| Despite the extensive use of Geosynthetics-Reinforced Pile-Supported embankments (GRPS) of High-Speed Railway (HSR), their mechanism researches still lagged far behind the engineering practices. Based on the current codes of calculative theory, the settlements of HSR over Medium-Compressibility Ground (MCG) generally couldn’t meet the design requirements. Therefore, the foundation of dedicated passenger lines with unballasted track in Wuhan-Guangzhou, Zhengzhou-Xian, Beijing-Tianjin, Beijing-Shanghai, etc, were strengthened by using a mass of reinforced techniques such as CFG piles, concrete-reinforced GRPS structures and pile-plank structures. In view of the immature reinforcement techniques for the foundation of medium compressibility, few research results of Geosynthetics-Reinforced Floating-Pile Supported (GRFPS) embankments have been obtained. There are many uncertain factors about the post-construction settlements of subgrade and arrangement of construction period of HSR.The objective of this study was to improve the understanding of load transfer and foundation settlement mechanism of GRFPS embankments through both theoretical analysis and field test. A full-scale model test with six fully instrumented embankment sections built over silty clay of medium compressibility was firstly performed along the Liuzhou-to-Nanning High-speed Railway (LNHR) in China. Based on the indoor physical and mechanics tests, centrifugal model tests, in-situ tests, numerical analysis, long-term field tests, the physical and mechanical characteristics of MCG were mastered firstly. Combined with monitoring of pile-soil stress and foundation settlements, the characteristics of settlements and additional stress of foundation were comprehensively analyzed. Afterwards, a new analytical arching model and a simple analytical solution derived for predicting the lateral displacements were proposed for equilateral triangle arrangement of piles. The selections of deformation parameters of MCG were optimized. The computation and prediction of ground settlements suitable for MCG was proposed, which provided experiences for the settlement control and ground reinforcement of domestic HSR.Based on literature investigations, the basic physical properties such as compression properties, consolidation characteristic, strength characteristics, swelling-shrinkage characteristics, microstructure features, and mechanical parameters including strength parameters and deformation parameters of undisturbed samples of MCG were obtained by in-situ tests and physical mechanics tests. In order to couple the deformation response compatibility in two KO consolidation states of undisturbed silty clay, KO consolidation tests and one-dimensional consolidation tests were carried out. The deformation response characteristics of compression modulus (Es) and compression index were analyzed and their relationships were established. The influence of stress state and history on KO and Es was investigated and accuracy of empirical correlation equations was probed into. In addition, for investigating the influence of local deformation measurement on deformation parameters of undisturbed silty clay, triaxial consolidation drained and undrained shear tests were carried out to analyze the behaviors of strain-stress and effective shear parameters. The investigation also concerns the characteristics of Poisson’s ratio and stiffness under small strain.Afterwards, vertical settlements between pile and subsoil, lateral displacements, efficiency of pile caps, dissipation of pore pressure, strains of geosynthetics and distribution of additional stress, etc, were obtained relying on the long-term monitoring field tests of LNHR. By means of the auxiliary centrifugal model tests using floating piles, the effects of strengthening MCG by using GRFPS were checked and evaluated. Based on the field measurements, the characteristics of ground settlements and mechanisms of soil arching and membrane effects as well as ground reaction contributing to load transfer were analyzed. The field results indicate that arching occurs within the embankment during its construction due to the development of differential settlement between piles and surrounding soils. As a result of the soil arching and membrane effects, most of the embankment load is transferred to the piles. Accordingly, the pile efficiency bilinearly increases with increasing of embankment height with a transition at the height of around 6 times the net pile spacing. The pile efficiency increases with increases in both replacement area ratio and pile length. Consequently, the small pile spacing is more effective in reducing the settlements. Meanwhile, the excess pore water pressure at depth of 10 m rise significantly, which is 2 m below the bottom of pile tip. This confirms again that the piles can transfer most of the surcharge load to the substratum while the additional stress over the subsoil within the piles length is substantially reduced. The ground reaction modulus decreases with increasing settlement between piles and surrounding soils. It exhibits lower for the reinforced sections than for the unreinforced section. At the end of embankment construction, the ground reaction modulus k could reach 0.69 to 1.57 kN/m3 It tends to decrease when increasing ground replacement area ratio due to the decrease in settlements.Based on the field measurements, soil arching and membrane effects as well as ground reaction contributing to load transfer were analyzed. Afterwards, a new analytical arching model in both 2D and 3D cases was proposed for equilateral triangle arrangement of piles, which was based on the well-known model developed by Hewlett & Randolph (1988) while considering square arrangement of piles. The vertical equilibriums of arching forces at the crown and above the pile cap were derived and solved based on the limit-state equilibrium of arch model. In the proposed model, two main refinements were made by introducing new parameters:an elastoplastic state parameter of soil arching(α) and a coefficient of equivalent uniform stress (β). The former was used to satisfy the load equilibrium in case of partial arching while the latter was adopted to allow possible nonuniform vertical stress acting on the ground surface. From equilibrium of vertical and horizontal stress distribution on geosynthetics, the tension strength in the geosynthetic was given, which coupled with the arching above. The ground reaction method was first-time incorporated to take into account the reactive support of the subsoil beneath geosynthetics-reinforced layer when calculating the required tensile strength of geosynthetics. A parametric study was carried out on the proposed arch model in order to show its main characteristics of efficiency. Finally, the performance of the proposed analytical model was demonstrated against both the several widely-used existing models and the field measurements. Results showed that the proposed model is capable of well predicting the pile efficiency while slightly overestimating the required tensile strength of geosynthetics.Based on Boussinesq’s formulas, a simple analytical solution was firstly derived for predicting the lateral displacement of a soil foundation with a Poisson’s ratio μ< 0.5 under embankment loading. Based on back analysis of lateral displacements measured in situ by using the analytical solution, a simple method for estimating the stress concentration ratio of GRFPS embankments was proposed. In order to validate the proposed method, a full-scale HSR embankment with four instrumented subsections over medium silty clay was constructed in three stages. The field deformation analysis shows that CFG pile reinforcement performed well in terms of reducing settlements and lateral displacements, and the lateral displacement is a good indicator for evaluating external stability of GRPS embankments.Based on the proposed analytical solution for lateral displacements, the composite modulus of the reinforced zones of pile-supported embankments were estimated by the back analysis method, which is similar to conventional curve fitting technique for matching the measured lateral displacements by varying the equivalent modulus used in the analytical formula. Finally, the back-calculated stress concentration ratios using the proposed method were compared with those determined from field measurements. The comparison shows that the proposed method is capable of reasonably estimating the stress concentration ratios. Moreover, the appropriate equivalent elastic modulus in the proposed method can be obtained based on the stiffness-strain curve of the foundation soils from laboratory triaxial tests while considering the strain level determined based on field measurements of lateral displacement.In addition, the adaption of different settlement computation methods referred to GRFPS embankments over MCG was compared. Anew method of settlement computation considering over-consolidation state was put forward and statistical analysis of settlement modification coefficient was analyzed. The principle of deformation parameter selection was also optimized. Finally, various numerical simulations for comparing and optimizing field cases were developed. The rules of foundation reinforcement aiming at GRPS embankments over MCG of HSR were summarized and formed, which provided effective references for engineering design and application. |
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