Study On Water Retention Characteristics Of Expansive Soil Considering Cumulative Damage Effect Of Drying-wetting Cycles

The engineering properties of expansive soil are mainly over-eonsolidation,expansion-shrinkage and multiple fissures,which often lead to instability and slump of expansive soil slopes during the engineering cycle.Expansive soil fissures are mainly caused by external causes such as tectonic movement and internal causes such as swelling and shrinking.During the period of project service life,tectonic movement is often not the main cause of the development of fissures in expansive soils.Therefore,more and more scholars focus their research on the relationship between dilatancy and fissure development.The volume of expansive soil expands when the water content increases,and the volume of expansive soil decreases when the water content decreases.In addition,the low permeability of expansive soil also causes the water evaporation rate of the surface soil to be higher than that of the inner soil,so there is an obvious water content gradient between different thicknesses inside the soil.When the shrinkage stress caused by the drying shrinkage of the soil mass on the surface is greater than the tensile strength of the weak part of the soil mass,cracking occurs on the surface.The new evaporation surface aggravates the evaporation of water in the soil near the crack surface,shrinks the soil in the horizontal direction,and promotes the development of cracks in the vertical direction.Fractal dimension is a statistical parameter to study local and whole,function and information.The existing research shows that the crack network and the morphological characteristics of expansive soil have good statistical self-similarity,and the fractal dimension can describe the morphological characteristics of expansive soil fissures in detail and accurately.Damage is an important factor affecting the mechanical properties of materials.Previous studies have mostly used mechanical and geometric indicators to describe the damage effect of materials.Using the fractal dimension of soil fissures to construct a functional expression to represent the damage variables,it can accurately describe the damage to the soil caused by the development of fissures caused by dry-wetting cycles.In this paper,a soil column with a diameter of 70 mm and a thickness of 65 mm made of Nanyang medium expansive soil was used as the research object,and the Yuba lamp was used to simulate the illumination in the natural environment and the water film transfer method was used to conduct a dry-wet cycle(0-3 times)test.The images of soil column fissures were obtained by photographing and CT scanning,and the damage calculation of apparent fissures and internal fissures was carried out by using image processing technology.A soil-water characteristic curve model including cumulative damage effects was developed.The research conclusions are as follows:(1)The fractal dimension was used to evaluate and analyze the apparent cracks and internal cracks of the expansive soil column.It was found that the apparent fractal dimension of the cracks increased with the decrease of the water content,and the apparent fractal dimension appeared in turn during the drying process.It is the fast-increasing stage,the slow-increasing stage and the stable stage.Both the apparent fractal dimension and the internal fractal dimension increased with the increase of the number of drying and wetting cycles.(2)A damage variable with fractal dimension as a factor was constructed by applying damage theory,and the cumulative damage of expansive soil under the action of drying and wetting cycles was described by apparent damage variable and internal damage variable.With the increase of the number of dry-wetting cycles,the apparent damage variable increased continuously,but the increase decreased in turn.Under the action of dry-wetting cycle,the internal damage variable first increases and then decreases along the thickness direction,this is because the cracks near the surface(0-6mm)of the soil column heal to a higher degree during the wetting process,but the upper region of the soil column(6-20mm)heals to a higher degree during the wetting process,under the repeated action of dry-wet cycles,burst damage occurred and the degree of healing was small,and less cracks were generated in the lower part of the soil column under the action of dry-wet cycles.(3)The saturated water content and the overall damage variable of the fractured expansive soil are defined.Under the action of 0-3 dry-wetting cycles,the saturated water content of the fissured expansive soil decreases continuously with the increase of the number of dry-wetting cycles,and the decreasing range increases continuously.The increase of the overall damage variable increases continuously during the 0-3 dry-wetting cycles,which means that the overall fissure structure of the soil column has a significant impact on its water-holding capacity.(4)Combined with the soil suction measured by the three methods,the influence of the overall damage variables on the air intake value,residual value of the soil column,the saturated water content of the fissured expansive soil,and different parameters in the soil-water characteristic curve was analyzed.A soil-water characteristic curve model with global damage variables and suction as variables.