Field Observations and Analysis of the Subgrade Response Beneath GRCS Embankments at the Council Bluffs Interchange System

Abstract

Geosynthetic-reinforced column-supported (GRCS) embankments partially transfer fill loads through stiff vertical elements to competent ground at depth, reducing the embankment stress applied to weaker, more compressible soil. Previous studies have focused on load transfer via soil arching and engagement of geosynthetic reinforcement above the columns. The role of the subgrade response to system efficacy—including the influence of the downdrag mechanism to load transfer—has received less attention. Vertical deformations were measured with settlement plates and multipoint borehole extensometers beneath full-scale GRCS embankments constructed with drilled-displacement columns at the Council Bluffs Interchange System (CBIS). The subgrade response was analyzed via the load-displacement-compatibility (LDC) method in combination with measured subsurface vertical deformations, a proxy for load transfer at depth, and measured values of interface friction from full-scale column load tests. A stiff upper layer of overconsolidated clay transferred embankment stresses imparted at the base of the fill to the columns via downdrag, shielding the underlying softer clay. This mechanism limited the efficacy of the geosynthetic reinforcement. Stiff shallow clay and crust that partially adheres to the columns via interface friction at shallow depths can effectively function like a subsurface load transfer platform. Based on these observations a design sequence to determine the necessity of geosynthetic reinforcement and to optimize column spacing—using familiar GRCS concepts—is recommended.