Research On Calculation Of Soil-slope Safety Factor By Strength Reduction Finite Element Method

Compared with the conventional limit equilibrium method and the limit analytical method, the finite element method based on shear strength reduction (SSR) technique has many evident advantages in analyzing the stability of slope. The primary advantage of the Shear strength reduction finite element method (SSRFEM) is that it can obtain the most dangerous slip surface and the corresponding safety factors of the slope without predetermining the shape and location of the slip surface, but only by reducing the shear strength parameters of the rock and soil of the slope continuously until the shear stress exceeds the shear strength and consequently result in the failure of the slope. Shear strength reduction finite element method has the advantage of combining numerical analysis method with traditional limit equilibrium methods. However, there are also many problems of the SSRFEM. In order to solve above problems and deeply investigate the SSRFEM, this thesis mainly takes the homogeneous soil-slope as case studies, and analyzes their stability safety factors by making use of the SSRFEM. Some useful conclusions and improved measures are as follows:①Elastic-Perfect plastic model and Non-associated flow rule can satisfy the precision of slope engineering.②This paper proposes a new transform formula between any different yield criterions based on Drucker-Prager(D-P) criterions according to their semidiameter of projection circles on theπPlane. The ratio of the semidiameter of the projection circles on theπPlane of different D-P yield criterions is equal to the ratio of their corresponding safety factors. Because the ratio is only relative to the friction angle of the soil, this paper lists the transform tables about the frequently used friction angles and their corresponding safety factors under different D-P yield criterions, so that the safety factors of the slope can be found conveniently through the tables when different D-P yield criterions are employed in analyzing the stability of the slope in ANSYS. The example validates the correctness of the proposed transform method in this paper.③This paper compares the results of different D-P yield criterions under three different calculated conditions, and the conclusions indicate that it is more appropriate to take the D-P yield criterion which is matched by the Mohr-Coulomb criterion under the plane strain condition as the yield criterion of the homogeneous soil slope.④For the purpose of practical use in slope engineering, this paper makes a conclusion of the available method for determining the value of every parameter.⑤The criterions of slope failure in strength reduction finite element model were discussed. At present, there is no uniform understanding in this aspect. Combining with slope destroy feature and concrete embodiment of slope failure in the finite element calculation, this thesis points out that in the practice of safety factor calculation using SSRFEM the criterion of slope failure could base on whether the nodal displacement change and the nodal equivalent plastic strain change on slide surface is abrupt or not. And the slope failure criterion could also base on the force and displacement convergence curve develop trend in iteration process. This paper's author thinks that in engineering practice only depends on one kind of slope failure criterion to judge whether the slope will collapse and when the slope will failure or not is fallible. Taking traditional three failure criterions which are calculation divergence,plastic zone developed run-through from slope toe to top and dramatic increase of key nodal displacement all into account is recommended. And combine the slope destroy feature and concrete embodiment of slope failure in the finite element calculation to carry out systematic judgment of whether the slope will collapse and when the slope will failure or not. Using the viewpoint of systematic judgment, the dispute and disaccord of slope failure criterions may be solved.⑥Different calculating parameters have various influences on the analysis accuracy of slope stability: First, the variation of cohesion C and friction angleΦaffects the analysis results evidently, and the variation of E andνaffects the analysis slightly, which can nearly be ignored. However, the value of E can't be too little, or it will cause too large value of deformation of the slope, which results in the divergence of numerical analysis and affects the safety factor accurately. In case of no practical data, it's better for the Young's modulus E to be greater than equal to 1.0Mpa. The advantages and disadvantages of soil bulk density on slope stability should be systematically judged by concrete engineering geological condition and slope shape.