Engineering geology considerations for realignment of Interstate 70/76 across the landslide at New Baltimore, Somerset County, SW Pennsylvania

The Pennsylvania Turnpike Commission plans to widen I-76 due to increased traffic in the Somerset County. The New Baltimore landslide, which has caused serious problems for the turnpike since its construction in 1939, is located within this area. The landslide extends 2000 feet (610 m) upslope and 1000 feet (305 m) laterally, and moves 5 to 10 inches (13 to 25 cm) per year. The Turnpike Commission plans to cut into the slope between mile markers 128 and 129 on the south side of the turnpike, thereby passing through the New Baltimore landslide. When the Pennsylvanian Turnpike is realigned, there could be new instability, reactivating the landslide and adding to instability. The bedrock geology of the New Baltimore landslide site consists of the Upper Devonian Catskill Formation which contains interbedded sequences of harder and softer strata including siltstone, shale, and grayish red sandstone units.;This research project was conducted to determine the engineering properties of rocks and stability of the slope located between the mile markers mentioned above. A subsurface investigation was conducted on the New Baltimore landslide by the American Geotechnical and Environmental Services, Inc. of Pennsylvania. Field work was conducted to find the upper limit of the landslide and to collect discontinuity data. The discontinuity data were analyzed to evaluate various modes of slope failure. Lab work conducted in this study included the dry density test, slake durability test, unconfined compression test, and the shear strength test. The results of the study show that slake durability index of the claystone units ranges from 72 to 99% whereas the unconfined compressive strength of the sandstone/siltstone units ranges from 8,900 to 25,600 psi (61.4 to 176.5 MPa). Direct shear test results show that the claystone has a cohesion of 28,455 psf (1362 kPa) and a friction angle of 17 degrees.;The inclinometer data shows that there is small movement along the weaker claystone units (where gouge was encountered during drilling) and this is being closely monitored. Stability analysis indicates that the factor of safety for a dry slope is 1.1 and for a wet slope it is 0.4 with a average pore pressure 546 psf (26 kPa). Selected remedial measures are proposed for the New Baltimore landslide so that the interstate highway can be realigned safely.