Upgrading of Route 87/287 from two to four lanes to accommodate increased traffic flow required the installation of a replacement bridge abutment approximately three feet in front of the toe of the embankment supporting the existing west abutment at the Meadow Street overpass. Since the new abutment foundation would be up to 14 feet deeper than the existing abutment foundation, an excavation support system would be required to support the embankment soils beneath the existing abutment during this work. Moretrench was awarded a subcontract by the General Contractor for New York DOT to design and build the earth retention system.
Soil Nail Design and Installation
The embankment soils consisted of silty glacial till with large boulders. Given the presence of these boulders, coupled with the low headroom working conditions beneath the overpass, Moretrench elected to use soil nailing to support the soils. With the active abutment bearing directly on soil rather than deep foundations, the soil nail wall was designed to take into account the large vertical surcharge, as well as lateral earth pressures from the embankment.
Three levels of soil nails were installed on a 4.5-foot high by 5.0-foot vertical grid to support the 14-foot deep, 200-foot long embankment. Given the nature of the embankment fill, Moretrench chose to first reinforce the cut face with wire mesh and a three to four inch thick layer of 4,000 psi shot- crete. After the shotcrete had cured, the soil nails were drilled and grouted in place. Specialty drilling techniques were required to penetrate the large boulders during soil nail instal- lation. Sacrificial test nails were used to verify bond values.
The soil nail wall was installed in one mobilization, with no removal of overhead superstructure required.