Study On The Evolution Law Of Crack And Hydro-mechanical Properties Of Expansive Soil Under Dry-wetting Cycle Effect

Expansive soil has complex properties,such as swelling,shrinkage,crack and overconsolidation,which can easily cause engineering hazards.Expansive soil is widely distributed in subtropical and temperate regions with low rainfall,and it is in unsaturated state all the year round.With the alternation of rainfall and evaporation,the state of water and gas in the soil body is in a dynamic change.During the alternation of water and gas content,the soil mass expands and shrinks inhomogeneously,leading to the development of fractures,which directly affects the mechanical and hydraulic properties of the soil mass,and poses a severe challenge to the stability of the expansive soil slope.Therefore,in order to make a correct analysis of such engineering problems,it is necessary to systematically study the water-gas transport law and the evolution mechanism of strength and crack of expansive soil in the process of repeated wetting and drying cycles.Taking hefei expansive soil as the research object,to experience different dry-wet cycles of expansive soil specimen on a CT scan test,the test,the soil water characteristic curve of unsaturated triaxial shear test and triaxial permeability test,to explore the expansive soil under dry-wet circulation effect of fissure and evolution law of hydraulic and mechanical characteristics,in order to provide basis for the expansive soil slope stability analysis.The main research contents and results are as follows:（1）CT scan tests were carried out on the undisturbed expansive soil that underwent different dry-wetting cycles（0-5 times）,and the CT images of samples with different fracture development states were obtained.Combined with CT image 3D reconstruction technology and plane digital image processing technology,the evolution law of swelling and shrinkage cracks of expansive soil under dry-wetting cycle effect is systematically analyzed from qualitative and quantitative levels,from two-dimensional plane to three-dimensional space.The results show that under the action of the dry-wetting cycle,the cracks in the weak parts such as the initial pores and micro-cracks on the top of the sample are the first,and then spread from the top to the depth direction,gradually forming point-like local macropores and strip cracks,and finally forming a three-dimensional fracture network throughout the soil.Along with the advancement of dry-wet circulation role,crack volume fraction,the average area rate,crack length and crack the average width of the index is nonlinear growth trend,especially in 1～3 times of the dry-wet circulation during the growth of the largest,and after five dry-wet circulation role,the width of crack are narrow,the crack of volume fraction and the average area rate is slightly reduced,the relationship between the crack index and the dry-wet cycles are available quantitative characterization of logistic function.With the mass propagation of dry and wet cyclic cracks,the compactness of the soil structure gradually degenerated,which resulted in the attenuation of the gray value average value of the crack CT image,and the gray value distribution characteristics of the crack image in the plane of the sample also changed significantly.（2）The gray level co-occurrence matrix image texture feature analysis method was used to construct the gray level co-occurrence matrix based on expansive soil crack CT image and extract the corresponding eigenvalues to quantitatively study the evolution law of expansive soil crack texture under the effect of dry-wetting cycle.The results show that the Angle second moment（ASM）,entropy（ENT）and contrast（CON）,as the eigenvalues of the gray co-occurance matrix with high applicability,can well describe the variation law of the texture of expansive soil under the disturbance of dry-wet action,which can provide a new idea for quantitative research of the texture characteristics of expansive soil.（3）Using pressure membrane type 1500 meter acquisition under different dry-wet cycles（0～5）fracture expansive soil of soil water characteristic curve,the quantitative analysis of dry-wet circulation role the influence regularity of the characteristics of the expansive soil moisture,and establish the corresponding constitutive model of soil water curve.The results show that under the influence of the dry-wetting cycle,with the initiation and expansion of cracks,a bimodal pore system composed of cracks and soil matrix pores is gradually formed inside the soil,which leads to the"bimodal"development of soil-water characteristic curve of soil samples.At the same time,the soil-water characteristic curve of expansive soil tended to shift downward,the values of all characteristic parameters of the curve gradually decreased,and the water-holding capacity of soil decreased.The grey correlation analysis method showed that the change of crack volume fraction had the greatest correlation with the decline of soil water-holding capacity.Due to differences in initial pore structure,reshape the expansive soil in the initial stage of soil water characteristic curve characteristic values are higher than that of undisturbed expansive soil,and the"twin peaks"morphological characteristics are relatively weak,but with the deepening of the dry-wet circulation function,two kinds of curve shape of soil water of soil gradually approaching,the gap between curve characteristic value also has a decreasing trend,show that two kinds of soil pore structure is more and more similar;Based on the segmentation idea of Fredlund&Xing model and Burger bimodal model,a bimodal soil-water characteristic curve model was constructed with the number of dry-wet cycles and matric suction as variables.The accuracy and rationality of the model were verified by the measured data.The fitting parameters showed a good linear trend with the number of dry-wet cycles.（4）Using GDS unsaturated triaxial test system,to experience different number（0）,1,3,5times dry-wet circulation effect of expansive soil specimen of different matrix suction（50150250 k Pa）and net confining pressure（50100200 k Pa）under unsaturated triaxial consolidation drained shear tests,the dry-wet circulation effects on the mechanical characteristics of unsaturated expansive soil are studied.It is found that during the first three dry-wetting cycles,with the increase of the number of dry-wetting cycles,the stress-strain curve of the sample gradually transitions from the strain hardening characteristic to the strain softening characteristic,and the sample is also prone to dilatancy failure,and the greater the matric suction,the more obvious this phenomenon is.However,at the later stage of the dry-wet cycle（N=3～5）,the stress-strain curves gradually developed into superstrain hardening or ideal elastoplastic type,and the dilatancy phenomenon also converged.The failure strength of expansive soil decreases with the increase of wetting and drying cycles.The confining pressure and matric suction can improve the strength of expansive soil to a certain extent,and the decay trend of failure strength is more intense.The matric suction can increase the total cohesion of expansive soil,but the effective internal friction Angle is not affected by the matric suction.The effective cohesion decreases non-linearly with the increase of wetting and drying cycles,while the internal friction Angle decreases first and then increases with the increase of wetting and drying cycles.In contrast,the effective internal friction Angle is not affected by dryness or wetting.According to the results,based on the Fredlund double stress state variable strength formula,the shear strength correction formula of unsaturated expansive soil considering the number of dry-wet cycles and matric suction was constructed.Analysis the change trend of nonlinear model parameters,the dry-wet circulation and the matrix suction on the initial tangent modulus of deformation parameters（n,k,m₁）and tangent bulk modulus(K_t,⁰)have great influence,through the relevant function fitting respectively puts forward the thinking of dry-wet cycles and matrix suction of the tangent modulus of deformation and tangent bulk modulus of the modified formula.（5）The effects of dry-wetting cycle,confining pressure and overburden pressure on the permeability characteristics of expansive soil are discussed through triaxial permeability tests.The results show that with the increase of confining pressure and overburden pressure,the soil structure becomes denser and the permeability of soil decreases.Under the action of dry-wetting cycle,the soil permeability coefficient shows a nonlinear increasing trend with the initiation and development of cracks,and the increasing trend is the most obvious during the 1-3 dry-wetting cycle.However,confining pressure and overbearing pressure can alleviate the disturbance of dry-wetting cycle effect to some extent,thus inhibiting the increasing trend of permeability coefficient to a certain extent.