Since 1916, State Route 718 has crossed the Shenango River in Sharon, PA via the State Street Bridge. Eventually, the open-grate structure had deteriorated to the point that weight limits had been imposed on vehicular traffic and pedestrian traffic was prohibited. As part of a five-year upgrade plan for Sharon, Pennsylvania Department of Transportation (PennDOT) initiated demolition of the old bridge and construction of a new, 2-span concrete bridge at the same location.
Since installing driven piles to support the abutments and central pier would create unacceptable vibration resulting in the potential for damage to buildings surrounding the bridge, micropiles were specified to transfer the structural loads to a competent bearing stratum. Moretrench was awarded the contract for micropile installation by general contractor Delta Constructors, Inc. of Hermitage, PA.
Complex Soils, Difficult Access
Subsurface borings had encountered up to 90 feet of alluvial overburden with cobbles, boulders and heaving sands, overlaying siltstone and sandstone. At each abutment location, the foundation design called for two rows of seven, 124-kip micropiles spaced at nine feet horizontal and 5.75 feet vertical, in plan. Similarly, the central pier would be supported on two rows of micropiles, but on a 4.5 feet horizontal spacing. All piles were designed to extend I 0 feet into the underlying rock. Compressive load testing conducted on a production pile recorded a maximum deflection of 0.308 inch at 200% of the design load, verifying the pile design parameters.
Pile installation at the abutments was completed first and required a high level of precision and attention to safety while working around and beneath live overhead power lines. Duplex drilling techniques were used to advance the 9.625-inch OD wall fiush joint N80 steel casing through the overburden and to create the 8-inch diameter rock socket. One No.20, 2.5-inch diameter, centralized, 75-ksi, inner steel reinforcing bar was then placed for the full, I 00-foot length of the micropile and 4,000 psi Type I Portland neat cement grout was tremied in place.
Micropile installation for the center pier support was accomplished atop a sheetpile cofferdam accessed via a rock causeway, approximately I 00 feet long, constructed through the shallow river water by the general contractor. Crane mats provided Moretrench with a stable working platform for the drilling equipment. Grout was pumped via hoses from the land based grout plant.
Despite the complex subsurface conditions and challenging drilling access at both the abutment and center pier locations, the foundation work was successfully completed on time and within budget.