Replacement of Pennsylvania Turnpike Bridge No. NB-391, Lehigh County, Pennsylvania

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Overview: Not long after completion, the abutments of the Pennsylvania Turnpike Bridge no. NB-391 began to exhibit signs of instability. Differential settlement and outward rotation of the abutments over time jammed them against the deck and opened wide cracks next to the cheek walls, requiring the Pennsylvania Turnpike Commission to maintain a long-term monitoring and repair program. GeoStructures was responsible for the geotechnical design aspects of the bridge replacement.

The bridge abutments were supported on steel H-piles driven through 80 ft of rocky embankment fill. The bridge piers were supported on shale- and slate-bearing footings. The original pile driving records, as well as previous subsurface explorations, indicated that some piles terminated above design tip elevationson large slabs of old slate dumped before the approach fills were constructed. Other piles, driven deeper than design tip elevations, may have deflected off buried obstructions. Supplemental explorations previously conducted by GeoStructures identified a 30-degree sloping rock surface below the south abutment as a likely contributor to the erratic pile lengths under that particular substructure unit. Further complicating the subsurface picture at the south end of the existing bridge was the presence of a 60-ft high, 700-ft long, overly steep, distressed fill embankment.

GeoStructures was responsible for the geotechnical design aspects of a much wider replacement for the bridge including improvements to a half-mile of approach roadway at each end. Like most projects in mountainous terrain, this one transitioned from deep rock cuts at each end to high benched fills toward the middle. Added to this were unusual challenges of stabilizing a distressed slope and understanding enough about the existing bridge instability to design a new foundation that would remain stable over the long-term. GeoStructures was also responsible for designing five 1H:1V geogrid-reinforced fill slopes in areas where right-of-way will not accommodate standard 1.5H:1V slopes.

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