Finite Element Analysis Of Slope Stability Considering Tensile Stress And Strength Anisotropy

Slope stability has always been an important research concern in geotechnical engineering.Conventional limit equilibrium method requires to assume the sliding surface geometry and the interaction of inter-slice force,and the limit analysis method cannot consider the slopes with heterogeneous material and irregular geometry.The finite element method(FEM)can overcome the above limitations or assumptions effectively.On the one hand,previous FEM analysis often focuses on the shear failure of the slopes,losing sight of the effect of tensile stress on the crest of slopes.Practically,theoretical analysis,physical model experiments and field observation have confirmed the occurrence of tensile stress on the crest of slopes.The real failure pattern cannot be indicated if the shear failure assumption continues to be conducted.On the other hand,geotechnical materials often exhibit different strength characteristics along different shear directions during experiencing shear action,that is,strength anisotropy.Existing studies have made many explorations on the anisotropy of soil cohesion under saturated and undrained conditions,but the effect of the anisotropy of internal friction angle on slope stability under drainage conditions is not clear.Based on the above issues,this paper performs the self-programming MATLAB finite element code to analyze the slope stability.Firstly,the soil slope stability with shear failure occurring is compared with the existing research based on the Mohr-coulomb strength criterion,thus verified the feasibility of the program,and the high-precision element shape,plastic stress integration algorithm and slope instability criterion selection method in the finite element method analysis are given to establish the foundation for subsequent research.Secondly,aiming at the fact that the classic Mohr-Coulomb strength criterion overestimates the tensile strength of soil,this study proposes a Mohr-Coulomb tension cut-off criterion to achieve the purpose of modifying the tensile strength,and combined with strength reduction methods,to analyze the relationship between slope stability and the tension cut-off degree under different flow criterion,respectively.Thirdly,by utilizing the anisotropic Mohr-Coulomb strength criterion,combined with the associated flow criterion,this study discusses the variation of the strength parameters of the soil under different stress combinations,analyzes the development and evolution of the plastic zone under the strength anisotropy,and reveals the instability process and failure pattern,aswell as the correlation between the anisotropy parameter and the variation of the stability.Finally,the main research results can be concluded as follows:(1)The explicit modified Euler algorithm with sub-steps has higher precision and efficiency than the modified Euler algorithm;The accuracy of triangle element with equal side length is lower than that of the quadrilateral isoparametric element,and the latter has wider application range;the sudden increase in displacement of the feature points of the slope after the connection of plastic zone is selected as the instability criterion,which has higher precision and can better reflect the actual failure mode of the slope;The self-programming MATLAB code proposed by this study to calculate the slope stability is feasible.(2)The modified Mohr-Coulomb cutoff criterion proposed in this paper can well verify the effect of tensile strength on slope stability;The safety factor of soil slope calculated by the non-associated flow criterion only considering the shear failure is 9.65% greater than that considering the tensile and shear failure(ft=0k Pa),The safety factor of soil slope calculated by the associated flow criterion only considering the shear failure is 9.4% greater than that considering the tensile and shear failure(ft=0k Pa).The tensile strength(ft?0k Pa)with lower value has a more dramatical influence on the slope stability.(3)Tensile stress has a significant influence on the failure mechanisms of slopes.Considering the tensile cutoff strength criterion in the slope stability analysis,the obvious tensile stress zone will appear at the crest of slope.The tensile yield will occur while the tensile stress exceeds tensile strength.Then,the tensile plastic zone and the shear plastic zone progressively connect,forming a slope instability mode of controlled by the tensile-shear failure mechanism.Meanwhile,this paper discusses the influence of flow criterion on the slope stability,suggesting that the result of associated flow criterion is larger than that of non-associated flow criterion,with a difference smaller than 4%.(4)The stability of the slope is discussed based on the anisotropic yield criterion,which utilizes two shape parameters n and ? based on the Mohr-Coulomb yield criterion,and considers the influence of initially anisotropic strength of soil masses.Thereinto,n is the primary causative factor,which can result in a more significant characteristic of anisotropic strength of soil masses with a small value,thus leading to the lower safety factor of slope.Specifically,when the anisotropy parameter n=0.707,?=22.5°,the safety factor(Fs)of the slope is 32.9% lower than that under the homogeneous isotropic condition.(5)The anisotropic parameter ? is the azimuth angle corresponding to the maximum internal friction angle,and its influence cannot be ignored.When the value of n is determined,the change of ? can lead the difference of slope safety factor to reach 13.73%(slope consisting of weak interlayer).The smaller the value of n,the greater the influence of ? on the slope stability.