Research Of Group Foundation With Unequal Piles Of Multi-span Cable-stayed Bridge With Ultra-high Piers

Group piles foundation is one of the most common foundations in the designing of substruction. The available analysis theories are based on the case that the length of piles under same cap is basically or exactly equal. However, in case of pile group foundation with unequal pile length, in actual projects, the theoretical research relatively can't meet actual engineering's needs at present. Therefore, based on the current project of"ChiShi bridge"of the RuChen highway under construction, the author has done certain research on design and calculation of the pile group foundation with unequal pile length of the cable-stayed bridge with ultra-high pier and multi-tower. Its purpose is to improve the design theory of the cable-stayed bridge with high pier and multi-tower, and provide reference to designers when case is similar. In view of this, the author in this paper focused on the following aspects:(1) It has introduced the design and calculation of the pile group foundation with unequal pile length's concrete contents and basic steps in detail, and the analyzed method for solving the internal forces of the pile group foundation is discussed. Then the internal forces result of pile under different action mode of load is provided, and the internal force distribution of the pile group foundation with unequal pile length is summarized by analyzing the results. The result shows that the internal force distribution of foundation is uneven, and some additional internal force would be generated under the load except the effect of moment, witch could result in series adverse effects, and should be calculated emphatically.(2) It has introduced general steps and methods of the pile group foundation with unequal pile length in finite element analysis, and deeply discussed and studied on issues such as element selection, boundary condition simulation, calculating width determination, calculating pile length selection and pile-soil interaction. On basis of this, the method using concept of equalized pile length for consideration of the pile side frictional resistance's effect is suggested. The result shows that this method will disrease the sum of the finite elements greatly, and the analysis efficiency is improved.(3) The analysis of the tower 6's foundation is carried out after the finite element model is set up by the program Midas. And the result of finite element analysis and the result obtaind through the calculate method posed in above aspect 1 are compared, the comparing indicates that the results of the two methods show good agreement.(4) It has analyzed possible problems of the pile group foundation with unequal pile length, and carried on detail calculation on the problem of pile cap bearing uneven reaction, cable tower deviation caused by foundation uneven subsidence, foundation settlement influence on internal force of the whole bridge by finite element analysis program Ansys (or Midas). Based on this, the cause of the problem has been analyzed, and some relevant conclusions have been summarized.(5) It has provided some improvements when it can't meet design or construction requirements leading by the problem of unequal pile length of the pile group foundation; Several improvement scheme also have been offered using the example of cable tower 6, the comparing of the analysis results of before and afer the improvement shows that each improvement proposal is effective and feasible. Finally, the advantages and disadvantages of each method have been summarized, when corresponding improvement suggestion given.