INSIGHTS INTO HYDROLOGIC CONTROLS ON REGIONAL LANDSLIDE ACTIVITY NEAR PITTSBURGH, PA FROM MONITORING AND FIELD OBSERVATIONS IN A NATURAL LABORATORY ALONG A HIGHWAY RIGHT-OF-WAY

We have been monitoring a deep-seated, translational rockslide in a 2-km stretch of the Pennsylvania Dept. of Transportation right-of-way on the east side of Interstate Route 79 northwest of Pittsburgh. Our monitoring results since 2013 have provided insights into conditions associated with both deep-seated and shallow landslide movement in the region. This site is an important natural laboratory for landslide research based on its representative physiographic and geologic setting, and variety of landslide types and processes. The right-of-way includes areas of both modified and forested natural slopes and contains several prehistoric landslides that reactivated during highway construction. A deep-seated rockslide at Aleppo remains active despite subsequent mitigation efforts. Landslide deposits and colluvium overlie bedrock units of the Pennsylvanian-age Glenshaw and Casselman Formations, including the landslide-prone Pittsburgh red beds near the formation contact. Landslide types include deep-seated, translational and rotational rockslides; a shallow slow-moving earthflow; rock falls; and debris flows. Bedrock fissures in one location indicate slow failure of a high cut slope.

Hydrologic monitoring of shallow colluvium since March 2016 reveals seasonal variation in soil moisture and short-term response during rainstorms. A period of elevated soil moisture (Volumetric Water Content >30%) extended from late October through early May. The rapid transition from the annual low soil moisture in late September (VWC <20%) to elevated soil moisture corresponded with a storm having a multiday rainfall total of 64 mm. The hydrologic response to storms varied depending on antecedent soil moisture and storm rainfall amounts. The magnitude of the change in soil moisture induced by rainstorms was highest during dry conditions in the summer and fall, but the storm-induced peak values were higher during periods of elevated soil moisture in winter and spring. Movement monitoring of the rockslide using cable extension transducers and total station surveying revealed storm-induced millimeter-scale slip. The maximum average velocities occurred shortly after the peak rainfall intensity and corresponded to the reported times of other landslides in the Pittsburgh region during a June 15, 2015 storm.