Geotechnical Engineering Online Library

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.

This paper describes a ground improvement case study where preloading and prefabricated vertical drains (PVDs) were used to accelerate foundation settlements. The case study is used in a classroom setting with the learning objective of introducing engineering students to methods for estimating settlement of shallow foundations on compressible soils. The project site was developed for a corporate retail chain planning to open a new facility in San Luis Obispo, California. Up to 2.5 meters of fill were needed across much of the site to raise foundations and improvements above the flood elevation. Loads from the fill and the structure were expected to cause total and differential settlements that exceeded the allowable values established by the retailer. To mitigate settlement, the geotechnical engineer developed a preloading plan. Although the soil conditions were complex (e.g., interlayering, dipping strata, variable compressibility), the preloading plan was successful in achieving the desired settlement within 3 months, and subsequent site performance has been exemplary. This case study has been used for several years within a quarter-long shallow foundation design course to teach settlement performance. Learning outcomes from the assignment are summarized in the paper. Students are given the subsurface information and test results originally acquired by the geotechnical engineer. The students, working in teams, try to estimate how much primary consolidation settlement will occur due to the fill plus the preload, and the PVD spacing needed to achieve 90% of that settlement in 3 months. The assignment and relevant data are included herein along with the grading policy. The project culminates with the geotechnical engineer of record presenting in class the results of site monitoring during preloading and consolidation. These results include settlements across the 16,908 m2 site, which were tracked up to three times a week at 20 locations. This project affords students a case study experience that is rich in the “messy” details of a complex and local (i.e. familiar) geotechnical project. Included is a discussion of lessons learned by the instructors who have taught several iterations of this case study.

A container yard was constructed for handling of loaded containers at Chittagong Sea Port in Bangladesh covering an area of 60700 m2 over a sub-soil that included a layer of soft stratum of clayey silt/silty clay at the depths of 0 to 3.5 m below grade. Thicknesses of the soft stratum varied from 3 m to 7 m. Ground improvement using pre-loading with prefabricated vertical drains was undertaken to pre-consolidate the compressible sub-soils, which was followed by field monitoring. It is revealed that the classical theories can effectively be used in calculating the consolidation settlement and the time for consolidation. Predicted settlements and the consolidation time matched reasonably with the measured values. To account for the smear effects, the coefficient of consolidation and the coefficient of permeability were taken as those for vertical flow. Predictions with the smear diameter as two times the equivalent drain diameter provided an upper bound of the consolidation time while prediction without consideration for smear effects provided a lower bound of the consolidation time for the container yard project.