Application of Hybrid Drained-Undrained Model for Analyzing the Stability of Reinforced Soil Structures Over Soft Foundations with Prefabricated Vertical Drains

Abstract

Prefabricated Vertical Drains (PVDs) are typically used for embankment construction over saturated soft cohesive soil deposits to accelerate consolidation and reduce construction time in the field. PVDs accelerate consolidation of thick soil deposits by reducing the drainage path from tens of meters to 1-2 meters depending on PVD spacing in the field.  Current design methodologies typically consider the increase of shear strength due to accelerated consolidation, but still use undrained shear strength for the entire cohesive soil layer even after PVD’s are installed.  However, for cases in which PVDs are closely spaced, which allows excess pore water pressure to dissipate relatively fast, the assumption of undrained conditions for design may be overly conservative and, in some cases, this assumption may render an embankment construction unfeasible, unless additional ground improvement techniques are used to significantly enhance the foundation strength. This paper presents a Hybrid Drained-Undrained (HDU) model for construction of embankments over soft soils that accounts for the improved soil drainage conditions after installation of PVDs in the assessment of the shear strength used for design.  A field case study is presented where the HDU methodology was used for the design of a 2.4-km long MSE berm constructed over a PVD-improved soft soil site, allowing for significant cost savings. The HDU approach was implemented using limit equilibrium models during the design stages to analyze the global stability of the MSE berm at different stages. Finite element models calibrated using field monitoring data collected during construction showed factors of safety comparable with that calculated using the HDU approach, which further supports the suitability of the HDU approach for PVD design.