TECTONIC FAULTS AT ENGINEERING PROJECTS AND ELSEWHERE IN SOUTHWESTERN PENNSYLVANIA

Southwestern Pennsylvania includes the Pittsburgh Plateaus and Allegheny Mountain Sections of the Appalachian Plateaus Physiographic Province. The Pittsburgh Plateaus have flat-lying sedimentary rocks with dips less than 5^o. The Allegheny Mountains to the east have sedimentary rocks in low, open folds with flank dips of 5^o to 20^o. Very few tectonic faults have been reported in near-surface rocks of the Pittsburgh Plateaus and Allegheny Mountains in older geologic literature describing general stratigraphy and structure with emphasis on mineral resources.

Recent engineering projects involving dams and sewage treatment plants, along with hunting treks, have disclosed numerous faults in near-surface Pennsylvanian and Mississippian age rocks of this region. Other faults have been reported in surface and underground mines and caves.

Locations of engineering projects, hunting treks, mines, and caves show the geographic extent of faults from the Allegheny Mountains westward to a “Transition Zone” in more gently folded rocks along the east side of the Pittsburgh Plateaus. These many locations suggest that faults are ubiquitous in this region and most streams here developed along faults.

Surface manifestations of these faults are often obliterated or obscured by erosion or construction activities. Faults can sometimes be recognized as ground lineations, sharp bends in streams and rivers, and irregular topography on air photos, topographic maps, and in the field. Faults are also indicated in the field by resistant sandstone scarps, ground fissures, pressure ridges, scattered boulders, and boulder piles. Faults may be discovered in surface and subsurface investigation programs, but their extents and characteristics can only be determined from exposures in large surface or underground excavations. Where such faults exist, they have both geologic and engineering implications. Geologic implications include tectonics of the region; erosional patterns and landforms; groundwater flow; rock breakdown, weathering, and erosion; and soil and cave formation. Engineering implications include layout and interpretation of subsurface investigation programs; rock excavation and support, both surface and underground; slope stability; groundwater flow, dewatering, and water supply; and foundation preparation, treatment, and support characteristics.