'WHERE IS THE ROCK?' - SUBSTRUCTURE REMEDIATION DURING CONSTRUCTION OF NORFOLK SOUTHERN BRIDGE MI-130.35 OVER THE SHENANGO RIVER IN SHENANGO, PA

摘要

Norfolk Southern (NS) Bridge MI-130.35 over the Shenango River was a two-span lattice truss bridge with masonry abutments and one masonry pier founded on piles and/or spread footings. Bowman, Barrett & Associates Inc. (BB&A) was retained to design a three-span through plate girder replacement bridge supported on two new piers, with each pier founded on two six-foot-diameter concrete drilled shafts. The shafts were designed to be rock-socketed approximately 15 feet below the mud line, based on soil borings. During construction the shafts were drilled to a depth of 25 feet below the mud line without encountering rock. Construction was immediately halted and a conjunctive redesign was conducted among NS, BB&A, D'Appolonia (sub-consultant), Ruhlin (contractor), Parks Drilling (subcontractor), and Hayward Baker (subcontractor). Based on equipment limitations, a rock depth 65 feet below the mud line, sandy soils, and available material, a retrofit involving five micropiles installed within each shaft and extended 50 feet below the drilled shafts and then rock-socketed, was approved for construction. Additional challenges during implementation of the micropile retrofit, including grout encroachment within the shafts, lack of bearing for drilling, and settlement of the shafts, were overcome during construction through swift communication, design and approval. The paper features the "lessons learned" throughout the design and construction to completion of this bridge.