Study On The Relationship Between Structural Properties And Unsaturated Characteristics Of Loess

Loess is a typical unsaturated structured soil with both structural properties and unsaturated characteristics.The structure plays a leading role in affecting the mechanical properties of soil,the unsaturated characteristics are also the important factors that affect the mechanical properties of loess.There is a close relationship between the structural properties and unsaturated characteristics.The change of unsaturated characteristics of loess makes the mechanical properties more complicated.It is necessary to study the relationship between the structural properties and unsaturated characteristics of loess,and then revel the mechanism of unsaturated characteristics affecting the structural properties.The quantitative characterization of soil structure is the current emphasis of the research on structural properties.While developing quantitative index,the formation of structural properties should be analyzed from the perspective of mechanics,so as to clarify the mechanism of the mechanical effects of structural properties.The unsaturated characteristics of soil can be characterized by matric suction and soil-water characteristic curve.Matric suction reflects the mechanical effect of unsaturated characteristics as a mechanical index,the decrease of matric suction is one of the main reasons for the reduction of structural properties and the collapse of loess.The soil-water characteristic curve reflects the water-holding capacity of soil,which is a comprehensive reflection of the unsaturated characteristics of soil.When the same matric suction applied,the water content of the loess with different unsaturated characteristics is different,the difference of water content causes the cementation in loess to be strengthened or weakened in different degrees,thus changing the structural properties.Therefore,it is significant to explore the correlation between unsaturated characteristics?matric suction,soil-water characteristic curve,etc.?and structural quantitative index,so as to reveal the mechanism of unsaturated characteristics affecting the structural properties,they are helpful to solve the problems of engineering safety and geological disasters in the loess distributed area.In this paper,the loess in Shaanxi Tongchuan is taken as the research object.The structural parameter?stress ratio structural potential?and structural stress?connection stress,arrangement stress?that quantitatively describe the loess structural properties are defined,matric suction and soil-water characteristic curve are chosen to characterize the unsaturated characteristics loess,the relationship between structural properties and unsaturated characteristics of loess is studied.In this paper,the change rules of stress ratio structural potential and structural stress of loess are explored when the matrix suction changes.Structured loess with different structural properties and unsaturated characteristics is prepared by artificial method,these samples are taken as the research object to study the relationship between stress ratio structural potential and parameters of soil-water characteristic curve model,and the function between them are established.Finally,combined with computed tomography and PFC^3D simulation,the evolution of loess structural properties is analyzed when stress acts.The main results are as follows:Based on the triaxial shear test,the parameter,stress ratio structural potential,is established to describe the structural properties of soil quantitatively,the rationality of the parameter is verified by triaxial shear tests of undisturbed loess with different water content.The value of stress ratio structural potential is in the range of[0,1),and the closer it is to 1,the stronger the soil structural property is;for the soil with very weak structural property,the stress ratio structural potential is 0.The triaxial shear tests of undisturbed loess with different water content show that the stress ratio structural potential evaluates the structural properties effectively.Triaxial shear and tensile tests are carried out on the undisturbed and remolded loess,the matric suction is controlled at the target value when the tests are conducted.The stress ratio structural potential and structural stress of undisturbed loess under different matric suction are obtained through the tests,then the influence of matric suction on structural properties of loess is investigated.In the range of matric suction in the tests?0-450 kPa?,the stress ratio structural potential increases with the increasing matrix suction,the structural properties of loess strengthened continuously.In the range of lower matric suction?0-25 kPa?and higher matric suction?400-450kPa?,the structural properties change little,however,there is a positive linear correlation between the stress ratio structural potential and matrix suction when matric suction increases from 25 kPa to 450 kPa.When the matrix suction increases,the connection stress increases continuously,but the growth rate decreases gradually.The change of growth rate can be divided into three stages,the critical matrix suction is 50 kPa and 350 kPa.The variation of arrangement stress with matrix suction is similar to the connection stress,but it is significantly smaller than the connection stress.The research on the preparation of artificial structured loess is carried out,the differences between the properties of artificial loess and undisturbed loess,such as material composition,microstructure and mechanical properties,are compared,which proves the reliability of the preparation method.CaCO3 in artificial loess consists of three types:cubic,spindle and film wrapped on the surface of particles.It exists at most of contact points of soil particles.CaCO3 is widely distributed in artificial loess,and has a strong cementation.The shear strength of artificial loess is much higher than that of undisturbed loess and remolded loess.The artificial loess with low density is prone to collapse,but does not collapse when the density is high.The collapsibility of artificial loess is weaker than that of undisturbed loess with same density.The artificial loess is taken as research object,the variation of structural properties is analyzed when the parameters of soil-water characteristic curve model change.Van Genuehten?VG?model is chosen to fit the soil-water characteristic curve of artificial loess with different density.The results show that parameter a and residual water content??_r?in the model are strongly correlated with the stress ratio structural potential of artificial loess.Taking parameters a and?_r as independent variables and the stress ratio structural potential as dependent variable,the function is established by multivariate nonlinear fitting.The function is proved to be reliable through the test data of undisturbed loess.Sensitivity analysis of parameters is carried out on the function,the result shows that the sensitivity and interaction of parameter a are stronger than that of?_r.The industrial computed tomography is used to detect the artificial loess samples which are sheared under different stress in triaxial shear test,the triaxial shear tests of artificial loess with different density are simulated by PFC^3D.The numerical simulation is proved to be reliable by the results of computed tomography.Then,the evolution process of structural properties is analyzed when stress increases.The results of computed tomography show that the structure of sample is intact when the stress is 100 kPa,but obviously damaged at 200 kPa,and the connected cracks appeared when the stress is 250 kPa.Results of PFC^3D simulation show that the shear zone of sample is formed when the stress reaches the peak strength,connections between particles are not damaged in the early stage of elastic deformation,but rapidly exert in the middle and late stage of elastic deformation,most of the connections has been damaged when the peak strength is reached.In the initial stage of stress,the force chain is relatively dense,the distribution of arrangement effect is uniform.In the stage of plastic deformation before peak strength,the number of strong force chain increases and a stable stress structure is formed.The stronger arrangement effect is concentrated in the area where strong force chains distribute.After peak strength,the force chains are destroyed gradually,therefore,the arrangement effect is weakened.