Case Study: Vertical Drain and Stability Analyses for a Compacted Embankment on Soft Soils

This case study describes the failure of an interstate connecting-ramp embankment during construction and investigates the failure mechanism, performance of the prefabricated vertical drains (PVDs) installed to accelerate consolidation of the weak embankment foundation soils, embankment shear strength parameters, and design slope stability analyses. The weak, fine-grained foundation soil experienced less drainage, and thus less consolidation and strength gain, than expected via the PVDs because of an overestimate of the design horizontal coefficient of consolidation. As a result, the inverse analyses show the failure was caused by lower than expected shear strength of the foundation soils and an overestimate of the compacted embankment shear strength. The compacted embankment fill strength was characterized using an undrained shear strength, i.e., cohesion, without a tension crack, which inflated the calculated factor of safety. Recommendations to estimate embankment shear strength parameters, depth of embankment tension crack, bearing capacity factor of safety for comparison with limit equilibrium values, and horizontal consolidation properties for PVD design for future embankment projects are presented.