Study On The Structural Parameter And Structural Constitutive Model Of Loess

Loess is a Quaternary sediment formed under arid and semi-arid conditions.It is widely distributed in Northwest my country,North China and Songliao Plain in Northeast China,of which the Loess Plateau is the most typical.As a typical structural soil,loess has unique collapsibility due to its special structural characteristics,which often causes engineering accidents and geological disasters such as ground subsidence,building foundation collapse,and loess landslides in loess areas.With the implementation of my country’s"One Belt,One Road"strategy and the major strategy of ecological protection and high-quality development in the Yellow River Basin,basic engineering,ecological engineering and water conservancy construction and other engineering activities in the Loess Plateau have gradually increased,and engineering construction is constantly facing new requirements and challenges.Because the structural characteristics of soil control and affect its macroscopic mechanical properties,the engineering properties and phenomena of loess in various engineering geological environments are related to its structure.Therefore,it is necessary to conduct in-depth research on the structure of loess,and to further understand the mechanism of different influencing factors on the structure of loess and the evolution mechanism of loess structure under different external forces.At present,the research on soil structure focuses on the quantitative characterization of the structure and the constitutive model of the soil.It is of great theoretical and practical significance to establish the connection between the quantitative system of the structure and the constitutive model of the soil.In this paper,the loess in Tongchuan,Shaanxi Province,which is located in the Loess Plateau,is taken as the research object,and the material composition,basic physical properties and collapsibility characteristics of natural loess are clarified through indoor geotechnical tests.Based on the understanding of soil structure and the theory of critical state soil mechanics,and considering the characteristics of stress ratio of soil samples,a new parameter for quantitative research of structure stress ratio sensitivity is proposed.The rationality of structural parameter is verified by the triaxial test results of loess samples with different water contents.A new method for preparing structured loess samples is explored,and the reliability of the method is proved in terms of basic physical properties,microstructure and mechanical properties.According to this method,loess samples with different Ca CO₃ contents were prepared and their structure was studied in depth.Meanwhile,the coefficient of variation method is used to clarify the degree of influence of different factors on loess structure.Based on the modified Cam Clay model,the proposed structural parameter is introduced,and the structural constitutive model of loess is established and verified.The main work includes the following aspects:From the perspective of sensitivity,considering the stress ratio characteristics of undisturbed soil and remodeled soil,a new structural parameter-stress ratio sensitivity is defined.Triaxial tests were carried out on undisturbed loess samples and remould loess samples with different water contents to verify the rationality of the proposed structural parameter.The stress ratio sensitivity parameter decreases gradually with the development of the strain and finally tends to be stable.The change curve of the parameter can be divided into three stages:fast decay,slow decay and stable development.The decay rate of the parameter is smaller under the condition of high water content than that under the condition of lower water content.The initial structural parameters of the samples are determined to describe the initial structural state of the sample and reflect the strength of the original structural properties of the sample.Stress ratio sensitivity and initial structural parameter are affected by confining pressure and water content conditions,both of which decrease with the increase of confining pressure and water content.The change of cohesion is the main factor for the change of loess structure.The cohesion difference（the difference between the undisturbed and the remould cohesion）reflects the difference between the original structure of the undisturbed and the remould.The initial structural parameter of the samples have a monotonous exponential relationship with the cohesion difference.A certain amount of Ca O and distilled water were added to the soil material from which Ca CO₃ was removed,and the soil was placed in a CO₂-filled environment to prepare loess samples with different structures.The rationality and reliability of the sample preparation method are evaluated by Ca CO₃ content test,liquid-plastic limit test,microscopic test,energy spectrum test and mechanical test.With the increase of Ca CO₃content,the clay content in the prepared samples gradually decreases,the silt and sand contents gradually increase,and the plasticity of the sample gradually weakenes.The newly generated Ca CO₃ mainly includes four types:cubic,fusiform,reticulated and maculiform.They are widely distributed in the prepared samples and mostly aggregate at the contact between particles,forming effective cementation and greatly improving the connection strength between particles.The Ca CO₃ content has a certain influence on the microstructure type,pore distribution characteristics and pore morphology of the prepared samples.The higher the Ca CO₃ content,the greater the prepared sample cohesion and internal friction angle.The prepared structured loess samples have collapsibility.The degree of collapsibility and the collapsibility coefficient of the samples are closely related to the Ca CO₃ content.Based on the proposed preparation method of structured loess samples,loess samples and remould loess samples with different Ca CO₃ contents were prepared and triaxial tests were carried out.The influence mechanism of Ca CO₃ cementation on loess structure is explored by using the proposed structural parameter of stress ratio sensitivity.The results show that Ca CO₃ content and confining pressure have important effects on the shear strength,curve shape and failure mode of the loess samples.The sensitivity of stress ratio can better describe the structural evolution process of loess samples with different Ca CO₃content in the shear process,and the change curve of parameters can also be divided into three stages:rapid decay stage,slow decay stage and stable development stage.Confining pressure and Ca CO₃ content have important effects on the initial structural parameter of the samples.Under the same Ca CO₃ content,the cohesion and internal friction angle of the loess samples are greater than those of the remould samples.With the increase of Ca CO₃ content,the differential cohesion increases exponentially,and its growth rate increases gradually.The relationship between the initial structural parameters and the difference cohesion under different confining pressures is logarithmic,indicating that the higher the Ca CO₃ content,the stronger the structure of the sample.The weight of influence of different factors on soil structure is determined based on the coefficient of variation method.Compared with Ca CO₃ content,the influence of water content on soil structure is more obvious.Considering the influence of soil structure,the structural constitutive model of loess is constructed by introducing four parameters related to structure,including initial structural yield stress,initial structural strength,structural attenuation and structural strength,on the basis of the Modified Cam Clay model.Based on the laboratory test results of loess samples with different water contents and loess and saline soil samples in the research literature,the basic parameters of the model were calculated.The ABAQUS software was used to carry out numerical calculation on the established model,and the calculated values of the model were compared with the test results of the selected samples to verify the rationality and applicability of the established model.