Study On Crack Disturbed State Variables And Its Characterization Function Of Unsaturated Expansive Soil

Overconsolidation,swelling and shrinkage,fissure and structure are the prominent features of the original expansive soil,and they have important influence on the strength,deformation and stability of the soil.During the drying-wetting cycle of expansive soil,the fissures initiated and the soil structure was obviously disturbed.How to characterize the disturbing effect of expansive soil on the soil in the process of fracture development is a hot topic of study.Selecting the appropriate disturbance state variables directly determines the rationality of the constitutive model of the disturbance state.In this paper,CT scan is used to obtain the internal structural information of soil,and the relevant eigenvalues in the process of crack development in expansive soil are quantitatively studied to provide structural parameters for the study of dislocation variables in expansive soils.Mainly studied the following aspects:(1)The drying-wetting cycle test of Hefei undisturbed expansive soil was carried out.Combined with CT scanning,the CT images of the samples with different disturbance states were obtained.CT image processing program written in MATLAB was used to quantitatively study the variation of the eigenvalues of crack area,fracture volume,total crack length,gray value and CT number with the drying-wetting cycle,and then discussed their rationality as the disturbance state variables of expansive soil.At the same time,the distribution of these eigenvalues in the axial direction of the specimen with the wet-dry cycle was analyzed,and then the evolution law of cracks was discussed.The evolution of two-dimensional fractures in the plane of soil,the distribution of fractures in the axial direction,and the three-dimensional reconstruction of fractures are combined.Fissure research has evolved from qualitative to quantitative,from two-dimensional to threedimensional.(2)This paper proposes a method based on the gray level co-occurrence matrix(GLCM)to study the texture features of expansive soil.The effects of three different construction factors of gray level,distance and direction on GLCM eigenvalues were studied and screened.Based on the CT image after the wet-dry cycle of expansive soil,the gray level co-occurrence matrix was constructed and the GLCM eigenvalues were extracted.Combined with the stress-strain relationship of triaxial shear test soil,the fracture texture change of Hefei undisturbed expansive soil under different wet-dry cycles was studied.Then,quantitative analysis of the disturbing effect of cracks on the structure of expansive soils is performed;the GLCM eigenvalues are proved to be reasonable for studying the texture features of expansive soils.(3)Based on the GDS triaxial test system,the effect of one-dimensional initial damage at low moisture content on the strength of expansive soil was studied.Triaxial shear tests were performed to control the suction and confining pressure.The effect of dry-wet cycle on the expansive soil of Hefei was studied.The different effects of the stress-strain relationship of remolded expansive soil,and the disturbance of the combined effect of suction and wet-dry cycles on the strength characteristics of the original expansive soil.Predetermined three groups of samples with different particle sizes were subjected to a wet-dry cycle test to study the effect of the combined effect of particle size loss and cracks on the stress-strain behavior of expansive soil after the wet-dry cycle.The main findings are as follows:(1)Through the three-dimensional reconstruction of CT images,qualitative study of the three-dimensional fracture evolution law within the soil found that: the development of fractures began at the weak spots of the initial pores and micro-cracks within the soil,and thus extended the extension to form the fracture surface,fissure body,and finally formed.Complex three-dimensional fracture network.Quantitatively extracting eigenvalues such as fracture area,fracture volume,total crack length,gray value and CT number,it can be found that the variation of these eigenvalues with the disturbance of dry and wet cycle can be described by Logistic function.The distribution of fracture area along the axial direction of the specimen has obvious fluctuation and periodicity.The amplitude A increases first and then decreases;the fracture area of the specimen is distributed between the radial and axial distribution and the cumulative dry shrinkage volume change.The approximate relationship can predict the development and distribution of cracks in three-dimensional space.(2)The variation of ASM,ENT,and CON values in the GLCM eigenvalues during the wetting and drying cycle and the cumulative shrinkage volume change can describe the disturbance of expansive soils in the process of dry and wet cycles on the expansive soils.Logistic functions can be used to represent them.The values of ASM,ENT,and CON in the triaxial shearing process can well correspond to the stress-strain curves of soils in expansive soils.They can describe the texture changes of the soil in the axial loading process,and are well-adapted textures.Describe the eigenvalues,which can provide a new reference for the texture study of soils,especially the study of fracture texture in the wetting-drying cycle.(3)The original expansive soil is structurally significant.Before the wet-dry cycle,the strength is significantly greater than that after the remodeling disturbance.When the fissures are not fully developed,the strain characteristics of the original expansive soil and the remolded expansive soil are all softened,and the softening characteristics of the original expansive soil are more obvious.After the fissure is fully developed,the structural effect on the strength of the soil is significantly reduced.The strength of the undisturbed and remolded soils almost reaches the residual strength,and the dry-wet cycle no longer has a significant effect on the strength of the expansive soil.(4)In the early stage of the wet-dry cycle,due to the effect of suction,the crack width is closed,the strength is partially repaired,and the magnitude of the decrease is smaller.After the particle size is missing,the coarse particles are high and the strength is higher under low confining pressure.In the early stage of the wet and dry cycle,high fine particle content,rapid strength decay,low fine particle content,slow strength attenuation.At the later stage of the wet-dry cycle,the strength of many fine particles does not change substantially,and the strength of large particles increases continuously.(5)The perturbation function of the eigenvalues of the crack perturbation of expansive soil was compared and analyzed.It was found that the eigenvalues ASM,ENT and CON of the gray level co-occurrence matrix were more consistent with the characterization of the strength of cracks in the original expansive soil.The study of expansive soil texture based on eigenvalues of gray level co-occurrence matrix provides a new approach for the study of soil deformation and mechanical properties.