Study On Soil Desiccation Cracking Based On Digital Image Correlation (DIC) Technique

Desiccation cracks are ubiquitous in soils under dry environments.The existence of cracks will destroy the integrity of the soil,reduce the strength of the soil and may lead to various geological and geotechnical problems.It is of great significance to study the process of soil desiccation cracking in order to accurately grasp the response characteristics of soil engineering properties in arid climate environment.The generation of cracks is related to the stress state of soil.By analyzing the evolution characteristics of stress field or strain field on the soil surface in the drying process,then establishing the relationship between strain/displacement field and cracks,it helps to better understand the generation and evolution mechanism of cracks.Therefore,in this paper,expansive soil and Xiashu soil are used as the main test materials,initial saturated homogeneous mud samples and compacted soil samples with different moisture content close to the natural state are prepared respectively,then a series of laboratory drying tests are carried out.The dynamic development process of soil desiccation cracks was recorded by digital camera in real time,the surface strain field and displacement field of soil were analyzed by digital image correlation(DIC)technique.Through the analysis of the evolution characteristics of strain field and displacement field in different stages and states,some laws and mechanisms of soil during drying process are revealed.The main work and research results are as follows:(1)During the drying process of the initial saturated mud sample,the cracks usually appear in the tensile strain concentration area first.After the cracks are generated,the surrounding strain field redistributes rapidly and the displacement field changes accordingly.The new strain field and displacement field will act on the cracks again,which will affect the development direction of the cracks.Under the interaction of strain/displacement field and cracks,the crack tends to intersect other ones at right angles.In addition,influenced by the existing cracks and the container boundary condition,the cracks will show different degrees of deflection in the development process.There is a phenomenon of shrinkage center in the process of shrinkage deformation,the position of the shrinking center is continuously move with the drying time.Based on the information of strain field and displacement field on the soil surface,the characteristics of desiccation cracking in the local area of soil can be predicted in advance.In addition,whether the cracks or soil clods are relatively active or relatively stable can be directly judged.(2)It is observed that the crack usually develops at the defect point on the surface of the sample first,which is due to the defect point will lead to obvious stress concentration on the soil surface.The results show that the unevenness of the soil surface,the existence of bubbles,or any man-made defects of the soil sample will lead to stress concentration,which is reflected in the strain field with large area of tension strain concentration area and compression strain concentration area.Single defect often induces three cracks at the beginning,these three cracks expand in three different directions with an angle of 120°,forming a "Y" shape.For multiple defect points,the crack will first appear in a "Y" shape from a certain defect point;then the second defect point begins to develop cracks,forming a "T" shaped distribution of cracks;last the third defect point will forms a "I" shaped cracks.The crack starts to develop from a certain defect point,it will be affected by the stress field around other defect points during the process of expansion,which will change the development direction.(3)When the soil samples are compacted at optimal moisture content and on the dry side of optimum moisture content,aggregate structure will be formed.The surface of the soil samples will produce scattered tensile strain concentration points,resulting in the cracks develop almost at the same time and distributed evenly.While the soil samples compacted on the wet side of optimum moisture content are mainly formed dispersed structure.The continuous tensile strain concentration will be formed during the drying process and the cracks are generated by grading.This is because the aggregate structure is formed by the aggregation of clay particles.There are large pores between the aggregates,which is the weak part of soil sample connection,so it is easy to develop many cracks at the same time.The desiccation cracking process of the mud clay sample and the compacted soil sample under the condition of the moisture content of the wet side is similar,both of them developed the primary and secondary cracks orderly,which shows that the initial structure state of soil sample has an important influence on the crack development process.In addition,with the increase of water content,the number and density of cracks gradually decrease,the average length and width of cracks also gradually increase,and the geometry of cracks tends to be more simplify.(4)Under the action of non-uniform temperature field,the cracks will first appear from a certain location near the high temperature area,then expand to both sides in an arc shape.The cracks form from the high temperature area to the low temperature area layer by layer.The difference of temperature leads to the obvious difference of drying rate on the soil surface.There is a drying-wetting interface,which will move from the high temperature area to the low temperature area with the continuous evaporation.In the whole process of desiccation cracking,the strain and displacement are very uneven,but most of them decrease from the place with high temperature to the place with low temperature.The evaporation rate of moisture is fast in the high temperature area,and the water escapes more,which results in large strain and shrinkage due to the rearrange of particles.(5)Digital image correlation technique can track the shrinkage deformation and crack development state of the soil in real time during the drying process.It can effectively and quickly obtain the information of strain field and displacement field on the soil surface,visually reflecting the dynamic process of crack development.As a high-precision non-contact monitoring method,digital image correlation technique has unique advantages different from the traditional soil deformation measurement method,which provides an effective way to better study the formation mechanism of soil desiccation cracking and has great promotion value.