Performance Based Design of EPS Embankments in Regions of High Seismicity
In Vancouver, Canada, the last three major infrastructure projects have included large Expanded Polystyrene (EPS), or geofoam, embankments. Most recently, the Port Mann/Hwy 1 Highway Improvement Project (PMH1) included the design and construction of 22 large EPS embankments, using over 300,000 m3 of EPS blocks. EPS blocks are ideal for light-weight embankments that can avoid the need for lengthy pre-loading of soft foundation soils. This has made EPS embankments particularly attractive for design-build projects that always have compressed construction schedules.
On PMH1, EPS embankments were designed for two performance objectives: repairable damage for the 1/475 year and no collapse for the 1/975 year ground motions. The corresponding peak ground accelerations were 0.26g and 0.36g, but with amplification due to soft ground some of the embankments could experience horizontal accelerations in excess of 1g. At these levels of shaking, sliding between EPS layers is expected; however, there are currently no guidelines that deal with this situation.
To evaluate the sliding displacements within embankments, a simple non-linear analysis method was developed based on the Newmark sliding block approach. The simple analysis uses time-history records of surface ground motions at each location and includes amplification from the natural vibration of the embankment itself. The simplified analysis is compared to a nonlinear dynamic analysis using discrete-solids modeling of individual blocks. Since there are no guidelines relating sliding displacements to performance objectives,specific design criteria had to be developed for the PMH1 project.