Theoretical And Numerical Study Of Column-Supported Embankments

Very soft soils are often avoided in construction due to their high compressibility and low shear strength. Embankment constructed on soft soils, where the structure impose a large load onto the ground raises several concerns on factors like bearing capacity failure, differential settlements, lateral pressures and instability which results in long construction delays and/or premature failure. Special construction methods are adopted when embankments are constructed on very soft soils. Geosynthetic-reinforced and column-supported (also known as pile-supported) earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. In this dissertation, the theoretical analysis and numerical simulation of column-supported embankment have been performed.In past ten years, composite foundations, especially a multi-column composite foundation has been extensively used. A multi-column composite foundation is a method of ground improvement that involves using different column types with varying lengths and diameters. The long columns are generally much stiffer than short columns and they are used in a manner similar to that used for conventional columns; that is, to mobilize bearing capacity from the deeper soil strata. On the other hand, the short and flexible columns strengthen the shallower soil strata. Consequently, the strength and stiffness of the shallow soil strata can be improved and utilized together with contribution from the deeper soil strata.Firstly, a new theoretical analysis similar to the analysis proposed by Low was presented for the analysis of an embankment of granular fill on soft ground supported by a rectangular grid of columns and geosynthetic. The main refinements were: the inclusion of a uniform surcharge loads on the embankment fill, individual square caps have been used, and taking into account the skin friction mechanism, which contributes to soil-geosynthetic interface resistance. In particular, the proposed method has the advantage over existing method in that it is simple to get the solution of the developed equations and there is no need for using trial values to get the solution of unknown parameters. The method showed that inclusion of geosynthetic reinforcement can increase the fill load carried by columns, as indicated by efficiency. The method also showed that the portion of the fill load carried by columns increases with the area ratio of column caps. For a given area ratio, the efficiency reached a maximum value when the ratio of the thickness of the fill to the spacing of the column caps is large. Comparison of results of the present method to that of current design methods showed a good agreement with the results of Low method. It seems that the methods for 2D situation in BS8006 and Guido overpredicted the vertical stress acting on the geosynthetic. As a result, BS8006 and Guido methods underestimated the efficiency and stress concentration ratio.Secondly, a full scale load test on multi-column composite foundation was provided using the finite element software PLAXIS 3D FOUNDATION in order to investigate the behavior of the composite foundation under various load distributions. The parameters investigated include the length and diameter of columns and the thickness of the cushion. The influence of the cushion on load-settlement behavior was also studied. The results of these analyses were summarized into a series of design charts, which can be used in engineering practice.Finally, a series of two-dimensional (2D) finite element analyses on the mechanism of consolidation behavior of multi-column supported embankment using the finite element software PLAXIS. Parametric studies were presented that shed light on effective combinations of columns. Development and dissipation of excess pore pressure, settlement, horizontal displacement, differential settlement, and axial force of columns were also presented and discussed in details.The achievements in this dissertation can be valuable for theoretical study and practical application of column-supported embankments.