INSTRUMENTATION AND ANALYSIS OF THE NEW BALTIMORE LANDSLIDE, SOMERSET COUNTY, PENNSYLVANIA

The New Baltimore landslide is located along I-76 (Pennsylvania Turnpike) in Somerset County, Pennsylvania, and has been problematic since the Turnpike was constructed in 1939. The landslide extends approximately 2000 ft upslope and 1000 ft laterally, and moves toward the east-bound lane of I-76 at a rate of 5-10 in/yr. Overall, the landslide can be classified as a translational failure with localized rotational slides, rock falls, and flows. The geology at the site consists of the Upper Devonian Catskill Formation that includes sandstone, siltstone, mudstone, and shale units.

The New Baltimore landslide was mapped in detail to show the presence of major and secondary scarps, tension cracks, drainage channels, and depressions. A subsurface investigation was conducted by American Geotechnical & Environmental Services, Inc. of Pennsylvania. It consisted of 18 borings, ranging in depth from 39.4 ft to 118.1 ft, and installation of 15 piezometers, 11 Time Domain Reflectometry (TDR) cables, and 3 slope inclinometers. These instruments were monitored quarterly to investigate the pore water pressures at different depths and determine the location of the failure plane. Additionally, three transect lines were installed within the slide area to monitor any near surface movement. The core from the boreholes was logged to establish the stratigraphy and to select samples for laboratory testing. The factor of safety against sliding was determined for varying drainage conditions.

Results of the study indicate that the primary failure is located along the bedding plane at an approximate depth of 10 ft in the toe area and 75 ft near the crest of the slope. The material along the failure plane is a nondurable (slake durability index < 37%) soil-like claystone of low shear strength (cohesion=0 psf; friction angle=21 degrees). The instrumentation data show that most movement and higher pore pressure occur in the months of February through May. For varying drainage conditions, the factors of safety range from 1.0 (for dry slope) to 0.8 (for maximum pore pressure measured along the failure plane). A number of stabilization alternatives were evaluated as possible remedial measures including rock dowels, drilled piles, and slope re-gradation.