Forensic Investigation of Backyard Landslide with a Sewer in Bethel Park, Pennsylvania

In a southern suburb of Pittsburgh, Pennsylvania (PA), a portion of a housing development was constructed in the 1950s on an unrecognized, marginally stable prehistoric landslide mass. Grading practices of that era included placement of poorly compacted fill over a sanitary sewer behind houses on the downslope side of a street across the upslope part of the prehistoric landslide mass. A shallow landslide developed in fill and near-surface colluvium (old landslide debris). This shallow landslide was fully active by 1973 and continued to move, at least intermittently, thereafter. Landslide movements observed by a homeowner behind his house circa 2003–2004 led him to sue the Municipality, alleging that sewer leakage had caused the landslide. A detailed forensic investigation with heavy emphasis on observational engineering geology, including but not limited to historical aerial photographs and high-quality sewer inspection videotapes, clearly showed that the sewer was not leaking in the landslide area. Rather, the sewer was acting as a drain to reduce groundwater levels, thereby improving stability of the landslide.