Study On Strip And Slab Structures In Geotechnical Engineering By FEM

Pile and cut-off wall are widely used in geotechnical engineering. This kind of structures is in the form of strip or slab. In engineering design, people usually concern about the numerical result of displacement and internal force, especially bending moment of structure. But with finite element method (FEM), only the element stress and nodal displacement can be obtained .What is more, Shear locking is likely to occur in this kind of structures, which can induce smaller dislacement result. So it is necessary to study on how to calculate bending moment utilizing the result of FEM. Based on the research of a cantilever beam and a passive pile, the author investigated the shear locking, bending moment calculation, equivalent stiffness and resistance capacity of rectangular element for big ratio of length to width.Several types of element were used in the research of cantilever beam's computation. Differences between beam element and solid element were studied when concentrated force and uniform load are applied on the beam, respectively. The computational results indicated that shear locking occurs when Timoshenko beam element is used, while it can be avoided if Euler beam element is used. For solid element, the precision of displacement and bending moment can be improved as the element partition quantities are increased in length direction. The precision of calculating bending moment with stress can be improved as the element partition quantities are increased in section direction, but the precision of calculating bending moment with displacement may be reduced in the same way. Shear locking could be avoided when higher order and reduced integral element are used, but linear element can't. Serious oscillation occurs when bending moment is calculated with displacement, but the computational results are steady when bending moment is calculated with stress.Typical passive piles were analyzed with FEM following the above work. The research concentrated to the differences of displacement calculation and bending moment calculation in different conditions, such as different meshing, different load and stiffness. It showed that the results have the same characteristics with that of the cantilever beam. But shear locking phenomenon is not serious in passive pile. Slight oscillation occurs when bending moment is calculated with displacement by using high order element. Equivalent stiffness has little effect on the displacement calculation, but it has a big effect on the result of bending moment. Porper modulus and bending stiffness is necessary in the calculation. The author also analyzed displacement and bending moment calculation on cut-off wall of GAO LE HAN reservoir and pile of JIN TANG bridge. It indicated that oscillation is more obvious when the soil property is not uniform in the case of bending moment being calculated with displacement.The research illuminates that shear locking is not serious for the strip or slab structure when they are embedded in the soil. Better result could be achieved by proper meshing. Meshing in length direction has a remarkable effect on displacement calculation. Oscillation occurs when bending moment is calculated with displacement, and the result is not reliable in certain condition. Meshing in section direction needs more quantities in order to get satisfying precision. In addition, rectangle element has a good resistance capacity on big ratio of length to width.