Column Supported Embankments with Geosynthetic Encased Columns: Validity of the Unit Cell Concept

Column supported embankments (CSEs) are used to overcome common problems associated with the construction of embankments over soft compressible soils. The use of granular columns as deep foundation elements for CSEs can be problematic in soft soils due to the lack of adequate lateral confining pressure, particularly in the upper portion of the column. Using a high-strength geosynthetic for granular column confinement forms geosynthetic encased columns (GECs); the confinement imposed by the geosynthetic increases the strength of the column, and also prevents its lateral displacement into the soft surrounding soil. This paper presents the results of finite element analyses of a hypothetical geosynthetic reinforced column supported embankment (GRCSE) (i.e., a CSE underlain by geosynthetic reinforcement) that is constructed with GECs as the deep foundation elements. Full three-dimensional (3-d), 3-d unit cell, and axisymmetric unit cell analyses of the GRCSE were carried out to investigate the validity of the unit cell concept. The effect of the degree of nodal constraint along the bottom boundary when numerically modeling GRCSEs was also studied in this paper. Numerical results show that a full 3-d idealization is required to more precisely determine the tension forces that are produced in the geosynthetic reinforcement that underlies the GRCSE. A number of design parameters such as the average vertical stresses carried by the GECs, lateral displacement of the GECs, and the maximum settlement of the soft foundation soil, however, can be successfully calculated using unit cell analyses.