Numerical Analysis And Experimental Study On The Large Diameter Belled Pile

With the construction of large municipal structures, wide-span bridges and high buildings, the large diameter belled piles have been used widely due to their high bearing capacity. The study of the bearing characteristic, the deformation mechanism and the bearing behavior of large diameter belled pile in deep soft soil has been expanded from analysis of field test data, analytic study and numerical calculation.Based on the static load tests and stress measurement of large diameter cast-in-place piles and belled piles in deep soft soil foundation, the bearing capacity behavior and load transfer mechanism of large diameter cast-in-place piles and belled piles are studied. The difference in the bearing capacity behavior and load transfer between large diameter cast-in-place piles and belled piles are emphasized in the analysis. The action laws of pile end resistance and lateral friction resistance are obtained. The lateral friction resistance of large diameter belled piles firstly takes effect. The belled part affects not only the pile end resistance but also the lateral friction resistance. Also the frictional resistance of large diameter belled piles will be degraded under higher load level. The bearing behavior of large diameter belled pile is that of friction piles under low load level and that of end-bearing friction piles under heavy load level.Based on the generalized parabolic model and the load transfer method, coupling with the Mindlin solution and the Rusch model which considering the settlement produced by the shaft resistance and the plasticity of concrete under heavy load respectively, the analysis model of large diameter belled pile is set up. Its rationality is proved by the contrast of the calculating and the field test results. The difference in the bearing capacity behavior large diameter cast-in-place piles and belled piles is compared by using this model. And the effects of shaft geometry on bearing and deformation characteristic of large diameter belled pile are studied by using this model. The changing regularities of end bearing ratio of large diameter belled pile are obtained.Considering the factors, such as slipping and disengage between pile and soil, and the nonlinear behavior of soil, the analysis model is set up by using the finite element method. The changing regularities of pile top settlement, axis force in pile, the displacement around the pile, axial and frictional load of pile and stress of soil under pile are studied by using related software. Also the factors affecting the vertical bearing behavior of pile are analyzed. A case study is performed on the basis of analyzing data from field tests and the difference in Q-s curves between the large diameter cast-in-place pile and belled piles is compared. The results show that the bearing behavior and mechanics of pile can be simulated by the finite element methods. The bearing capacity increases with the increasing of diameter, expanding diameter and length of pile. And the bearing capacity has an optimal value between the diameter, expanding diameter and length of pile. The soil stress under pile is affected by the top load of pile. The module of soil under pile can obviously affect the bearing capacity of pile, but the module of soil around pile affects less.Finally, considering the fact that the large diameter belled piles bear not only the vertical load but also the lateral load in engineering practice, the lateral bearing behavior of large diameter belled piles should be studied. In order to study, a numerical model is adopted for taking into account the nonlinear geometric properties of the pile-soil system, the interaction between concrete and steel and the nonlinear properties of concrete. The regularities of pile plastic strain, bending moment, shear force, stress distribution, lateral displacement of pile, separated state between pile and near soil are studied under different loading levels through an example. At the same time, some factors, which affect the lateral bearing behavior of large diameter belled pile, are discussed by using the numerical model.