ASSESSMENT OF FEM CALCULATION RESULTS ACCURACY WITH THE USE OF SYMMETRY PLANES IN GEOTECHNICAL PROBLEMS

Abstract

To calculate different geotechnical objects (foundations of unique buildings, pile foundation grillages, slab foundations, embankments, etc.) we often need to simplify geometric model. Use of symmetry principles allows to simplify the model without reducing its detail. A relevant issue with the use of symmetry planes is the assessment of calculation results accuracy. The article considers the research of such an assessment for two traditional geotechnical problems: a shallow foundation slab and a shoring of deep excavation. Calculations have been performed with various options of symmetry planes usage: a full-scale model, using one symmetry plane, using two symmetry planes; differences in the ways of representing building structures: 2D and 3D continuum elements. Generating the finite element mesh, we took into account the fact, that the degree of mesh grinding significantly influence the accuracy of the calculation results. That’s why the finite element mesh was divided into two areas: the area of «interest» represented by small sized elements, and the «peripheral» area created by larger elements. We also considered the sequence of technological processes: the initial stage of the calculation is the creation of the initial stress state, the next stage is the creation of the structure (foundation slab, shoring of excavation), the final stage is the force application. The calculation results were evaluated according to the criteria related to the second group of limit states. The controlled parameters for the shallow foundation slab were: maximum and average settlement, relative difference of settlements; for the shoring of deep excavation - the maximum displacement of the enclosure structure. It was found out that to present the construction in the form of 2D-elements it is necessary to introduce additional boundary conditions. They prevent rotation in nodes located on the plane of symmetry. The authors make conclusions and recommendations for creating design models with the use of symmetry planes.