Desiccation Cracking Mechanism And Mechanical Properties Of Fissured Clay Soil

In dry environment,the volume shrinks after the soil loses water,and it is prone to desiccation cracking.This phenomenon is very common in natural clay soil.Desiccation cracking leads to the formation of crack network in the soil,which destroys theintegrity and uniformity of the soil and directly changes the soil hydraulic properties such as strength,deformation and penetration of the clay.In recent years,the global extreme drought has occurred frequently,which has further aggravated the occurrence of engineering disasters in many fields such as geotechnical,environmental,geological,hydraulic and agriculture.Therefore,it is necessary to understand the desiccation cracking mechanism of soil.The study of desiccation cracks by test methods often has many interference factors and poor representativeness,while finite element methods are difficult to reproduce the development process of cracks.Discrete element method(DEM)has been widely used in the study of crack propagation because of their advantages of simulating large deformation and material damage.This paper will use PFC2 D discrete element software to simulate the evolution process of desiccation cracks,and try to analyze the desiccation cracking mechanism of clay soil considering crack evaporation from the microscopic point of view.The influence of fissure parameters on the mechanical properties of the specimens was studied by biaxial compression test.The quantitative relationship between the fissure quantitative index and the shear strength is established.The research content mainly has the following aspects.(1)Based on the previous research results,the water transfer model corresponding to the one-dimensional heat transfer equation is derived by using the heat conduction theory,and the Water-Mechanical coupled discrete element model is established by combining the expansion and contraction formula of soil particles.Compared with the theoretical solution,it is proved that the simulation of water movement by PFC2 D thermal module is reliable within a certain range.A water evaporation model considering crack effect is proposed by introducing fissure ratio R and total fissure depth H.That is,the crack surface is also involved in the evaporation of water.After considering the evaporation effect of the crack in the simulation process,the average depth and average width of the crack are increased,and the expansion of the crack is accelerated.(2)The whole process of desiccation cracks evolution under evaporation is simulated.According to the simulation results,there are two characteristics of water content variation.From the time point of view,the water content of the upper soil(less than 5 cm in depth)can be divided into three stages as a function of time:(1)Rapid decrease of water content at the beginning of crack formation.(2)Declined at a low rate in the medium term.(3)In the later stage,the water content remained stable,and the variation law of the water content of the upper soil is the same as that of the evaporation rate.From a spatial perspective,the internal water content of the soil appears as a “parabolic” distribution pattern along the depth direction.The water loss rate of the upper soil is fast,and the water loss rate of the lower soil is slow,resulting in uneven spatial distribution of water content,causing uneven shrinkage deformation.(3)In the same horizontal direction,the water content of soil around the crack increases with the distance from the center of the crack,and a “water content downward funnel” is formed on both sides of the crack.In the vertical direction,the spatial distribution of water content at the tip of the crack is similar to the "hyperbolic" form.The water content of the cracking point increases gradually along the depth direction.When the crack is close to the bottom of the soil sample,the water content of the crack tip decreases.(4)The effects of permeability coefficient,evaporation rate,tensile strength and expansion coefficient on soil moisture content and crack propagation are discussed.Reducing the permeability coefficient or increasing the evaporation rate will increase the heterogeneity of the spatial distribution of water content.However,adjusting expansion coefficient and tensile strength does not change the distribution of water content.At the same time,reducing evaporation rate or increasing tensile strength can significantly delay the formation of dry shrinkage cracks.In addition,increasing permeability coefficient or decreasing evaporation rate,tensile strength and shrinkage coefficient will lead to smaller cracks,which can provide solutions for engineering prevention and control.(5)Based on the biaxial compression discrete element simulation test of fissured soil samples,the quantitative relationship between the measurement indexes of fissure dip angle,width,spacing and fissure rate and shear strength parameters was established.Firstly,the cohesion and internal friction angles show a "V" distribution with the change of the dip angle of the fissure.The dip angle corresponding to the minimum cohesion is 55 degrees,which is close to the failure shear surface of the sample and meets to the Mohr strength criterion.Secondly,the shear strength decreases linearly with the increase of the fissure width.The cohesion of the sample is negatively correlated with the fissure width and fissure rate,while there is no significant correlation between the internal friction angle and the fissure width and fissure rate.Thirdly,by comparing three types of typical multi-crack models: parallel oblique model,horizontal and oblique model and pinnate model,the mechanical properties of soils are greatly weakened under complex crack combination mode,which is also the reason that the development of crack network endangers engineering safety.