| With the implementation of the Yangtze River Economic Belt development strategy in China,expressway construction along the Yangtze River is increasing day by day.During the expressway construction along the river in Anhui Province,a large amount of slope instability occurred after several rainfall-evaporation cycles,which brought substantial hidden danger and economic loss to expressway operation.Under the cyclic change of temperature,relative humidity,and other environmental factors,the soil's highly nonlinear behavior makes the study of cracking very complicated.The initiation and expansion of cracks reduce the shear strength of the soil and cause the change of the slope's seepage field,leading to a significant decrease in the slope's stability.Therefore,a systematic study on the mechanism of crack initiation and evolution under rainfall-evaporation can enrich the research results on the mechanism of crack propagation in clay and reveal the failure mode and disaster mechanism of crack slope.The main work and research results of this paper are as follows:(1)According to field investigations,literature investigation and laboratory tests,the engineering physical and mechanical characteristics of cracked soil in areas of the Yangtze River of Anhui province.The genesis,mineral composition,microstructure,swelling and shrinkage characteristics are explored,and the field distinguishing characteristics of cracked soil in study area are put forward.The results show that the clay minerals in the cracked soil samples are mainly montmorillonite,accounting for69%,showing significant expansion.Due to the cold-dry paleoclimate characteristics,the silicate minerals such as feldspar and mica in Zongyang,Chaohu and Hanshan samples were depotassified and enriched with illite clay minerals,accounting for 43%to 57%,respectively.The free expansion rate of the crack soil in study area ranges from 27.5% to 42.5%,the plastic limit ranges from 18.2% to 29.0%,the standard moisture content ranges from 1.398% to 2.898%,and the expansion force ranges from52.0 to 72.1k Pa in natural state.All these indicate that the soil in the area is weak in expansion,and the soil in the state of higher moisture content is more prone to shrinkage deformation.It is the key reason for the rapid cracking of a large number of cracked soil slopes in study area after short evaporation after rainy season.(2)Through the quantitative analysis of the microstructure of crack soil samples under different dry-wetting cycles,the micro-mechanism of crack soil cracking under different dry-wetting cycles is explored.It is found that the soil undergoing dry-wetting cycle and cation exchange in the early stage reduces the negative potential on the surface of the lamellar structure between the particles,and the suction between the soil particles is dominant so that the soil particles can get close to each other and flocculate into the flocculated structure of small unit clusters.After leaching and evaporation,the loss of soluble salts and the loss of capillary pressure lead to the decrease of the cementation and capillary cohesion between particle units,leading to the structure's loosening.The sharp increase of microfracture indicates the development of macroscopic fractures,and the increase of microfracture content may be the main reason for the complex distribution of the average shape of pores.(3)Based on the triaxial test and quantitative study of microstructure under the action of wetting and drying cycles,the regression analysis of micro-parameters and crack rate,internal friction angle,cohesion,and elastic modulus of cracked soil under different wetting and drying cycles was carried out.The linear regression model and nonlinear regression model were established,respectively.Within the statistical damage theory framework,the strain hardening statistical damage constitutive model of cracked soil under cyclic wet and dry loads based on Laplace distribution is established and derived.The assumption that the micro-element strength of cracked soil follows Laplace distribution was put forward,and the influence of the initial damage threshold was considered.The dual damage variables were introduced to investigate the macro-micro damage mechanism of cracked soil under the action of wetting-drying cycle cracking and stress level.The model simulates the total stress-strain curve of cracked soil under the wetting-drying cycle's combined action and confining pressure.The more the wetting-drying cycle,the higher the confining pressure,and the better the model fits.(4)Based on the one-way drying test of crack soil,the law of fracture initiation and evolution was explored.In the framework of online elastic fracture mechanics theory,the applicable fracture criteria for various fracture evolution stages are proposed.Simultaneously,the stress field and displacement field in the shrinkage and fracture stage of cracked soil are solved.Based on the COD fracture criterion and the Cohesive fracture model,the numerical simulation of the cracked soil's crack propagation stage was realized by using FISH language in PFC5.0.The results of numerical analysis are in good agreement with the experimental phenomena.It is found that fracture initiation is easy to occur in weak spots of depressions,and the number of fractures in samples with higher initial water content is significantly higher than that in samples with lower water content.In experiment study,cracking occurs initially at the interface between the container wall and clay,which is only a manifestation of boundary effect,and the existence of boundary constraint effect can significantly affect the crack propagation.In the process of dehumidification,the soil produces a high elastic shrinkage strain energy,which needs to consume part of the elastic strain energy through the formation of crack surface energy,and the residual elastic strain energy is absorbed in the form of crack expansion.After forming the leading crack network in the sample,only the main crack widens and deepens with time growth,while the probability of the expansion of the secondary crack in the residual shrinkage stage is small.(5)Based on the saturated-unsaturated infiltration theory,the influences of cracks on the slope's transient saturated zone,the depth,Angle,distribution location,and number of cracks on the transient seepage field of the slope were investigated,respectively.A conceptual model of preferential flow was then proposed,and the governing equation of preferential flow was derived based on the Green-Ampt infiltration model.The influence rules of cumulative infiltration amount under different rainfall intensity,fracture area ratios,and fracture zone saturated permeability were expounded.It is found that the direction,degree,and distribution of anisotropy induced by crack have a specific influence on rainfall infiltration of slope.The Green-Ampt infiltration model was introduced into the double-pore infiltration theory,and the preferential infiltration model was established considering the area ratio of dry-shrinkage fractures.The form is simple,the calculation is convenient,the physical meaning is clear,and the effect of dry shrinkage fissure on the rainwater infiltration process is well-reflected.Cracked soil's preferential flow to rainfall intensity is not significant and is mainly controlled by crack area ratio and crack saturation permeability.(6)The full-scale model test system was independently developed to conduct whole-process and multi-physical joint monitoring of the progressive failure of cracked soil slope.The spatial-temporal characteristics of water transport and the instability mechanism of cracked soil slope under rainfall-evaporation action are explored.Moreover,evolutionary model of the progressive deformation and failure of cracked soil slope under rainfall-evaporation action are revealed.The results show that the volume of the slope decreases by 4.84% and 47.2% respectively in the initial deformation and final failure,and the maximum cumulative horizontal displacement reaches 207.4cm in the final failure.The slope with large cracks is far more destructive than the slope without cracks.The development and evolution of crack is the main reason to control the deformation and instability of crack soil slope.The repeated deformation of crack soil cutting slope is not a simple repeated cycle of shallow deformation,but a progressive deterioration failure.It is found that the dominant flow effect of cracks can easily lead to the occurrence of a large area of connected saturated zone in the slope,which is the main reason for the overall instability of the cracked soil slope. |
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