree.Initial samples with different compaction degrees underwent tests for pore structure,shrinkage characteristics,matrix suction,capillary rise,and expansion under different conditions.Observations of crack development during single drying processes for each sample were conducted,and the effects of various sample factors on the drying cracking process and crack parameters were analyzed.Four stages of crack development during drying were identified: initial stage without cracks,development stage of crack length,stage of crack width variation,and stable stage of cracks.The sample surface area had minimal influence on cracking time and crack ratio.The larger the soil thickness,the slower the drying ratio,corresponding to larger crack widths and intact soil block areas.Greater compaction resulted in slower drying ratios for samples,leading to fewer crack segments,crack intersections,surface shrinkage rates,crack widths,and intact soil block counts after drying.(3)Based on the analysis of the effects of sample preparation factors on crack characteristics,circular samples with a compaction degree of 90 %,a diameter of 39 cm,and a thickness of 2 cm,as well as rectangular samples measuring 20 cm in length,1 cm in width,and 2 cm in height,were prepared.Different degrees of drying conditions were then applied to observe the evolution of cracks on the plane and longitudinal sections.The influence of different drying conditions on crack development during WD cycles and WDFT cycles was analyzed in conjunction with microscopic structural features of the samples.The results showed that with increasing drying degree,the primary crack ratio increased with each WD cycle,while the plane crack ratio and crack width decreased,and the cross-sectional crack ratio increased.When the drying degree was low,cracks mainly developed into numerous dense sub-cracks,and the plane and cross-sectional crack ratios remained relatively constant after each cycle.The freeze-thaw process stabilized the structure of samples with low water content but caused dispersion and reorganization of particles and aggregates in samples with high water content.The double-sided effect of the freezing-thawing process resulted in different final crack characteristics of the samples after multiple WDFT cycles compared to the WD process alone.(4)Cylindrical samples with a height of 20 cm and a diameter of 10 cm were prepared to undergo unidirectional WD cycles and WDFT cycles,allowing for the study of spatial distribution characteristics of cracks in deep soil layers.Observations of crack status on the top and side surfaces of the samples were made,and real-time monitoring or stratified sampling tests of water content,temperature,electrical resistivity,particle composition,and permeability of different depth soil layers were conducted to assess crack development at each stage.Using CT scanning technology,the three-dimensional characteristics of cracks after 10 cycles were obtained,and the crack development results based on soil properties were verified.Cracks initiated from the top surface during the drying process and gradually extended downward.As WD cycles progressed,shallow cracks healed during saturation,while deep cracks did not fully heal during saturation.The plane crack ratio decreased with increasing depth.In WDFT cycles,horizontal freezing expansion enhanced the horizontal expansion capability of cracks,resulting in deeper cracks.(5)Based on the crack development process determined by indoor experiments and the crack morphology characteristics obtained from CT tests,a numerical model of crack development in dispersive cohesive soil under WD cycles and dry-wet-freeze-thaw cycles was established.The model simulated crack characteristics of dispersive cohesive soil in the Qian’an region under WD cycles and WDFT cycles.The reliability of the model was verified by comparing the crack depth and crack distribution results of each soil layer with the experimental results.The model monitored particle contact quantity,coordination number,and properties of normal stress in various directions of each soil layer to explore their relationship with crack development and evolution. |
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