ROCK SLOPE STABILIZATION OF FEDEX DRIVE, ALLEGHENY COUNTY, PENNSYLVANIA

Several rock slope failures have occurred over the past 15 years along the north side of FedEx Drive in Moon Township, Allegheny County, PA. The rock slope was cut through existing terrain in 1992 as part of the construction of FedEx Drive which leads to the FedEx Ground Package System Inc. (FedEx) headquarters. As constructed slope ratios varied from 0.5H:1V in the highest reaches of the slope, near the intersection of FedEx Drive and Montour Run Road, to 2H:1V near the entrance to FedEx headquarters. The slope was constructed by mass excavation, as well as presplit blasting methods where more durable rock was encountered. The rock units present in the slope are Pennsylvanian in age and belong to the Casselman Formation. Specifically the units are, in descending order, the Morgantown Sandstone, Wellersburg Claystone, and the Birmingham Shale stratigraphic units. Over time differential weathering between the claystone and overlying sandstone has resulted in undercutting, generating rockfall and planar failures within the Morgantown sandstone. These events have created a recurring hazard to the public and adjacent facilities near the intersection of FedEx Drive and Montour Run Road.

Gannett Fleming, Inc. was retained by FedEx to evaluate the nature of the failures occurring in the rock slope and to prepare a mitigation plan to reduce the hazard. The deltaic depositional environment has contributed to the variable and channelized nature of the Morgantown Sandstone. Depositional features including cross bedding within the sandstone and slickensides within the claystone were revealed during construction of the cut, which created irregular fracture patterns after the blasting. Given the variable nature of the rock and unpredictable fracture patterns that resulted from the blasting, the recommended mitigation plan included a combination of excavation, rock scaling, anchored wire mesh, and shotcrete. The construction was completed during the fall and winter of 2010. This presentation highlights the findings of the evaluations, as well as the various rock slope mitigation techniques utilized to stabilize the slope.