SEWAGE TREATMENT PLANT ON A MAJOR FAULT ZONE, SEWICKLEY TOWNSHIP, PENNSYLVANIA

A sewage plant is being constructed in Sewickley Township 18 mi southeast of Pittsburgh. Pennsylvanian age sedimentary strata in this area typically dip less than 2º and tectonic faults are rare. The site is 1 mi west of the axis of the Port Royal Syncline. Published maps show maximum dips of 4º toward the syncline axis in this vicinity.

Abandoned room and pillar workings in the Pittsburgh Coal underlie the site at a depth of 90 ft. These workings were grouted prior to plant construction.

A subsurface investigation involving 14 borings and 11 test pits was done in 2007. Results showed mine voids and artesian water at the south end of the site and near-surface fault disturbed rock with breccia and gouge throughout the site. The south end of the site was filled several feet above artesian water level prior to mine grouting.

One hundred thirty-nine 6 in. air rotary holes for mine grouting were drilled on a 25 ft grid pattern in 2009. These holes were carefully logged for stratigraphy, voids, wet zones and water levels. This drilling showed the south end of the site had mine voids, rock faulting and fracturing, and high groundwater levels with large inflow rates. The north end of the site had more coal left in place, less faulting and fracturing, and lower groundwater levels with smaller inflow rates.

Excavations for the pump station (40 ft deep) and process tanks (15-20 ft deep) in 2010 showed the site to be located on a major tectonic fault zone at least 500 ft wide and 20 ft thick. This zone contained sandstone blocks up to 15 ft wide along with finer breccia and gouge. The fault appears to have involved both thrust and transverse movements and to have occurred as a brittle response to gentle folding of interbedded strong and weak sedimentary rocks here, west of the more intensely folded rocks of the ridges to the east, during the Appalachian Orogeny.

Faults of this type should be considered in investigation and design of future engineering projects in the region. Surface manifestations of these faults are obscured or obliterated by erosion. The faults may be discovered in surface and subsurface exploration programs but their extents and characteristics can only be determined from exposures in large excavations. Where such faults exist, they have engineering implications regarding excavation methods, slope stability, dewatering, and foundation treatment.