| For a high-speed railway bridge built in soft soil, excessive settlement can seriously affect the railway safety of operation. So, how to strictly control the settlement has become a common concern in this filed. As a new type of bridge foundation, lattice shaped diaphragm wall (hereinafter for LSDW) provides a potential new solution to this problem. In order to investigate the applicability of LSDW as bridge foundations in soft soil, firstly, three models (pile group, LSDWs with single chamber and two chambers) under similar material quantity are studied by small-scale model tests and numerical simulation. Secondly, the model test of LSDWs under large settlement is carried out to figure out the influence of chamber numbers on the vertical behavior such as wall group effect for LSDW. Meanwhile, several numerical simulations by FLAC3D are used to study the behavior of LSDW with six chambers and the chamber number effect on the behavior of LSDW. In addition, the soil arching effect of LSDW is discussed by PFC2D. At last, a simplified method for the settlement prediction of LSDW is proposed based on the load transfer method which can be used in the initial phase of designing LSDW. The main results in this paper can be drawn as follows.1. In order to investigate the vertical bearing behavior and applicability of LSDW as bridge foundations of high speed railway in deep soft soil, three foundation models (LSDW with single chamber and two chambers as well as group piles, respectively) with similar material quantity are studied by model test. It can be found that the settlement of LSDW with single chamber and two chambers are quite similar while the settlement of group piles is relatively large under the same loadings. Compared to group piles, the ultimate bearing capacity of LSDW with single chamber and two chambers are improved for 16.7% and 20.4%increases respectively considering the settlement limits. For a practical bridge foundation, the bearing capacity and the settlement will be improved by utilizing LSDWs instead of group piles in soft soil.2. The parameters sensitivities of the side interface and the bottom interface are investigated by taking the piles under compression and tension in sand as basic numerical examples, respectively. Based on the sensitivity analysis of interface parameters, a simplified method for the determination of pile-soil interface parameters in layered ground based on FLAC3D is proposed and verified by an example of specific project. Meanwhile, the prototype of the model test are simulated based on the determine method of interface parameters. It can be found that although the Q-s curves of three prototypes changes linearly, the varying trend of settlement with loads of prototypes is basically the same with which of three testing models. Namely, the settlement of LSDWs with single chamber and two chambers are almost the same during the whole loading process but are both larger than which of pile group.3. In order to investigate the vertical behavior of LSDW with large settlement, three foundation models (LSDW with single, two and four chambers, respectively) are studied by model test. It can be found that the settlement of LSDW with single chamber and two chambers are quite similar while the settlement of LSDW with four chambers is relatively small under the same loadings. Under the same depth and wall thickness, adopting fewer numbers of chambers of LSDWs is more useful to weaken the effect of wall groups than enlarging the edge length of chambers, so that a better bearing efficiency of foundation can be achieved.4. By using the finite difference software FLAC3D, the foundation of Aomori Bridge (LSDW with six chambers) is simulated. The results show that the distributions of the inner and the outer skin friction are affected by the geometry character of LSDW. In which, the outer skin friction is relatively large at the angular part and the inner skin friction shows a relatively small value at the angular location. The chamber number effect is also studied by FLAC3D; it can be found that the area of inner wall increases with the growth of chamber numbers which can turn the loading proportion of inner skin friction to be a relatively large value. Therefore, the properties of smaller chamber size and less chamber number can be useful to improve the supporting efficiency of LSDW.5. Based on the particle flow theory, the developing process of soil arching, the soil arching shape for LSDW and the parameters influencing the soil arching effect are studied thoroughly by utilizing the software of PFC2D. It turns out that the shape of soil arching is basically a hemisphere which can be explained by the theory proposed by Hewlett and Randolph. The soil arching effect is significantly influenced by the distance of two adjacent wall elements and the foundation depth, and a relatively large or small value of these factors is disadvantageous to the exertion of soil arching. The soil arching effect increase with the growth of ball stiffness (kn and ks) and the bonding strength of particles has insignificant influence on the development of soil arching effect.6. On the basis of load transfer method which was used widely to calculating settlement of pile, a simplified method for predicting settlement of LSDW in soft soil is proposed considering its own properties on structure and load transfer mechanism. In which, the concepts of equivalent shear stiffness and the equivalent shear stiffness ratio are presented so that a fit formulas can be used to estimate the inner skin friction based on the results of model test. The whole analysis process of the simplified method for settlement prediction can be realized by the software of MATLAB r2010b. Finally, iterative calculation method proposed programming operation and visualization. Finally, taking the foundation of Aomori Bridge (LSDW with six chambers) as an example, the proposed simplified method for predicting settlement are verified and the calculating settlement is closed to the field monitoring data.. |
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