Research On Substratum Settlement Characters Of Composite Foundation Under Embankment Load

Based on three sections DK559+606and DK559+671of Liunan Passenger Railway and DK67+666of Haidong Passenger Railway, this paper researches the physical and mechanical parameters of medium compressible foundation soil through laboratory and in-situ tests, calculates substratum settlement of composite foundation and compares the calculation with the measured results and the Plaxis finite element analysis results.And the main research contents are summarized as follows:1. The laboratory and in-situ tests show that: most of the foundation soil compression coefficient of Liunan Passenger Railway and Haidong Passenger Railway test section is between0.1-0.5Mpa-1, a moderate compression of the soil;2. This paper analyzes mechanical properties of foundation soil in situ stress state through in-situ tests. The comparison of results indicates soil physical paraeters obtained a variety of testing methods are not consistent, but a variety of experimental data with variation of depth is roughly the same, where formation parameters obtained through the standard penetration and static cone penetration test are consistent, namely deformation parameters by in-situ testing can better reflect the soil formation parameters;3. The law of additional stress in soil between piles of the CFG pile and cement-soil mixing pile composite foundation:with addition of the ground depth, the additional stress decays rapidly; approaching the end of pile, the additional stress begans to spurt, and it reaches maximum below2m of the end of pile, then along the depth the additional stress slowly decays, which is quite different from the natural foundation additional stress attenuation law:4. According to the Mindlin-Boussinesq law and the correction base stress formula proposed by Jiang Guanlu[52], this paper calculates substratum settlement of CFG pile composite foundation which the deformation parameters of the moderate compression of soil are determined through combination of the standard penetration and static cone penetration test and the result of calculation is consistent with the measured values. Compared to the cement-soil mixing pile composite foundation, substratum settlement of the CFG pile composite foundation which is calculated through Mindlin-Boussinesq law is higher accurate, which is illustrated the Mindlin-Boussinesq method based on the pile lateral friction assumptions in this paper is more suitable for substratum settlement calculation of the bigger relative stiffness of the pile and soil; for moderate compression soil,the natural ground settlement result of calculation using the0.1method to determine the thickness of the compressed natural ground is larger;5. The predigestion of the constitutive model and the test parameters as well as the choice of the model in the numerical simulation will lead to the results have some errors with the measured values, but the law of settlement-load-time by the numerical analysis develops in good agreement with the measured results, we can predict the settlement after construction according to the simulation results and the settlement data obtained during the monitoring;6. Through the numerical simulation analysis, it is important that the selection of the appropriate pile length to control the settlement and optimizate the reasonable project and economic conditions and that the consideration of the relative modulus between the pile and soil to material design in the design of pile-net composite foundation.