| In this dissertation, the measurement results of stress and deformation acquired at the CFG pile-raft and pile-net composite foundation test sections at Liyao and Fengyang in Beijing-Shanghai high-speed railway were analyzed comprehensively based on the key research project of MOR, "Integrated technical study of CFG piled high speed railway embankment" (No.2007G046). Combining with mathematical analysis and numerical simulation, the load transference mechanism, computation model of additional stress and the settlement calculation method for CFG piled railway embankments were proposed systemically.1. Based on numerical simulation for foundation stress, combined with test result of settlement tubes and soil stress under the embankments, a flare angle model for calculating foundation stress under railway embankment was created.2. The measured result of test sections shows that, for the two kind of composite foundations, the soil stress at the top of pile or pile cap and the soil stress in fillings is relevant to load, and the soil stress under the same load has little change with time. LTP of pile-raft composite foundation regulates pile and soil stress at pile top mainly according to the cushion constrained by stiff plate to make sure the piles support more loads. LTP of pile-raft composite foundation transfers more loads to piles by soil arch effect on pile top and pile-caps collection effect with their extending area. The deformation of geogrid among LTP of pile-net composite foundations in Liyao and Fengyang test sections is little, it doesn't exert obvious membrane effect.As far as the stress ratio of pile to soil at pile top or pile cap top is concerned, usually, the stress ratio of pile-raft composite foundations is obviously bigger than that of pile-net composite foundations. But as far as the pile efficacy is concerned, pile-net composite foundations are not inferior to pile-raft composite foundations. At Liyao and Fengyang test sections, the stress ratio of pile-net composite foundations is about 1/10 times as likely as that of pile-raft composite foundations, while the pile efficacy of both of the two kind of composite foundations are similar. At every section, the CFG piles is shared 65～79% of all load with the area replacement ratio less than 7%.3. In-situ test results of the axial force of piles at Fengyang show that, the pile-raft and pile-net composite foundations have the same character at the respect of load transfer. That is there is negative friction at the upside of pile. In the effect of negative friction, part of the load supported by piles is transferred to soil, so the axial force of pile is increased, and the additional stress of soil is reduced. At a depth where the axial force of pile grows to its maximum, the additional stress of soil decreases to its minimum. Below this depth, the axial force of pile began to reduce. The calculated results indicate that below the depth the vertical stress of soil between the piles is approximately equal to the vertical stress of soil foundation before filling.Lateral friction at pile top and pile bottom is brought into full play. Within a limited range of both sides of neutral point (equi-settlement plane), the relative displacement between pile and soil is small, so the lateral friction is restricted to a small value. The segment of negative friction is short comparatively, and neutral point is at the upper place of pile.4. Based on the simplification, which includes one-dimensional compression, lateral friction of pile is brought into full play and the additional stress of soil at neutral point is reduced to about zero, it proposed a spring-set model for LTP of pile-raft composite foundation to simulate and analyze the load transfer effect of LTP consisted of crushed-stone +steel reinforced concrete slab. Pile and soil stress, penetration deformation at pile top and so on can be calculated with this model.5. It pointed out that the soil arch effect was the mainly load transfer method for LTP of pile-net composite foundation, which consisted of pile cap+geogrid+crushed-stone layer. By research on in-situ test and mathematical analysis of soil arch at pile top of pile-net composite foundations, it also pointed out that the soil arch of top-pile was close to parabola, and then built a spherical cavity expansion method and a parabola arch method to calculate the stress of pile and soil at pile top.6. Based on Mindlin's solution, it proposed the parallel model between the reinforced body and the substratum of CFG pile composite foundation. The parallel effect is affected by modulus ratio of reinforced body to substratum. The stress ratio of reinforced body to substratum is almost equal to modulus ratio. Because reinforced body has some characters of integrality, the ratio of its width to depth also affects the parallel effect. The bigger the ratio of width to depth is, the greater the stress of substratum has. Combining the two influences produced by modulus ratio of reinforced body to substratum and the ratio of width to depth of reinforced body, the additional stress coefficientΨof substratum can be solved.Ψis the ratio between additional stress in substratum and equivalent deep foundation Boussinesq's solution.7. It proposed a simplified calculation method systematically for additional stress of CFG pile composite foundation under the condition of non-sparse piles. That is, the additional stress of reinforced body is calculated according to simplified model of interaction of pile-soil under confined compression. By analyzing the additional stress in substratum using parallel effect between reinforced body and substratum, the additional stress coefficientΨof substratum can be solved. And then, we can get the additional stress of substratum by multiplying Boussinesq's solution under strip load withΨ.8. It proposed a calculation method for total settlement (final settlement) and consolidation of foundation based on above calculation method for additional stress. It is the final settlement of foundation can be computed by layer-wise summation method. During settlement calculation, the laboratory test indexes and in-situ test indicators should be analyzed comprehensivly. Deformation parameter of foundation soil should also be established combined with local experience. Consolidation of CFG pile composite foundation is mainly consisted of the consolidation of substratum. It can be calculated based on Terzaghi's one-dimensional consolidation theory.9. The measured settlement-time curve could be divided into three stages as follows: rapid developing stage, developing stage and stable stage. By analyzing the change trend of fitting residual in stable stage of settlement-time curve, the deviated direction of predicted settlement values can be obtained. Thereby, the reliability of predicted settlement can be improved.
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