Primary and Post-Surcharge Secondary Settlements of a Highway Embankment Constructed over Highly Organic Soils: A Case History

A new segment of U.S. Highway 14 was constructed over highly organic clay near Nicollet, Minnesota, by using prefabricated vertical drains and surcharging in 2016. The highly compressible soils were predicted to settle 3 feet at the maximum fill area over the duration of 2 to 3 years. During the design phase, coefficient of consolidation (c_v) values obtained from 1-D consolidation tests were used to estimate the time-rate of settlement curve. The estimated primary settlement (S_p) of the embankment was about 37 inches after 6 months of surcharging. Nonetheless, a back-analysis using permeability change index (C_k), which has the advantage of representing decrease in permeability due to decrease in void ratio, was performed to model the primary settlement response of the highly organic soils. The computed settlement and pore water pressure show a good agreement with the field data. After the removal of surcharge, the post-surcharge settlement behavior, including rebound and compression, were continuously measured and recorded for more than a year. By using laboratory secondary compression index (C_α) and post-surcharge secondary compression indices (C'_α and C"_α) derived from the field data, the projection of embankment performance over the pavement design life was made.