Study On The Evolution Laws Of Expansive Soil Cracks And Its Quantitative Characterization

Expansive soils with crack-prone properties are widely distributed around the world.With the increasing instability of global climate and the frequent occurrence of arid climate,expansive soils shrink significantly during the continuous drying and water-loss process,usually accompanied by various kinds of cracks.The original structural properties of the soil can be destroyed by cracks,resulting in attenuation of soil strength,deformation,and even significant changes in seepage characteristics.This series of changes accelerates the deterioration process of the soil,leading to the shortening of the life span of geotechnical structures and the occurrence of disasters.In considering these problems,the investigation of swelling-shrinking cracks in expansive soils is a fundamental and necessary task.In this study,the development pattern and mechanism of shrinkage cracks in expansive soil are investigated under dry environment,and a series of experimental studies are carried out based on important influencing factors such as size effect,ambient temperature,fiber action and boundary roughness condition,and local moisture content distribution of the soil.The crack evolution law under the action of these influencing factors is explored and quantitatively analyzed to further reveal the cracking and crack evolution mechanism of the expansive soil by combining the macroscopic crack evolution process and the microscopic soil particle-pore water interaction mechanism.The relevant research contents and conclusions are as follows:(1)Combining the characteristics of crack analysis and the adoption of relevant indexes,a set of soil crack image processing and quantitative analysis program is developed based on digital image processing technology and MATLAB platform.The results indicate that cracks can be effectively treated and quantitatively analyzed.(2)Considering the two types of effects of size and temperature,the characteristics and laws of crack development are investigated via water-loss shrinkage tests of expansive soils in the initial state of complete saturation.It is found that the shrinkage cracking of expansive soil has the following two processes: rapid expansion of cracks dominated by the release of tensile stress at the beginning of cracking,and widening and deepening of cracks dominated by continuous water-loss and shrinkage at the middle and end of cracking.The degree of influence of surface size,thickness and ambient temperature on the final crack index is ranked as follows: thickness > surface diameter >ambient temperature,whose effects are related to microscopic cracking of expansive soils as well as to the evaporation of water and stress release patterns during evolution.(3)Based on the fractal theory and the improved box dimension calculation method,the area fractal dimension and length fractal dimension of expansive soil cracks are proposed as there exists statistical self-similarity characteristics.Under the same water content,the area fractal dimension shows empirical quantitative relationship with crack degree,and the same empirical quantitative relationship between the length fractal dimension and the ratio of crack length to sample perimeter,which can be quantitatively characterized by logarithmic function.The length fractal dimension grows obviously in the early stage of crack evolution and tends to be stable in the middle and late stage,while the area fractal dimension grows gradually and stabilizes with the complete development of the crack,combining the two can quantitatively reflect the crack development process more accurately.(4)The shrinkage cracking and crack evolution process of expansive soils are significantly influenced by fiber reinforcement and boundary roughness.The crack development characteristics of expansive soil under the coupling of the two effects are investigated by setting three bottom boundaries with different roughness and four fiber doping levels corresponding to each boundary.It is shown that a very smooth boundary is conducive to the lateral shrinkage of the expansive soil,and the stress inequality is greatly relieved,which leads to the tensile cracks hardly generate in the center of the soil mass and the soil layer tends to shrink uniformly as a whole.On the contrary,the rough boundary is conducive to the generation of cracks,and the cracks formed in the soil are more complex.Fibers are able to increase the tensile strength of the soil and act as a significant pulling force at the internal cracks.However,fibers can also destroy the homogeneity of the soil and increase anisotropy,which in turn leads to large stress inequalities in the soil at the late-shrinking stage and even some tiny cracks sprouting on the soil surface.(5)The local moisture content distribution and lateral shrinkage characteristics of shallow expansive soils in the water-loss process are considered,and the formulas of tensile strength and surface tensile stress affected by the surface moisture content and soil depth are derived.Combining the macro and micro characteristics of crack evolution in shallow expansive soils,a quantitative discriminant formula applicable to describe three typical stages of shallow expansive soils before cracking,at the early stage of cracking,and at the middle and late stage of cracking was established.At the early stage of crack evolution,the quantitative formula of tension stress is linked to the crack index,which can characterize the stress release process.