FOUR FAMOUS HISTORIC LANDSLIDES OF THE PITTSBURGH AREA

The Pittsburgh area has long been noted for landslides. This results directly from its geology, topography, climate, and history. Flat-lying interbedded strong and weak sedimentary rocks have been acted upon by erosion, stress relief, weathering, creep, and sliding to produce masses of marginally stable colluvial rock and soil on many slopes. These masses are often reactivated by heavy precipitation and by human activity including fill loading, toe excavation, and surface and subsurface drainage changes. The area also has an Appalachian heritage of dumping fill on slopes. Four large, famous historic landslides are described.

The October 1920 Bigelow Boulevard Landslide involved uncompacted and poorly drained fill placed in the head of a ravine at the top of a 230 ft. high slope. This slide moved as fast as 1 ft. per day as an earthflow down onto a Pennsylvania Railroad yard. More than 200,000 cu. yd. of material were removed; damage was $800,000. General G.W. Goethals of Panama Canal fame, retained as a consultant, said “Let ‘er slide.”

On March 20, 1941, more than 100,000 cu. yd. of rock slumped from a 160 ft. high slope at Brilliant Cut. The slide toe displaced railroad tracks and a train was derailed. This slide is thought to have involved a partially excavated Pleistocene rockslide mass and to have been triggered by high water pressures in a stress relief joint at the rear of this mass.

Extensive Pleistocene age rockslide remnants and associated colluvium were not recognized during investigation and design of a section of Interstate Route 79 (I-79) in the 1960’s. When the toes of old slide masses were excavated in 1968-69, these slide masses were reactivated for a distance of a mile along the valley wall. Large quantities of slide debris were excavated to stabilize slide masses. Portions of these slide masses have been creeping ever since but pose no threat to the highway.

Site grading for a commercial complex ¾ mi. southwest of the I-79 slide area involved placement of extensive fill over unrecognized colluvium. In September 2006, a landslide 1,000 ft. wide and extending 600 ft. upslope dumped 500,000 cu. yd. of material on 4 lanes of Pennsylvania Route 65 and railroad tracks between this road and the Ohio River. The road was reopened in 2 days and the railroad was reopened in 3 days. Portions of the slide continued moving until at least 2012.