As part of a legal consent decree at this U.S. Environmental Protection Agency Superfund site, the Work Defendants at the former Operating Industries, Inc. (OII) landfill, located 12 miles east of downtown Los Angeles, were required to design the landfill cover on the steep northern face of the landfill to be able to withstand a design earthquake of Magnitude 6.9 located 11 kilometers directly beneath the site. Slopes as steep as 1.5H:1V were to be covered with six feet of soil for an evapotranspirative final cover. Seismic design criteria required that, during an earthquake, the soil cover would not fail at the cover/refuse interface and slide onto the freeway located at the base of the landfill slope. Therefore, the cover had to be constructed utilizing reinforcement that prevented through-going slump failures. The reinforcement design consisted of horizontal geogrids installed in the cover fill every five vertical feet on the steeper slopes. The geogrid was anchored in refuse at its distal end and terminated so that it did not daylight onto the slope. The regularly spaced geogrids were intended to prevent a through-going failure, maintaining cover stability on a macro scale during the design earthquake. However, termination of the geogrid a distance of two to three feet from the slope face resulted in an unanticipated problem that was initially hidden beneath the vegetation mat covering the surface.
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