Permeability and lateral earth pressure in granular backfill against bridge abutments

In bridge abutment design, Wisconsin DOT assumes the granular backfill material used behind bridge abutments as free-draining and no hydrostatic pressures are applied on the wall. This research study was undertaken to investigate if backfill materials meet the assumption of a freely-drained condition through a detailed laboratory and field study. Also, the viability of using recycled asphalt pavement (RAP) and shingles (RAS) for granular backfill was investigated. Laboratory testing involved characterizing the materials in terms of gradation/classification, erodibility, permeability, shear strength, volume change (i.e., water induced collapse). Laboratory tests revealed bulking moisture content for natural materials and collapse upon wetting. RAP and RAS materials exhibited collapse upon wetting and creep under constant loading. Scaled abutment model testing was performed to assess pore pressure dissipation rates for the different materials and calibrate input parameters to predict drainage using finite element analysis (FEA). Abutment model testing indicated that addition of geocomposite vertical drain can substantially increase pore pressure dissipation rates and avoid material erosion. Field testing involved in situ permeability, shear strength and moisture content testing, and monitoring lateral earth pressures and pore pressures behind abutment walls at four bridge sites. Results indicated that field conditions are more complex than the simple linear stress distributions typically assumed in the design for lateral earth pressures. Lateral earth pressures were greater than assumed in design over a majority of the monitoring period of this study. Pore pressures behind abutment wall were observed at one site following flooding. Predicted pore pressure dissipations using numerical analysis matched well with the measured values