Numerical simulation of geosynthetic encased stone columns bearing on a compressible soil layer used individually and in group configurations
Date
2021
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Publisher
University of Delaware
Abstract
Stone columns (SCs) are commonly used for soil improvement purposes. In very soft soils, insufficient strength of the soil surrounding the upper portion of the column often causes “bulging” failure (i.e., excessive lateral displacement) when compressive loads are applied to the SCs. In SCs, such failure is more common than sliding and general shear modes of failure. To reduce the impact of bulging behavior, SCs can be encased by high-strength geosynthetic to form geosynthetic encased stone columns (GESCs). GESCs can be used for foundation support applications or large-area ground improvement applications such as oil storage tanks, road embankments, and other types of structures that are not overly sensitive to settlement. ☐ GESCs can also be used as one possible option for deep foundation elements that are used in the construction of column-supported embankments (CSEs). CSEs can be supported by a wide variety of deep foundation technologies, including driven piles, deep mixing method columns, vibro-concrete columns, SCs, rammed aggregate piers, or other types of suitable support columns. Since most CSEs cover large areas, using flexible columns is often more economical than driving the numerous support piles that would be required in such applications. CSEs can also be strengthened by adding at least one horizontal geosynthetic layer to form geosynthetic reinforced column-supported embankments (GRCSEs). ☐ In most applications, GESCs should ideally rest on a rigid layer. However, soft soil layers can be thick, making the construction of the support column down to a rigid layer impractical. This raises the question about the effectiveness of using GESCs bearing on a compressible soil layer to improve the performance of structures that are constructed on soft soil. ☐ In the present research, a series of three-dimensional finite element analyses were first performed to numerically simulate the performance of a single loaded GESC bearing on a compressible soil layer. These analyses investigated the stress-settlement response and the lateral displacement distribution for GESCs subjected to compressive loads. ☐ The results of the aforementioned analyses showed that encasing SCs with a high strength geosynthetic material increases the strength and reduces the settlement and the lateral displacement of the column, even when the GESC rests on a compressible soil layer. The effect of variations in parameters such as the dilation angle, stiffness of the geosynthetic, column diameter, and length of encasement was also examined by performing a series of extensive parametric analyses. The load transfer mechanism due to the effect of the lateral confining stresses exerted by the surrounding soft soil was numerically examined for a single loaded GESC bearing on a compressible soil layer. The influence of the length of the GESC on the load transfer mechanism was examined for GESCs bearing on both a rigid and a compressible soil layer. ☐ In the next phase of the research, GRCSEs supported by GECSs were mathematically modeled and analyzed using the finite element method. Simulation cases representing a single row of columns (a 3-d slice of the embankment) supporting a GRCSE, with GESCs bearing on both rigid and compressible soil layers, were analyzed to examine the effect that variations in column spacing have on the behavior of the GRCSE. The vertical stress-settlement response of the GESCs, the lateral spreading associated with the simulated construction of the GRCSE, and the tensile force in the horizontal reinforcement geosynthetic layer were examined. ☐ In the final phase of the research, the validity of using a 3-d unit cell idealization as a computationally less expensive alternative to simulate the behavior of a single row of GRCSEs bearing on a compressible soil layer was investigated. The 3-d unit cell model showed a reasonable representation of the single row of GRCSEs supported by GESCs bearing on a compressible soil layer, especially in the encased material’s stress-settlement response and lateral deformation. Lastly, parametric studies were carried out for a 3-d unit cell bearing on a compressible soil layer and a less compressible layer to examine the effect that variations in the stiffness and the length of the geosynthetic encasement have on the behavior of the GESC.
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Keywords
Constitutive models, Geosynthetic encased columns, Group configurations, Soil improvement, Stone columns