The Evaluation Of Microstructure Stability Of Main Soils In The Loess Region Based On The Rheological Method

Loess plateau is the largest and most concentrated distribution of loess in the world.The soil parent material of loess soil is characteristic of large pore,vertical joints,poor cohesive,easily broken down by water,weak resistance to erosion and so on.Besides,the heavy rainfall and frequent human activities led to the ecological environment of the Loess plateau more sensitive;the landslide,collapse,soil deformation and other disasters frequently occurs.This could seriously affect the development of agriculture and construction of ecological environment.The macroscopic mechanical behaviors of loess soil are closely related with soil structural characteristics.At present,there are a large number of studies about soil structural stability in loess region,which mainly characterized by shear strength and other parameters obtained from direct shear test,unconfined compression test and triaxial compression test.However,the parameters obtained based on traditional soil mechanics methods are difficult to directly reflect the soil microscopic properties.Rheology is a subject that studies the flow and deformation behavior of viscoelastic material under external forces.As a viscoelastic material,the rheological properties of soil depend on the interaction between soil particles at the microscopic level.Thus,using the rheological method to study soil structural stability can provide more abundant information about soil structure stability.Therefore,our study takes four kinds of typical loess soil as the research object.Using the dynamic shear rheometer,we firstly explored the application condition of the rheometer in the loess soil.Secondly,we discussed the influence of soil bulk density,water content and ionic strength on the rheological properties of soils.This work can provide a new method and more abundant parameters for evaluating soil microstructure stability on the Loess Plateau.The main results obtained in this study are as follows:（1）The influence of different rheometer parameters on microstructure stability of typical loessal soil were studied.The shear strength and viscoelasticity of soil microstructure under different oscillation frequencies（0.25,0.5,0.75,1,1.25,1.5,2 Hz）and normal forces（0.5,1,3,6,10 N）were quantitatively evaluated by amplitude scan test.Results showed that Lou soil and Loessal soils were all featured with shear thinning characteristic;and in the selected shear strain ranges the soils transformed from elastic to plastic.Under the condition of small strains,soil microstructure stability increased with the increase of test temperature,but remained constant with the increase of the test time.The shear strength of Lou soil decreased firstly（0.25～1 Hz）,then increased（1～1.5 Hz）and finally decreased again with the increase of oscillation frequency;while the shear stress at linear viscoelastic end and yield point of loessal soil decreased linearly.With the increase of oscillation frequency the linear viscoelastic end decreased,while the shear strain at yield point and integral zone increased firstly and then decreased.With the increase of normal force,the shear strength（shear stress and storage modulus at linear viscoelastic end and yield point）and viscoelasticity（shear strain at linear viscoelastic end and yield point,integral zone）of the two soils increased significantly（p<0.05）.Therefore,considering the other studies,the vibration frequency of farm machinery and tools in the field and the timeliness of the experiment.We finally selected 20℃as the test temperature,15 min as the test time and 0.5 Hz as the oscillation frequency,when we studied the influence of normal force,soil bulk density and water content as well as ionic strength on the stability of soil microstructure.In addition,when the sample height was difficult to control,the normal force can set 1 N to ensure the contact between instrument rotor and test sample.（2）The effects of soil bulk density and water content on soil microstructure stability were quantitatively evaluated.The shear strength and viscoelasticity of soil microstructure under different bulk density(0.9,1.1,1.3,1.5,1.7 g·cm^-3)and water content（9,12,15,18,21,24,27%）were quantitatively evaluated by amplitude scan test.Results showed that with the increase of soil bulk density,the shear stress and storage modulus at linear viscoelastic end and yield point,as well as maximum shear stress of the four soils increased,the ability of soil to resist deformation increased.The shear strain at yield point of Hei Lu soil and integral zone of Lou soil increased firstly and then decreased,indicating that there was a peak value at 1.3 g·cm^-3,and the arrangement between soil particles was the most stable.The shear strength of the four soils was:Cinnamon soil>Lou soil>Hei Lu soil>Loessal soil.With the increase of water content,the shear stress at linear viscoelastic end and yield point as well as maximum shear stress of Lou soil,Hei Lu soil and Loessal soil reduced,indicating that the ability of soils to resist deformation was decreased;while the storage modulus at linear viscoelastic end and yield point of Cinnamon soil increased.High clay and organic matter content of Cinnamon soil can absorb more water,and thus had the stronger shear resistence under the same treatment.The linear viscoelastic end of Lou soil and Hei Lu soil increased with the increase of water content,while the integral zone of Lou soil and Cinnamon soil increased firstly and then decreased.The shear strength of the four soils was:Cinnamon soil>Lou soil>Loessal soil>Hei Lu soil,and the viscoelastic of the four soils was:Lou soil>Cinnamon soil>Hei Lu soil>Loessal soil.（3）The effect of ionic strength on soil microstructure stability were investigated.The shear strength and viscoelasticity of soil microstructure with different ions(K⁺,Ca²⁺,La³⁺)at different ion concentrations(0.001,0.01,0.1,0.25,0.5,0.75,1 mol·L^-1)was evaluated by amplitude scan test.Results showed that with the increase of ionic strength,the shear strength of Lou soil(except G’_LVE and G’_YP)and Cinnamon soil(except G’_LVE)increased firstly and then decreased,and the shear strength of Hei Lu soil and Loessal soil decreased linearly.The linear viscoelastic end of the four soils decreased with the increase of ionic strength,and the shear strain at yield point of Cinnamon soil and Loessal soil(except La³⁺)decreased firstly and then increased.La³⁺had the most obvious positive effect on the four soil shear strength,but the viscoelasticity was the lowest when the ion concentration increased（the shear strain at linear viscoelastic end and integral zone were the lowest）,indicating that the bridge bonding of polyvalent cations significantly increased the number of rigid bonding in soil and reduced the elasticity between soil particles.The results showed that Cinnamon soil had the highest shear strength due to its high content of organic matter,clay,specific surface area and Ca CO₃,but the lowest viscoelasticity due to its low content of montmorillonite.Loessal soil has higher sand content and the lowest shear resistance under saturation condition.