| Depending on the key project of Ministry of Railway "Experimental test on key technology of composite foundation in deep thick soft soil in Beijing-Shanghai high speed railway-Research on character and calculation method of settlement of composite foundation using ram-compacted piles with bearing base" (NO.2008G32-A), basing on the section of site test at Lang fang in Beijing-Shanghai high speed railway, this dissertation conduct the theoretical research on the pile-raft composite foundation of Ram-compacted Piles with Bearing Base (RPBB) aiming at meeting the settlement standard of high speed railway.1. Some summaries are presented to specify the skip-piling techniques, the down-the-hole hammer with casing techniques and other techniques during the construction of the RPBB composite foundation in high speed railway based on site construction, and construction parameters are introduced as well in this dissertation for RPBB used in high speed railway.2. Compacting effects of the soil around expanded pile end of RPBB is evident by analyzing and contrasting the test result of heavy dynamic penetration of RPBB and CFG pile as well as pipe pile, the behavior is also shown in the area where between the top surface and 1.5m above the surface of the expanded pile end, while the test result of CFG pile and pipe pile is not obvious.3. Post-construction settlements of pipe pile, CFG and RPBB composite foundation all perfectly meets the demands of the high speed railway based on monitored settlement. The deformation and the stress of RPBB are compared between pipe pile and CFG piles; then the results show RPBB composite foundation has a larger settlement than pipe pile and CFG pile, and takes more time to stabilize.the final settlement of RPBB composite foundation accounted for 23% of pipe pile and for CFG pile the number is 52%, when the test end. In this test, the foundation caps of the pile exert little on settlement.4. The deformation superposition principle and Hamilton system are adopted to obtain the analytical solution of raft in pile-raft foundation, in an effort to overcome defect of on-service simplified method subject to missing accurate analytical solution. First, get analytic solution of the rectangular plate with sliding support on Winkler elastic foundation, under the combined action of distributed load and point load. Consider two analytical solutions and a hypothesis that no deformation on pile top, one can find the point force on pile top. And then put that into the analytical solution of the point load and connect with the analytical solution of distributed load by the superposition principle, the analytic solution of the raft can be obtained. Data from in-situ field indicates the validation of solution, and a further sensitivity analysis for related parameters is presented and for construction practice consideration.5. Take into account the bearing performance of RPBB in high speed railway, the pile is regarded as a dowel bar and expanded end as an unenforced spread foundation. The dissertation discuss the settlement and deformation of the stabilized layer and put forward the settlement calculation pattern of the underlying layer of the RPBB composite foundation in high speed railway based on Specification for design of ram-compacted piles with bearing base. Equivalent entirety method is used to calculate the load of the underlying layer and Mindlin solution is adopted to find the additional stress. Settlement of each layer is found after modifying the elastic modulus of underlying layer soil with respect to depth and compression layer thickness is determined by strain control. Total settlement of the underlying layer can be thus obtained by adding the settlement of every layer based on the modified layer-summation.6. Plaxis 3D Foundation is applicable to further research RPBB composite foundation with the supporting measured values of the test site. Reasonability of FEM to simulate the site test is verified by comparison with the measured values and parameters sensitivity analysis is conducted for further study as to length, modulus and individual failing of pile. |
