Study On Behavior Of Integral Pile-Raft Structure And Its Design Method In Beijing-shanghai High-speed Railway

Depending on the successful application of piled raft foundation in construction engineering, the integral pile-raft structure has been used to handle the deep Quaternary soft ground of high-speed railway for the first time. The main purpose of this treatment is to increase structural integrity and long-term stability of the subgrade, control the total and uneven settlement, so as to achieve the stability of upper track structure and the condition which high-speed traffic demands. However, there is little theoretical research about the integral pile-raft structure in high-speed railway, and the information about the structure’s performance and design methods is scarce. Therefore it’s meaningful to explore and study such issues as the interaction among the structure’s raft, pile and foundation soil, the settlement controlling effect, the simplified analyzing model of pile-raft structure used in the designing process, and the design method which is advanced, safety and economic.With Beijing-Shanghai high-speed railway as the backgroud, the integral pile-raft structure was modeled using the static ABAQUS finite element method and the performance of each component was analyzed when considering the load characteristics of the high-speed railway’s upper structure and the interaction effect among raft, pile and foundation soil. The analysis results showed that the pile-raft structure greatly improved the deformation modulus and composite stiffness of the reinforced area. And the raft should be seen as two-way slabs which homogenizes the forces, piles were under a combination of force effects including compression, bending and shear.Key issues about the high-speed railway subgrade:the total settlement, layered deposition, lateral deformation and settlement after construction had been discussed through settlement analysis of the long-term field tests combined with finite element analysis and the systemic theoretical calculations. The results showed that the integral pile-raft structure had an obvious effect in controlling the subgrade settlement. The e-P curve and Es method had been proposed for the total settlement calculation of the pile-raft structure in high-speed railway. And corresponding settlement amending factors had been suggested. It was also suggested that the settlement after construction of the moderate compression soil should be calculated by the completed proportion of the total settlement in different stages.Field tests and theoretical analysis of the internal force and deformation of the integral pile-raft structure showed that there may be some surplus in the original design. So designing optimization has been done by using the simplified calculation of probabilistic limit state method which is used generally in concrete structure. According to the work property of each component calculated by the simplified SAP2000finite element model, one-dimension continuous beam method has been proposed for the raft designing and two-dimension frame beam method has been suggested for the pile designing. It’s also been found that three-dimension framework method could better reflect the spatial properties of the pile-raft structure. And it’s recommended to analyze the structure’s working behavior with the ABAQUS solid element model.Considering the characteristics of high-speed railway loads, load effects were calculated in conditions of pre-press and operation period respectively. The design was carried out under a combination of ultimate limit state and serviceability limit state of the components as well as the overall structure. All the research results indicated that the structure design would be more economical and reasonable when taking the probability limit state dsesign method and each component of the structure would have a clear physical meaning of reliability.The study of the pile-raft structure’s work properties, performance, calculation theory and design method provides a reference for the theory analysis, design application and the wider application of the integral pile-raft structure.