Three-Dimensional Analysis of Geogrid Reinforcement used in a Pile-Supported Embankment

Pile-supported geogrid-reinforced embankments are an exciting new foundation system that is utilized when sites are limited by a soft soil or clay. In this system, an embankment is supported by a bridging layer, consisting of granular fill and one or multiple layers of geogrid reinforcement. The bridging layer transfers the load to piles that have been driven into the soft soil or clay. The load from the embankment induces large deformations in the geogrid reinforcement, causing tensile forces in the ribs of the geogrid. Many of the current methods used to design geogrid reinforcement for this system simplify the approach by assuming that the reinforcement has a parabolic deformed shape. The purpose of this thesis is to thoroughly examine the behavior of the geogrid in a pile-supported embankment system, in an effort to determine the accuracy of the parabolic deformed shape, and identify the most important parameters that affect reinforcement design. The geogrid was analyzed using a three-dimensional model that included a cable net to represent the geogrid and linear springs to represent the soil underneath the geogrid. A larger pressure was applied to the geogrid regions that are directly above the pile caps so that arching effects could be considered, and the stiffness of the springs on top of the pile were stiffer to account for the thin layer of soil between the geogrid and the pile cap. A Mathematica algorithm was used to solve this model using the minimization of energy method. The results were compared to another model of this system that used a membrane to represent the geosynthetic reinforcement. Additionally, the maximum strain was compared to the strain obtained from a geosynthetic reinforcement design formula. A parametric study was performed using the Mathematica algorithm by varying the pile width, embankment pressure applied to the soil, embankment pressure applied to the pile, stiffness of the soil, stiffness of the soil on top of the pile, stiffness of the geogrid, geogrid orientation, rotational stiffness of the geogrid, and the layers of geogrid reinforcement.Master of Scienc

Analysis of geosynthetic reinforcement in pile-supported embankments. Part II: 3D cable-net model

ABSTRACT: In this second part of a three-part study, the geogrid reinforcement is modelled as a three-dimensional cable net. Square pile caps are considered. Again, symmetry is utilised, and numerical results are obtained by minimising the total energy. The cables are connected at nodes, which are represented as spherical joints with variable rotational stiffness. Nodal displacements are computed, and strains and stress resultants can then be calculated from them. The net stress reduction ratio is also found. The effects of variations in the following quantities are determined: the size of the pile caps; the stress on the geosynthetic over the soft soil between the piles; the stiffness of the soft soil; the spacing between geogrid nodes; the orthotropy of the geogrid; and the bending stiffness (modelled by rotational springs) at the nodes. In addition, the effect of rotation of the geogrid by 45° is analysed, in which each rib is parallel to one of the diagonals of the unit cell. Also, the case of two geogrid layers is treated. As for the plate model, large spikes in strain and stress occur near the corners of the pile caps, and three-dimensional effects are important. </jats:p