ELECTRICAL RESISTIVITY IMAGING OF A MULTIPLE ROCKSLIDE, PITTSBURGH, PA

Landslides in the Appalachian Plateau region of western Pennsylvania are a well-documented geologic hazard. During construction of Interstate Route 79 in the late 1960s near Pittsburgh, Pa., highway slope excavation reactivated a pre-existing rockslide. Despite remedial bench excavation and landslide mass removal, this slide continues to move slowly. Gradual upslope propagation of movement has resulted in a transverse horst-and-graben feature through the Morgantown Sandstone and deep, open fissures in various parts of the slide. In July 2016, electrical resistivity (ER) methods were applied to investigate subsurface geologic conditions, including depth to failure surface, displaced landslide material, and subsurface openings and fissures. Two downslope and two transverse ER lines totaling 350 m were acquired within the landslide footprint. Wenner, Schlumberger, and dipole-dipole arrays were collected for each line to determine the effectiveness of standard configurations in this geologic setting. Interpretations of inverted resistivity profiles correlate well with local stratigraphy and landslide morphology. Red beds, exposed at the toe of nearby slopes, were interpreted to contain the failure surface. Upslope ER lines show wide, shallow, high-resistivity anomalies that we interpret as buried Morgantown Sandstone blocks that have moved downslope. Transverse profiles near the toe show narrow, near-vertical, high-resistivity anomalies that we interpret as air-filled fissures. These anomalies correspond with observed depressions and sinkholes at the surface. Analysis of the three electrode arrays and spacing revealed differences in resolution and depth of measurement. Dipole-dipole arrays captured deeper features such as the failure surface and provided a broader view of slope characteristics; Wenner and Schlumberger arrays better highlighted shallow features. We conclude that integrating multiple ER array configurations and electrode spacings in conjunction with high-resolution mapping of observed surficial features is an effective way to characterize slope failure. This approach provides a comprehensive view of the slope and reduces non-uniqueness intrinsic to geophysical surveys, enabling effective decision-making for follow-up geotechnical subsurface investigations.