Stability Analysis Of Footing Above Void Using Upper Bound Finite Element Method

With the rapid development of urban and traffic construction, it is common to encounter footing above void on the process of design and construction. How to reasonably evaluate the stability of the footing above void becomes the key to success in the engineering construction. Improper handling can bring great potential safety risks and economic losses. However, currently the need of the practical engineering are far from satisfied by the calculation of the bearing capacity of the footing above void, the knowledge of the failure mechanism of the foundation is still less, and related standards are too simple. At present, the footing above void stability analysis,because of many influence factors, analysis and evaluation methods still rely on qualitative analysis and the engineering experience, as for the quantitative evaluation is still in the exploratory stage, the method to determine the bearing capacity of footing above void is very scarce. At the same time, a large number of engineering experiences show that the footing above void can be used as the structure of the foundation through appropriate treatment. Therefore, to carry out the footing above void ultimate bearing capacity and failure mode of the research has important theoretical significance and engineering practical value.Aiming at the footing above void stability evaluation method, this paper adopts a new method for analyzing the stability of geotechnical engineering―using upper bound finite element method to numerically stimulate the limit state during foundation damage. Firstly, this paper expounds the basic assumption and limit analysis upper limit theorem of limit analysis; On this basis, the upper limit value principle of the finite element method is introduced, including: numerical discretization method, the upper limit theorem of the optimization model and optimization model based on feasible arc interior point algorithm for solving; At the same time, the author introduce the theory of adaptive mesh refinement of upper bound finite element method. Secondly, the reasonable calculation assumption of the footing above void is put forward, a calculation model considering the influences of various factors is built, a method of the bearing capacity of foundation is presented and the calculation process of upper bound finite element method is briefly introduced.Thirdly, the upper limit solution of ultimate bearing capacity under various conditions was calculated by using upper bound finite element method, and bearing capacitycoefficients Nc 、 Nq 、 Nγ were computed by using the formula of ultimate bearing capacity, as Terzaghi suggested, and then their influencing factors were analyzed; At the same time, according to a lot of energy dissipation and velocity field pattern, this paper puts forward three kinds of typical failure mode, and analyzes its influence factors. Finally, ultimate bearing capacity of foundation when the void lies on the side of foundation and energy dissipation are calculated by upper bound finite element numerical calculation, and the impact of the void position are analyzed. Studies show that Nc and Nq increases with the increasing of φ, and when φ is larger, Nγ is positive number, and increases with the increasing of φ; Nc and Nq increases with the increasing of H/B, and when φ is smaller, Nγ decreases with the increase of H/B, whenφ is larger, Nγ increases with the increasing of H/B; Nc、Nq and Nγ decreases with the increase of D/B; footing above void has three typical failure mechanisms: punching failure mode, punching shear press failure mode and Prandtl failure mode; With the increase of the distance of the void from the center line X and the depth of the void Y,bearing capacity increases, the foundation damage develops to the Prandtl failure mode.