Numerical Predictions of Deformations in Geosynthetic-Reinforced Column-Supported Embankments: Validation of Manual Dissipation of Excess Pore Pressure Approach for Undrained and Drained Analyses

2020

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Design of column-supported embankments (CSE) requires the evaluation of global stability using the conventional limit equilibrium method (LEM). Yet, for CSEs using unreinforced concrete columns and load transferring geogrids, the failure mechanisms and corresponding soil-structure interactions are not well understood. There is increasing evidence pointing to large bending moments in columns and failure of columns in flexure, as opposed to a failure by shear as assumed in limit equilibrium analyses. In response to these design uncertainties, the failure height, failure mode, and deformations of eight column-supported embankment scenarios were investigated using three-dimensional (3D) numerical analyses. For the same embankment scenarios at failure height, factors of safety (FS) were then calculated using the two-dimensional (2D) LEM for investigating its applicability in evaluating global stability of CSEs. The 3D numerical analyses examined CSE stability for the limiting conditions at undrained end-of-construction and after long-term dissipation of excess pore water pressures. The numerical model included representations of flexural tensile failure in the concrete columns and tensile failure in the geosynthetic reinforcement. Scenarios consisted of a base case with typical concrete column design, five single-parameter variations using base case conditions, and two multiparameter variations using base case conditions. The undrained condition was the most critical, and two failure modes were found: (1) multisurface shearing in the embankment coupled with bending failure of columns and near-circular shear failure in the clay, and (2) multisurface shearing in the embankment coupled with bending failure of columns and shearing in the upper portion of the soft foundation clay. Both failure modes were accompanied by a rupture of the geosynthetic when included in the load transfer platform. Soilcolumn interactions were complex, and many columns failed in bending at lower embankment heights than those that produced collapse. The factors of safety calculated using the LEM were overstated. This is because the LEM assumes failure by shear, which has limited applicability for examining the complex mechanisms by which CSEs fail. The practical implication is that the LEM should not be used for evaluating global stability of this system type and, by extension, other system types in which soil-structure interactions result in failures controlled by mechanisms other than shear.

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Section: Undrained-dissipated Analysesmentioning

confidence: 99%

3D Numerical Analyses of Column-Supported Embankments: Failure Heights, Failure Modes, and Deformations

2020

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“…However, the analysis of deformation and pore water pressure histories could be computationally expensive, depending on the size of the numerical domain and the material properties. Huang et al (2018) established that lateral spreading in CSEs can be instead analyzed using limiting cases, based on case history records of vertical earth pressure distributions and geosynthetic strains. The limiting case of undrained end-of-construction calculates the largest increment of earth pressure in the foundation.…”

Section: Parametric Study Using Undrained-dissipated Analysesmentioning

confidence: 99%

Section: Parametric Study Using Undrained-dissipated Analysesmentioning

confidence: 99%

“…The two limiting cases can be represented numerically using the undrained-dissipated approach (Huang et al 2018), which computes an undrained end-of-construction state and a long-term dissipated state in succession. The method for computing consolidation was validated in one of the authors' previous works (Huang et al 2018) and is summarized here. First, undrained loading occurs by excess pore pressure development in the clay during embankment construction through the application of an effective stress model (i.e., Modified Cam Clay).…”

Section: Parametric Study Using Undrained-dissipated Analysesmentioning

confidence: 99%

“…A parametric study totaling 140 scenarios was performed using a half-embankment domain in FLAC3D 5.01. An undrained end-ofconstruction and a long-term dissipated state was computed for each CSE scenario using the undrained-dissipated approach (Huang et al 2018). Numerical parametric studies have been used to examine CSE system response under change of individual parameters (Bhasi and Rajagopal 2014;Borges and Marques 2011;Chai et al 2017;Farag 2008;Huang and Han 2010;Jennings and Naughton 2011;Liu and Rowe 2016;Mahdavi et al 2016;Shrestha et al 2015;Wang 2015, 2016).…”

Section: Scope Of Studymentioning

confidence: 99%

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Lateral Thrust Distribution of Column-Supported Embankments for Limiting Cases of Lateral Spreading

2020

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Lateral spreading analysis of column-supported embankments (CSEs) requires an understanding of lateral thrust distribution. This includes quantifying the portion of thrust that is resisted by tension in geosynthetic reinforcements installed in the load transfer platform. Results from a three-dimensional (3D) numerical parametric study using a half-embankment domain and totaling 140 scenarios are presented in terms of lateral thrust distribution. Forces examined include the lateral thrusts in the embankment and foundation soil, the geosynthetic tension, and the base shear at depth, and results are presented for the limiting cases of lateral spreading (i.e., undrained end-of-construction and long-term dissipated). Results show that lateral thrusts induced by embankment loading are significant in the embankment, foundation soil, and base shear beneath the columns. However, the portion of lateral thrust carried by the geosynthetic is limited, though it increases with the geosynthetic stiffness. Results also indicate that lateral spreading in CSEs is more critical at the undrained end-of-construction condition than in the long-term condition after excess pore water pressures have dissipated. Correlations for the thrust distribution at these limiting conditions and different embankment locations (i.e., centerline, shoulder, and toe) are provided.

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3D Numerical Limiting Case Analyses of Lateral Spreading in a Column-Supported Embankment

2019