| It is quite difficult to solve the problem of the uneven soft ground in large areas of highway and railway engineering, to which the technology of composite foundation proves to be an effective method. With the development of the technology of composite foundation, various innovative types have applied to the engineering field. But its designed computation theory is left far behind, especially in the aspect of bearing mechanism. Therefore, the research into composite foundation under embankment has an important theoretical and practical significance. The dissertation studied on the bearing capacity, settlement, soil arching effect, cushion design, etc. through theoretical analysis and numerical simulation analysis. The main researches and achievements are as follows:(1)Since the embankment is a flexible foundation, the formulas for the bearing capacity of ground developed from rigid foundation could not be applied mechanically. The combination of the Slip Line Method and the Limit Analysis Upper Bound Method was set forth to deduce new ultimate bearing capacity calculating formulas of ground under inclined loads. As regards to the composite foundation under embankment where the lateral earth pressure of the embankment, the gravity, and the tensile force of the horizontal reinforced body would bring inclined loads to the ground, the possible range of sliding width was first determined, then the most dangerous sliding surface of the ground was searched out with the optimization method, and finally the limit height of embankment adopting the deduced capacity formulas were calculated. The deduced formulas were proved reliable by sample engineering analyses. A new calculating method was provided for the ultimate bearing capacity of composite foundation under embankment.(2)To solve the problem of the ultimate bearing capacity calculations of horizontal reinforcement and gravel pile composite foundation under embankment, the interaction among horizontal reinforced body, the embankment, and the ground were first analyzed, in order to find out their relationships in terms of the acting force and speed. Then the calculating formulas, which considered the influences of replacement rate of pile and consolidation degree of ground soil, for the strength parameters of composite foundation soil were obtained after analyzing the effects which the gravel pile had on the bearing capacity of horizontal reinforcement and gravel pile composite foundation were deduced. And according to those formulas, the limit filling height of embankment were calculated. As the engineering case studies showed, the calculating results in the dissertation agreed with the practical measurements and turned out to be ideal. Meanwhile, based on the engineering cases, the dissertation made a detailed analysis of the effects which factors like the efficient coefficient of reinforcements and the consolidation degree, had on the ultimate bearing capacity of the ground. And the bearing mechanisms of horizontal reinforcement and gravel pile composite foundation were further studied.(3)First, the failure mode of horizontal reinforcement and rigid piles composite foundation under embankment was discussed. Then based on the Limit Analysis Theory and masking theory for passive piles, the dissertation relatively deduced a formula for ultimate bearing capacity of soil between piles, which took the masking effect of pile into account, and one for ultimate bearing capacity of rigid piles, which considered the slipping impacts of ground soil. Finally the total ultimate bearing capacity was received based on the joint force of composite foundation pile-soil. The sample analyses showed that the method used in this dissertation, rather than the normative one, was more in line with the actual bearing mechanism of rigid pile composite foundation.(4)When the composite foundation with a cushion pile was loaded, its pile would pierce into the cushion and a negative friction resistance would be elicited along at the upper side of the pile. With regard to the features of the relative slipping from the pile and soil of composite foundation and the mutual reaction between them, the elasto-plastic analysis method was improved by absorbing the Boussinesq solution and the Mindlin solution, and the method was introduced in the calculation of the settlement of pile composite foundation. Also, the method was extended in the case of groups of piles, so formulas for the piles with uniform forces and non-uniform forces were relatively received. Through model tests and comparison, it proved that the method in the dissertation was quite reliable. And a new method of calculating the settlement of composite foundation was found.(5)The ultimate bearing capacity of cushion was studied according to the Limit Analysis Lower Bound Method. And the formulas for calculating the ultimate bearing capacity of cushion with the bases of two-dimensional thick cushion, two-dimensional thin cushion and three-dimensional thin cushion under a circular base were relatively deduced. Also the formulas for calculating the maximum effective width of the thick cushion were deduced according to the geometric relationship. Then the relation among the internal friction of the cushion, the maximum width of the cushion, and the stress ratio of the cushion's pile and soil was studied, and also some inherent laws on the bearing mechanism of cushion were obtained. In the end, considering both the aspects of bearing capacity and settlement, a new design method of cushion was proposed, which was more comprehensive than the previous ones either studying from the perspective of the failure mode or the elastic deformation of cushion.(6)In double direction reinforced composite foundation under embankment, the deformation of the pile and soil between piles would bring a mismatch. Consequently it caused differences in the vertical deformation within the filling of embankment, leading to soil arching. Based on the analysis model of Marston's soil pressure theory and using Shear Displacement Method to analyze the stress and deformation rules of filling and geosynthetics, the dissertation deduced formulas for the stress ratio of pile-soil and settlement difference of pile-soil, of the composite foundation with geocell plus pile and composite foundation with multi-layer Geogrid(or geotextile) plus pile. At last, the formulas proved to be reliable through sample analyses. Meanwhile the way how the mechanical properties of embankment fillers(c,φ), pile radius, pile space, and tensile strength and bending rigidity of geosynthetics influenced the stress ratio of pile-soil were systematically discussed and some useful conclusions were made.(7)Based on field experiments on double direction reinforced composite foundation under embankment, an ADINA finite element analysis model was established. And by analyzing and comparing the finite element results, the theoretical results, and the measured results of the settlement of composite foundation, the finite element results proved to be reliable. Based on the finite element results, deep analyses were made into the deformation characteristics, such as the tensile force and friction of horizontal reinforced body, the axial force, bending moment and friction of the pile, and the lateral displacement of the soil between piles, in order to know further about the bearing characteristics of double direction reinforced composite foundation under embankment.
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