Study On Structural Evolution And Mechanical Properties Of Loess Under Plane Strain Unloading Conditions

Loess is found in arid and semi-arid regions of my country,mainly in the northwest.Loess is a special soil and is a typical structural soil with strong collapsibility and water sensitivity.In actual engineering,many projects such as embankments,retaining walls,foundations,and foundation pits can be similarly attributed to the plane strain problem.In the existing engineering design,the parameters of the conventional triaxial test and the direct shear test are mostly used,which is far from the strength and deformation,and the research under the plane strain condition is also mostly Concentrate on loading,and study less under unloading stress paths.This paper uses a true triaxial instrument after plane strain transformation to carry out plane strain unloading tests on loess with different water content,look around the development of structural parameters under the condition of plane strain unloading,and further reveals The influence of structural changes of loess on the strength and deformation properties of loess under the plane strain on unloading path need to be studied,and the plane strain structural mechanical properties of loess were expounded.The main research results are:（1）The stress-strain curves of undisturbed,remodeled and saturated loess under plane strain unloading are all hardened.When the strain reaches 1% to 2%,the soil will fail rapidly.The stress-strain curve increases gradually with the increase of consolidation confining pressure,the weakening of initial soil structure and the decrease of water content.（2）During the unloading process of plane strain,the middle principal stress and the middle principal stress coefficient both decrease first and then increase.As the lateral pressure decreases,the middle principal stress decreases rapidly;the curve turns when the vertical strain is 1%～2%,and then the curve rises.With the increase of the vertical strain,the middle principal stress increases in the plane strain direction.large until destroyed.The middle principal stress and middle principal stress coefficient curves of the remodeled loess show a nonlinear decreasing trend,which is similar to a hyperbola.The middle principal stress and middle principal stress coefficient curves of saturated loess are similar to those of undisturbed loess.The influence of water content on the evolution of the middle principal stress is weak,and its influence on the undisturbed loess is greater than that of the reshaped soil.（3）The bulk strain of the undisturbed loess soil sample is a dilatation type,and the lateral expansion first increases and then decreases.When the vertical strain reaches 1%,the curve turns and develops from lateral expansion to lateral contraction.The reshaped and saturated soil sample curves appear as straight lines.It decreases with increasing consolidation confining pressure,decreasing initial structural strength and increasing water content.In the initial stage of unloading,the strain of undisturbed loess is the largest,while that of remodeled and saturated loess is smaller.（4）The lateral deformation of undisturbed,reshaped and saturated loess under plane strain unloading conditions are all negative,showing different degrees of lateral expansion and extrusion.Under the conditions of low confining pressure and low water content,most of them show the slip failure of the obvious shear zone,and the other conditions can be attributed to the lateral expansion plastic failure.The higher the consolidation confining pressure,the lower the（-ε3）-ε1 curve;opposite on the water content.（5）The structural parameter of the stress ratio decreases with the increase of the vertical strain.Combined with the SEM test results and the destructive properties of the samples,the undisturbed loess has the strongest structure,and the types and shapes of the skeleton particles are relatively clear.With the increase of the confining pressure,the large particles and the aggregated structure are extruded into broken particles;with the increase of the water content,the pores of the sample increase and increase,the original skeleton structure is destroyed,and the strength decreases.（6）The shear strength parameter of loess decreases linearly,and the cohesion and internal friction angle of undisturbed soil decrease significantly with the increase of water content.,the attenuation trend of the internal friction angle is different from undisturbed loess,change of the internal friction angle of the reshaped soil is small.The shear strength parameter decreases due to the weakening of the initial structural state,and is significantly affected by the initial structural state.