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

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. Soil-column 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

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

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

Strengthening Low Plastic Soils Using Micro Fine Cement through Deep Mixing Methodology

Present research papers focuses to strength low plastic soil using deep cement mixing technique through model study. Soil column length of 10cm, 20 cm and 30cm was used with varying degree of saturation as 60%, 80% and 100% of OMC to determine settlement characteristics, unconfined compressive strength, modulus of subgrade reaction and modulus of elasticity of raw and treated soil. Cement dosage for UCS test and model plate load test was decided as per guidelines provided in FHWA 13-046 design manual and CDM-LODIC method respectively. Method of deep mixing the soil with cement was adopted from theory given by Filz et.al. (2005). The results depicted the cement deep mixing methodology based on soil particle-cement particle interaction with varying degree of saturation proved the efficacy for low plastic soils and maximum reduction in settlement was observed for 60% degree of saturation for column length of 20 cm. Modulus of elasticity was validated with provisions of FHWA whereas load carrying capacity of soil-cement column was validated with Broms empirical equation