Study On Soil Shrinkage Mechanical Properties And Cracking Boundary Effects

During the evaporation process,the soil will undergo shrinkage deformation and cracking,forming a dry shrinkage network in the soil.The development of cracking will destroy the structure of soil,change the physical and mechanical properties of soil,reduce the strength of soil,and increase the permeability of soil.It is a direct/indirect cause of many engineering geology and environmental geological problems.The soil in the natural is affected by the depositional environment.Different layers often have significant differences in thickness and material composition,and their shrinkage deformation behavior and crack development will also exhibit different characteristics.Therefore,it is of great significance to study the shrinkage deformation characteristics and mechanical mechanism of soil in the evaporation process,and to analyze the development process of the soil under the different boundary conditions,and to further master the soil engineering properties of the arid climate and guide the engineering geological disaster prevention and disaster reduction.In this paper,Xiashu soil and Pukou expansive soil in Nanjing area were used as test materials.Drying experiments were carried out under the conditions of controlled temperature and relative humidity.The effects of relative humidity and soil thickness on soil evaporation characteristics were explored.The change law of soil shrinkage stress with time and moisture content was tested.In addition,combined with digital image processing technology,the variation rule of soil fracture total length,average width,number of nodes,shrinkage rate and number of cracks with time and moisture content was obtained by taking regular photographs and weighing the sample.The effects of different soil thickness and interface roughness on the characteristics of soil cracks are described,and the boundary effect of soil cracking development is explained.The paper is completed under the support of the National Excellent Youth Science Foundation(41322019)and the National Natural Science Foundation Project(41572246,41772280)of Professor Tang Chao-Sheng.The paper has mainly obtained the following achievements:(1)The soil thickness and relative humidity have an impact on the evaporation rate of the soil,the duration of the evaporation stage,and the residual moisture content after stabilization.The influence of thickness on the evaporation characteristics of soil is obviously phased,and there is no obvious effect on the evaporation rate in the constant rate stage,but it has an influence on the duration of each stage.In the same evaporation period,the smaller the sample thickness,the lower the moisture content,and the less time required to reach the residual evaporation stage.The evaporation process of soil is very different in different relative humidity.With the decrease of relative humidity,the soil evaporation rate gradually increases,the shorter the time at the constant rate evaporation stage,and the lower the residual water content of the soil after evaporation is completed.(2)The shrinkage stress can be generated during the shrinkage process of the soil,and the shrinkage stress increases with time,but eventually becomes stable.The shrinkage stress increases with the decrease of moisture content,and the variation trend is different in different evaporation stages.In the early stage of evaporation,when the soil water content is high,the soil particles are arranged loosely and the interparticle voids are filled with pore water,shrinkage stress is slowly increasing.As the evaporation progresses,the particles gradually converge,the soil shrinkage stress begins to increase rapidly,and in the stage of rapid increase,there is a critical moisture content.When the soil moisture content is lower than the critical moisture content,the extruding between the particles leads to the rapid increase of the shrinkage stress.In addition,the smaller the relative humidity of the environment where the soil is located,the smaller the critical moisture content,and the greater the final shrinkage stress when the evaporation is stable.(3)The formation process of soil cracks is constrained by both soil conditions and interface contact conditions.Cracks can be formed either from the surface or from the bottom.When there is surface defect on the soil surface,cracks will start to appear on the surface defects and expand downward.If the soil is relatively homogeneous,for the thin layer of soil,the development of cracks will be affected by the interface friction conditions,and it will first form from the bottom of the soil and expand upward.the cracks will be generated from the bottom of the soil and expand upwards.The cracks extending from the bottom upwards can be divided into tensile cracks and shear cracks according to their generation mechanism.When the soil shrinks,it exhibits centripetal contraction and there is a pronounced phenomenon of nuclear shrinkage.(4)There is obvious size effect and boundary effect in the process of soil crack development.During the drying process,the greater the roughness of the interface,the faster the growth rate of the cracks;the greater the thickness of the soil,the slower the rate of crack development,and the more obvious the overall shrinkage effect of the soil.After drying,the greater the interface roughness,the higher the degree of crack development.The greater the thickness of soil,the lower the degree of crack development.Increasing the thickness of the soil can weaken the influence of interface roughness on the degree of crack development.Increasing the soil thickness or interface roughness can reduce the production of cracks.The number of cracks,the total length of cracks,the number of nodes and the number of soil blocks increase with the increase of interface roughness and decrease with the increase of soil thickness.The shrinkage rate,the average area of the soil blocks,and the average width of the cracks decrease with the increase of interface roughness and increase with the increase of soil thickness.