The Down Hole Dynamic Compaction Pile Composite Foundation Application Research On Collapsible Loess Ground Of Zhengzhou To Xian Passenger Dedicated Railway Line

Most of section along the newly built Zhengzhou-Xi'an Railway belongs to the geomorphic unit of loess accumulation. Due to the perforate and collapsible feature of loess, some technological methods are utilized to handle the collapsible loess foundation along the line of Zhengzhou-Xi'an Railway, such as, dynamic compaction, compaction, pile-board structure, down hole dynamic compaction (DDC in short) of composite foundation, and so on. Among all these methods, DDC pile composite foundation is a newer type of construction technology, domestic research on its service performance and mechanical characteristics still lacked at present. Therefore, it is very significant to research about its characteristics of settlement and stress.Based on the available research achievements, this paper undertakes a further discussion about the application of DDC piles to collapsible loess foundation, making use of centrifugal model test, numerical analysis, field monitoring and other methods.In the numeric analysis of this paper, we make use of the finite element analysis, construct four types of contrast models, and study the influences that stress deformation receives from reinforced layer, pile spacing, pile module and pile length. The following conclusions are finally reached: reinforced layer can reduce the settlement effectively to 75 percent of non-layer composite foundation. The sparse pile design is verified. The reduction of pile module does not much affect the settlement. When the pile length increases, the stress distribution angle of subjacent layer under composite foundation reduces and pile-soil stress ratio increases. However, the pile-soil stress ratio will not increase after the ratio of pile length to diameter is over 30, and it is clear that there exists an effective length of pile.In the centrifuge model test, it is proved that the foundation settlement turned steady after the placement of DDC pile composite foundation 6 months later and the total settlement was 5.2mm. In the 25th month after the railway being put into service, the foundation settlement turned steady and the total settlement was 4.7mm.According to the results of field testing, the foundation settlement was 8.4mm after the completion of filling embankment, while it was 6.9mm after the foundation became steady 6 months later.