Micropiles Ease Historic Pittsburgh Bridge Replacement

Over several decades, the superstructure of the 100-year-old Butler Street Bridge in Pittsburgh, PA had become severely dilapidated. In addition, the ravine beneath the span had been used as a dump site for ash from the City’s Herr’s Island incinerator to the point where it was filled almost to the underside of the deck arch. In 2013, the owner, Pennsylvania Department of Transportation District 11-0, began work on a long-awaited new bridge to entirely replace the existing structure. Each bridge abutment was initially designed to be supported on rock-socketed caissons. The original work plan was to remove the entire bridge superstructure and existing abutments and backfill up to the bottom of the new abutments prior to installation of the caissons. This would necessitate excavation of some 6880 m³ (9,000 yd³) of incinerator ash that was deemed landfill-sensitive. In an effort to reduce excavation quantities and efficiently streamline shoring operations, as well as to alleviate potential foundation element alignment issues that could arise from caisson drilling in the steeply sloping rock, the general contractor sought input from an engineering consultant, which developed an alternative micropile foundation design. This paper discusses the initial design, the re-design and its construction advantages, and installation and testing of the micropile foundation system.