| It is a global problem to construct high class railway on deep soft soil subgrade, especially for those under or pending construction projects of high-speed passenger railway and high speed railway in coastal areas, which are usually located in soft soil with the thickness exceeding the quality control depth range of composite-foundation treatment method. As a result, the PVD-DMM composite foundation Method is proposed. Based on the project of "High-speed Railway Ballastless Track Pile Raft Consolidation of Composite Foundation Characteristics and Settlement Control Mechanism Research" supported by National Natural Science Foundation, and project of "PDL Composite Reinforcement Deep Soft Soil Technology Research" supported by Railway Technology Research and Development Program, this paper sets engineering background in "Ningbo-Taizhou-Wenzhou" passenger special line located in Wenzhou South Railway Station, featuring large area of deep soft soil, and adopts model testing, workmanship tests of piles, filed tests in turn by experimental research methods. Combining theoretical analysis and numerical simulation method, this paper digs deeper into the reinforcement mechanism and engineering properties of PVD-DMM composite foundation. With major innovation results show as follows:1. Based on the similarity theory, it conducted a large indoor model study of PVD-DMM composite foundation, and gained the deformation and mechanical properties for composite foundation comprehensively; Combined with the testing results of numerical analysis, it worked out reasonable pile lengths and pile spacing for the design of foundation treatment, and guided the engineering practice in PVD-DMM composite foundation effectively.2. The measured data shows that the testing roadbed has been in a stable state constantly, and the post-construction settlement is in the allowed range of regulation. This shows that it is technically feasible to use PVD-DMM composite foundation in dealing with large deep soft soil, which provides a new method for subgrade treatment in coastal high-speed passenger railway.3. Synthesizing the analytical results of the model and field test, it clarifies the reinforcement mechanism for PVD-DMM composite foundation:when the mix pile foundation is reducing total settlement, it improves the foundation stability effectively at the same time, thus, which ensures the filling velocity during the construction period; Plastic drainage plate enhances the rate of the drainage consolidation and sedimentation for foundation soil remarkably, thus, which can shorten the construction period or reduce the post-construction settlement.4. Through model test, the pile-soil interface load transfer law of PVD-DMM composite foundation is system tested first time. Results show that the distribution shape of Skin friction are three straight line, the area of pile top bear negative friction, neutral point located in L/10 position; the axial force of pile body is small in the bottom, which proof pile of PVD-DMM composite foundation can be regarded as the pure friction pile.5. the compaction during the differential settlement between pile and soil of PVD-DMM composite foundation, the development law of pile top and pile bottom stroke amount are First completely tested, obtained the bearing deformation mechanism of pile-soil-cushion. Test results showed that there are negative friction in the pile top area of PVD-DMM composite foundation under the flexible loads, settlement of reinforced area pile primarily contain bottom pierce.6. Based on load transfer law and deformation characters of PVD-DMM composite foundation, Boussinesq-Mindlin joint solving method is used to calculate underlying soil layer additional stress of composite foundation, and a composite layer settlement analytical calculation of PVD-DMM composite foundation is established; a Composite foundation consolidation degree analytic calculation method of PVD-DMM composite foundation is advised.7. Based on the fluid-structure interaction theory, Using a large-scale finite difference software FLAC3D,considering pile-soil interaction, carried out numerical analysis to long board short pile composite foundation, systemic test the result proved in this paper and the theoretical analysis results, comprehensive contrast difference properties among plastic drainage plate foundation, mixing pile composite foundation and long board-short pile composite foundation, and take deeply research on the effect of mixing pile length and replacement rate of long board-short pile composite foundation engineering properties. |
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