Research On Structure Design Of Tubular Pile Composite Foundation Under The Embankment

The wild use of ballastless tracks on high-speed railway has raised a stringent requirement of post construction settlement. To build high-speed railway on deep soft soil the problem of inadequate foundation bearing capacity, excessive post construction settlement and differential settlement must be solved. How to reduce the construction investment under the premise of settlement control, the application of tubular piles with different structural types is one of the considerations. For this purpose, the paper carried out research work on the structure design of tubular pile composite foundation under the embankment.By3-D finite element simulation of tubular pile composite foundation, the impact of pile with and without cap, the cap size, pile spacing and geogrid on the settlement characteristics of composite foundation were analyzed. An appropriate design method for the tubular pile composite structure was then proposed on this basis, and the following conclusions were obtained:1) Pile cap has a great impact on the tubular pile characters. Tubular pile with cap can reduce ground settlement, adjust the differential settlement, and it also can prevent the upward penetration deformation of tubular pile. In practical engineering applications, it is recommended to set a cap on the top of tubular pile;2) In group tubular pile composite foundation the maximum settlement occurred at the centerline under the embankment. The settlement gradually decreases with the distance increasing from the embankment centerline. Under the embankment loading, lateral ground displacement will accumulate. Different pile spacing has significant impact on soil settlement of the reinforced area, but has less effect on the settlement of underlying layer and lateral displacement. Increased pile spacing will significantly deteriorate the total settlement of the foundations and differential settlement, hence in practical applications, large pile spacing is not suggested and the appropriate value S/D=3～3.5is recommended.3) Geogrid can strengthen and improve the overall behavior of the composite foundation. Therefore it has good effect on settlemental homogenization and constraint of the lateral deformation. The joint application of geogrid and pile cap was demonstrated to be a good type of tubular pile structure with integrated performance in terms of total settlement reducing, differential settlement adjusting and lateral displacement constraint.4) A new tubular pile was proposed, which has the feature of structural separation and elastic contact. Th elastic contact can avoid possible destruction of the punching. While using this novel pile, the total settlement consists of two parts, i.e. the settlement generated in the stage of "separation" and "combination". When loaded, the pile cap will move down with the soil in the separation stage, and this movement will help to reinforce the deep soil so less settlement in "combined phase" will be expected and the phenomenon of uneven settlement will be improved. This new type of tubular pile structure is worthy of further study.