PA State Route 33 Over Bushkill Creek: Structure Failure and Replacement in an Active Sinkhole Environment

In January 2004 a sinkhole formed under a pier of the three span bridge carrying northbound SR 33 traffic over Bushkill Creek. The bridge was located in a highly active sinkhole area. A limestone quarry is located just upstream of SR 33. Attempts to fill the sinkhole and halt settlement of the pier were unsuccessful, and necessitated closing and eventual replacement of both the northbound and southbound structures. The geology includes the Epler and Jacksonburg Formations, which are intensely folded and faulted with numerous joints and fractures. Weathering of these deformed units produce deeply developed karst topography. Subsurface exploration for the new structure included a combination of borings and geophysical methods. Geophysical methods included resistivity and seismic techniques. Rock encountered consisted of limestone and dolomite. Rock was highly voided with soil filled seams, and generally low recovery. The replacement bridges consisted of single span prestressed concrete I-beam structures supported on micro-pile foundations. The single span option had the advantage of construction time and cost savings, and the benefit of no substructure units down in or near the creek. Staying out of the creek had positive implications for environmental and stream health concerns, and major safety improvements of minimizing worker exposure to the most unstable areas of the project site, relative to sinkhole activity. Sinkhole development remained high in and around the site, during construction operations.