A Geogrid-Reinforced Timber Piled Embankment over Kettle-Fill Deposits near Chicago, Illinois

A new grade separation for the Elgin O’Hare Expressway required a widening and grade increase for a 185-meter long embankment section carrying Illinois Route 53 across a series of peat bogs. The subsurface investigation showed a wetland developed over a kettle basin subsided into glacial diamicton and subsequently filled with lacustrine clay, silt and organic soils. The organic soil sequence included fibrous peat sandwiched between upper and lower layers of soft, organic-rich clay and silt. As an alternative to the costly dry-land bridge concept an alternative was proposed to place the new embankment over a 900 mm thick aggregate platform reinforced with two layers of biaxial geogrid and supported on 9- to 12-meter long timber piles. The timbers were driven 7-foot on-square center into competent glacial diamicton. The system was designed for 300 mm settlement at the top of pavement. Local excavation of the existing fill was required to achieve the critical height. High groundwater and soft ground resulted in challenging but manageable working platform conditions. Pile-driving analyzer tests confirmed capacity of timber piles and established site-specific driving criteria.