Application Of Finite-element-based Limit Analysis With Mesh Adaption In Slope Stability Anaysis

Based on finite-element limit analysis theory,the failure modes of structures can be found out by calculating automatically without assumpting it like the traditional limit analysis method.To avoild the yield derivative of surface in Mohr-Coulomb yield criterion existing singularities and common nonlinear programming algorithm can not be directly applied,second-order-cone optimization for Mohr-Coulomb yield criterion generates an equality constraint and a second order cone constraints,combine that with he derivation of limit analysis. Assumpting material ideal elastoplastic,the study region is divided into two parts:plastic sliding region and elastic region,the margin of safety depends on the slide resistance which is related to the internal friction angle φ and cohesive materials c,the slide is plastic flow area in slope instability.calculating accuracy based on finite-element limit analysis is almost determined by the arrangement of grid,and calculating deviation of sarfety margin roots in plastic sliding region,mesh-adaption in in plastic region is a good way to improve the accuracy of the finite-element calculation.(1) Calculate and analysis the initial coarse meshed model, get the stress and strain data of distinct element, according to a preset stress / strain threshold,mark the objective distinct element by estimating the yield function slack and deformation with Mohr-Coulomb yield criterion of second-order-cone optimization,mesh optimization for marked element of the slide region doubles the number of units in plastic area in an iterative process,the mesh in quantity and quality is improved,then calculate again.That is an iterative process for finite-element limit analysis system with mesh adaption,repeat the above process until reaching the preset accuracy. Finite-element limit analysis with mesh adaption makes the mesh of local region automated and purposive, the 2-D construction illustrates Finite-element limit anysis with mesh adaption improves calcaulation in terms of efficiency and accuracy greatly.(2) Based the algorithms above, reveal the dynamic mechanism of slope instability in numberical simulation, with landslide taking place, the plastic deformation region develops from the slope angle to the top of sliding zone. when the plastic zone develops to a certain position, some of slide stress of elastic region and plastic zone up sliding zone is smaller thanthat resisting force, the slope tension began to be crested to supplement resisting force, after reaching the maximum tensile strength, the overall slope begins to slide.There is apparent turning point of external monitoring points- time curve when elastic deformation phase transiting to Elastoplastic stage, the monitoring point displacement- time curves began to be different linear at the up and down of sliding zone,it may provide the basis for prediction.(3) Sensitivity anaysis of slope stability to material parameters,with different elastic modulus and Poisson’s ratio, there is no difference in safety factor and stress / strain contours as well as the plastic region size.location and so on, elastic modulus and Poisson’s ratio is only regarded as intermediate variables in calculation, no impact on the calculation result. The value of unit weight directly affect the safety factor, but the location and size and the residual stress of the slide region seems no differentence.Effect of cohesion and internal friction angle on slide region is reversed, as the value increases, their sliding belts rotate downward and upward on its lowest position, greater cohesion means more possible to deep landslide,greater internal friction angle means more possible to shallow landslide, the opposite features help to explain in finite-element limit analysis that making both synchronous reduction in with strength reduction is important. The vaule of internal friction angle and cohesion also affects residual stress of sliding belt, greater cohesion means smaller residual stress of plastic region, with a big vaule close to the brittleness, residual stress is almost zero, which matchs facts,but greater internal friction angle means greater residual stress of plastic region.