Study On Deformation And Strength Characteristics Of Saturated Loess Under Different Stress Paths

Loess is a kind of structural soil with the characteristics of loose,porous and weak cementation.After soaking in water,the structure of loess is damaged and its strength decreases,which can lead to a series of engineering problems.Loess is widely distributed in the middle reaches of the Yellow River in China.Under the influence of natural environmental changes and human activities,changes in the internal stress state and water content state of soil can easily lead to disasters such as collapse and landslides.The indoor triaxial test is an important way to study the deformation and strength characteristics of soil,which can control various stress paths.The stress path used in the test should be consistent with the actual engineering conditions,so that the deformation and strength parameters obtained are of practical significance.In this paper,saturated undisturbed loess and saturated remolded loess were selected as the research objects,and two stress paths of conventional consolidated undrained（CU）and constant shear stress drained（CSD）with pore water pressure rising were selected for triaxial tests.The stress-strain curve,pore water pressure change curve,and stress path curve were measured.This paper not only discusses the mechanical properties and differences of saturated loess under two stress paths,but also compares the different characteristics between undisturbed loess and remolded loess under the same path.The main understandings of the study are as follows:（1）Under the consolidated undrained path,both undisturbed loess and remolded loess have a softening stress-strain relationship.The peak strength increases with the increase of confining pressure.The softening degree of the remolded loess is smaller than that of the undisturbed loess under each confining pressure.The rise of pore water pressure is an important reason for the strain softening of loess.According to the change rate of pore water pressure with axial strain,it can be divided into three stages:stable pressurization,decelerating pressurization and steady pressure holding.The final value of pore water pressure is close to the confining pressure.The peak strength of soil occurs at the turning point between stable and decelerating pressurization.（2）In the constant shear stress drained triaxial test with increasing pore water pressure,the stress path with constant shear stress is maintained by increasing pore water pressure at a constant rate.For the undisturbed loess in the same soil layer,the deviatoric stress instability descending points under all confining pressures fall on the same straight line,namely the effective stress failure line.The soil failure is sudden,that is,the sample hardly deforms for a long period of time when the pore water pressure starts to increase.When the pore water pressure reaches a critical state,the deformation starts and completely destroys in a short period of time.（3）Similar to undisturbed loess,the deviatoric stress instability descending points of remolded loess also fall on the same line under the constant shear stress path.The difference is that the deformation process of the sample is longer when the shear stress is unchanged before complete failure,and there is a transition section from the beginning of deformation to complete failure.In view of the characteristics of remolded loess,a method to determine the yield state line under this path is proposed,that is,the stress state point corresponding to the increase trend of the axial strain-average effective stress curve is the yield point,and the yield state line can be obtained by fitting the yield points under different confining pressures.（4）The total stress strength of saturated undisturbed loess and remolded loess under CU path is less than the effective strength.For loess L₁ and paleosol S₁ with low dry density,failure lines basically coincide under CU path and CSD path,and there is no significant difference in effective strength.The average effective stress failure line can be obtained by fitting failure points under two stress paths according to a straight line.The effective strength of remolded loess has the same characteristics as that of loess L₁ and paleosol S₁.The effective strength of loess L₂ and paleosol S₂ with high dry density under the CSD path is obviously greater than that under the CU path.（5）The failure modes of saturated loess under two stress paths are obviously different.remolded loess presents swelling failure under both stress paths.However,the undisturbed loess is dominated by swelling failure under the CU path.When the dry density of soil is larger,an inconspicuous shear plane appears.Three are three failure modes of swelling failure,shear failure and liquefaction occur under the CSD path.