Shallow Foundation Pile Reinforcement Of The Second Disease Model

There are many expanding foundations in the existed railway bridges. Various kinds of "diseases of shallow foundation" happened due to the insufficient estimates during construction or the changes of hydrology conditions after construction. For dealing with "the diseases of shallow foundation", there is a common method: to reinforce the expanding foundation by the method of pile-foundation replacement. It is an effective method. Traditional pile-foundation design and calculation theories recommended by the railway design standards for bridges and culverts of our country are commonly used by site engineers to cope with the calculations of shallow foundation reinforced by piles. It is the "m" method. Because the piles used in the bridges are mostly long and elastic, "a simple and quick method" to calculate the displacement and internal force is also allowed by《The Bridge Standards》. It is thought that the composed pile cap is rigid in the "simple and quick method". And analyses and calculations of piles under a rigid cap are carried out only taking the internal force caused by horizontal loads into consideration.In the actual projects, in the pile foundation with a composed pile cap after reinforcement, soft stratums or cavities may exist under the bottom of the pile cap with construction or hydrology reasons and so on. The existence of the soft stratums or cavities leads to fracturing of the concrete at the bottom of the pile cap which reduces the rigidity of the composed pile cap. Obviously, the "simple and quick method" in the "m" method which is in the engineers' common use is an approximate one. If the "simple and quick method" is used for the design and calculation of the reinforcement project, we will not only overlook the influence to the internal force of the pile shaft caused by the displacement of the pile cap and the vertical loads, but also probably lead to "second diseases" because of the insufficient estimate. For this reason, in order to make the analyses and calculations more accord with actual circumstance, the influence to the internal force and the displacement of the piles caused by the displacement of the pile cap and the vertical loads must be taken into consideration. Based on those problems mentioned above, the following works are carried out in this thesis:(1) Calculation and analysis theories existed which are used in the analysis and calculation methods together with the related pile foundation analysis and calculation theories are summarized.(2) A model used for simplify calculation of the reinforcement of the shallow foundation by pile is put forward with consideration of the displacement of the pile cap in this paper. Related formulas have been deduced and a program in Visual Basic language has been worked out.(3) In this paper, Visual Basic program is used for the calculation of a real engineering example and the influence of bending moment of the piles caused by the parameters such as flush depth, the thickness of the cap, the horizontal width of the cap, the diameter of pile and so on; Calculations are carried out through a finite analysis software ANSYS, and comparisons of calculation results and suitability are given among the mechanics model together with the "simple and quick method" and the ANSYS method.(4) Nonlinear regression analyses are carried out, and a series of empirical formulas have been worked out which are about the revised coefficient caused by vertical loads with each influence parameter on bending moment of the piles. Related charts which can be used for calculation are also provided, which can make a convenient use for the engineers.For calculations of the shallow foundation reinforcement by piles, engineers can still use "simple and quick method" under circumstance that the pile cap has a big rigidity; But, under circumstance that the pile cap has a small rigidity, calculations should be carried out by the model put forward in this thesis or make a necessary correction according to the results of the model, to make it more suitable for the actual conditions.