Disturbed Loess Under Cyclic Rotation Of Principal Stress Axis Research On Mechanical Properties And Micro Mechanism

The stress path of subgrade soil under traffic,earthquake and other loads is complex and changeable,especially the rotation stress path of principal stress axis is common.Only by completely simulating the stress state of the soil,can we respond to the actual stress path experienced by the soil and get more reliable experimental conclusions.In order to study the stress-strain characteristics of Qinghai remolded loess under complex stress paths,In this paper,the hollow cylinder torsional shear apparatus（HCA）was used to carry out a series of cyclic rotation test of the principal stress axis,shear test after cyclic rotation of the principal stress axis,directional shear test and SEM test of the specimen after shear failure.The main research conclusions are as follows:（1）The axial strainεz of remolded loess is closely related to the angular magnitude of the principal stress direction.The sample has the fastest accumulation rate of axial strain in the first two cycles.After 10 cycles,the sample finally accumulates as compressive strain.The radial strainεr of the specimen continuously fluctuates to the negative direction,and the angular amplitude of the principal stress direction has little effect on the circumferential strainεθ of the specimen.When the direction Angle of large principal stress α≤45°,the shear strain γ_zθ tends to increase positively on the whole,and when α>45°,the shear strain γ_zθ tends to increase negatively on the whole.The intermediate principal stress coefficient b has different effects on the strain of the sample in all directions.The samplesεz andεθ have realized positive accumulation,and the change of b presents a certain regular change.After a cyclic period,when b=0,the accumulated εz of the sample is the largest,andεγ continuously accumulates in the negative direction under different b,and εθ hardly accumulates.γ_zθ shows a continuous negative growth trend,and yzθ increases as b decreases.The deformation of the specimen under different deviator stress conditions is more obvious than that under other factors,and the amplitude ofεz gradually increases with the increase of deviator stress.The deviator stress level determines the fluctuation amplitude of εθ.The higher the deviator stress level,the larger the range of εγ and εθ changes during rotation;the increase of γ_zθ is the most obvious when the deviator stress is 145kPa,and the accumulation of plastic γ_zθ is the most.（2）The octahedral shear stress-strain curve of the sample under different α and b exhibits a highly nonlinear growth,showing a strain hardening phenomenon.Whenα=75° and 90°,the large principal strain ε₁ changes first to negative and then to positive,and the small principal strain ε₃ first changes to the positive direction and then to the negative direction;The phenomenon is more obvious at 90°.When b=0.5,the large principal strain produced by the soil is the largest,and when b=1,the large principal strain produced by the soil is the smallest.The normalized strength of remolded loess decreases continuously with the increase of α,and presents a parabolic decrease trend with the increase of b,and the deceleration rate increases with the increase of b.（3）In the process of directional shearing at β=45°,the development of principal strain ε shows ε₁>0,ε₂≈0.ε₃<0;ε₁ and ε₃ develop symmetrically around ε₂.The principal strain development law of the specimen undergoing the rotation of the principal stress axis shows ductility.The larger the average principal stress p,the better the ductility and the stronger the resistance to deformation.The specimen ε without principal stress axis rotation exhibits brittleness.The remolded loess with dry densityρ_d≤1.57g/cm3 exhibits ductile failure characteristics in the later stage of shearing,and the remolded loess with ρ_d≥ 1.62g/cm3 exhibits brittle failure characteristics.The influence of ρ_d on the normalized strength of soil samples firstly increases and then weakens,and the normalized strength is the largest when ρ_d=1.72 g/cm3.（4）With the increase of p,the pore in soil decreases gradually in axial,radial and circular directions,and the pore arrangement in soil becomes more and more disordered in three directions.When b=0,the soil has the greatest influence in the axial direction,when b=0.5,the change is more significant in the circular direction,and when b=1,the influence is more obvious in the radial direction.The gradual decrease of the pore area ratio of remolded loess under different p corresponds to the soil macroscopic phenomenon,while the normalized strength law of remolded loess under different b is exactly the opposite of the change law of pore area ratio.