INVESTIGATION AND ANALYSIS OF THE NEW BALTIMORE LANDSLIDE, PENNSYLVANIA TURNPIKE

New Baltimore landslide, located along Pennsylvania Turnpike in Somerset County, has been problematic for the Turnpike ever since its construction in 1939. Being an active landslide, it continues to move toward the east-bound lane at a rate of approximately 5-10 in/yr, which causes the shoulder to heave. Overall, the landslide is 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 landslide was mapped in detail to show the presence of major and secondary scarps, tension cracks, drainage channels, and depressions. A subsurface investigation, consisting 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, was conducted by American Geotechnical & Environmental Services, Inc. The instruments were monitored quarterly to investigate the pore water pressures at different depths and determine the location of the failure plane. 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. The results of the study indicate that the primary failure is located along the bedding plane at an approximate depth of 9.8 ft in the toe area and 75.5 ft near the crest of the slope. The material along the failure plane is a claystone of low shear strength (cohesion = 0 psf; friction angle = 21 degrees). The instrumentation data show that the most significant 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.