| As a natural geomaterial, soil has good bearing capacity, but it is of weaker tension or virtually no tension strength. So soil is not used directly in engineering sometimes and the field is almost limited. Geosynthetic reinforcement is a better option in many soil improvement methods because it has the advantages of enhancing shear strength, decreasing compressibility, improving the permeability and dynamic characteristics of foundation. Recently, this method is widely used in slope, embankment, dam and abutment etc. Especially geogrid-reinforced foundation is obviously better than other geosynthetics. However, because the working mechanism of the geogrid-reinforced method is different from that of traditional treatment to soft foundation, the special loading conditions, complex interaction mechanism between geogrids and soil and computational methods for the bearing capacity of reinforced foundation have not been well clarified. Thus, it will be theoretically important and practically significant to examine the working mechanism and work out effective methods for evaluating bearing capacity of reinforced foundation in soft ground under complex conditions. Therefore, this dissertation studies the interaction between geogrid and soil, the bearing capacity of reinforced foundation and numerical method. The main investigations and achievements in this PHD thesis are composed of following portions:(1) A series of experiments to investigate the friction on the surface of geogrids were conducted in the laboratory under various level of normal load. By analyzing experimental data, some conclusions were made that the friction on the surface of geogrids gradually transfers backwards along it, which rapidly increases in early time of the pullout test and approaches to a stable value ultimately. The relationship between the strain of the geogrids and pulling time was simulated by the sigmoid curve. Based on the experimental data of the strain of the geogrids at different embedment, a three dimensional strain surface that represents a function of coordinate and time was obtained. The time function T(t) for pullout force of geogrids can be deduced through integration of friction along total length of geogrids. In geogrids-reinforced earth structure, some strain gauge can be set on the surface of geogrids, then the tension of geogrids at any position can be predicted by this method.(2) Considering the reinforcement effects of geogrids in the silt not optimal, the pullout resistance behavior of geogrids in silt mixing gravel were conducted in the laboratory under different normal stress and gravel content. The effects of gravel content on the pullout resistance and friction coefficient of interface between geogrids and soil are investigated in various conditions. With the increasing of normal load on surface of samples, the pullout resistance of geogrids increases steadily, but the increasement is not large. This problem was mainly due to the smaller friction coefficient of interface between geogrids and soil. The passive resistance of transverse ribs can be enhanced by mixing some gravel in the silt. However, many cracks occurred on the surface of sample, and the whole resistance of geogrids decreased when the normal load was not enough to overcome the dilatation of soil. While the normal load and the gravel content increased at same time, the resistance of geogrids increased rapidly to a higher value. The normal load of sample can enhance not only the friction resistance of longitude ribs, but also the passive resistance of transverse ribs. The experiment data showed that the existing empirical formula in design about geogrids-reinforced earth structure is obviously conservative. Based on the relation of interface friction coefficient and gravel content, a fitting formula about them can be obtained. The experimental data is of referring value for design of geogrids-reinforced earth structure.(3) A series of experiments to investigate the resistance mechanism of transverse and longitudinal ribs are performed with newly developed individual-rib pullout devices under various normal load and pullout velocity. Some conclusions were made by analyzing experimental data that the friction resistance of longitudinal ribs rapidly develops in early time of the pullout test, and it increases appropriately linearly with the increase of normal load but is almost not affected by pullout velocity. The passive resistance before transverse ribs increases slowly with the increase of normal load comparing with frictional resistance. And relative displacement is necessary to the development of pullout force. The failure mechanism of passive resistance transforms from the punching failure mechanism to the general shear failure mechanism with the increase of normal load and pullout velocity. The ultimate pullout resistance can not be interpreted as the sum of the passive and shear components. The changes of load upon the ribs make the interface shear resistance gradually decrease, but the passive resistance obviously increase to 66% of whole pullout resistance. It is suggested that more attentions on reinforcement effect of transverse ribs of geogrids should be paid.(4) As for the ultimate bearing capacity of strip footings on homogeneous reinforced foundation, based on the limit equilibrium theory and the yielding criteria of Mohr-Coulomb, the equilibrium equation can be equivalently transformed to the format of functional extremum with undetermined boundary values by using the variational principle. The function of the sliding surface and the failure envelope of strip footings on homogeneous reinforced foundation are achieved when the boundary conditions are introduced. The effects of the angle of internal friction, tension angle of geosynthetics, layer numbers of geosynthetics and layer spacing of geosynthetics on the ultimate bearing capacity of strip footings on homogeneous foundation and the failure envelop are studied. With the increasing of the internal friction angle, the bearing capacity of reinforced foundation increases gradually. The sliding surface mainly develops along horizontal, rather than vertical direction. With the increasing of the layer numbers of geosynthetics, the bearing capacity of reinforced foundation increases obviously, but eventually tends to be a steady value. The sliding surface mainly develops along vertical, rather than horizontal direction. For some geosynthetics with higher strength, such as geogrids, it is not reasonable to assume the direction of its tension is always horizontal in design. In fact, the direction of tension is perpendicular to paler radius at the same point. The interfacial friction factors in chapter two were referenced in calculation to discuss the rational laying length of geogrids under the stripe footing. To meet the bearing capacity of foundation, the maximum spacing and minimum layers of geosynthetics can be obtained by this method. It is of referring value for design and construction of reinforced earth structure.(5) The interaction between geogrids and soil is the fundamental and crucial factor on stability of geogrids-reinforced earth structure. An improved FEM method considering the interaction between them is proposed. In this method, geogrids were simulated with entity elements, and transverse ribs were replaced with some spheres that can produce passive resistance. And the Eulerian elements used to simulate soil can solve the non-linear distortion problem. With the CEL (Coupled Eulerian-Lagrangian) technology, the pullout test and geogrid-reinforced foundation test are modeled by the finite element software ABAQUS. The results of FE analysis about pullout test can reflect the actual stress and displacement in the reinforced structure, which has proven that this simulation method is effective and reasonable. And the results of FE analysis about geogrid-reinforced foundation compared well with the experimental results, which has proven that this simulation method has accuracy and reliability. It is of referring value for FE analysis of reinforced earth structure.
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