Completed in 1842, the inactive 41-mile long masonry brick Old Croton Aqueduct and dam was once the sole supplier of fresh water to New York City. The aqueduct, which comes under the auspices of the New York Office of Parks, Recreation and Historic Preservation, was awarded National Historic Landmark status in 1992. In early 2015, a 70-ft section of the old gravity stone retaining wall supporting the aqueduct failed. Moretrench was invited to be a part of the engineering team assembled to develop the methodology to stabilize the 50-ft high slope, protect the buildings above and below, and design and construct a replacement retention system.
The 70-ft long failed section caused a landslide of some 500 cubic yards of soil, trees and boulders to crash into a senior housing apartment building at the foot of the slope, while the remaining portion of the 300-ft long wall had tilted and moved outward several feet. The building was immediately evacuated amid stability concerns, and a formal request was issued by the New York State Office of General Services (NYSOGS) for a design/build retention system to replace the entire original wall.
Constructability within the restricted access and steeply sloping work area was a major design consideration. Working closely with Engineer of Record McLaren Engineering of West Nyack, NY, NYSOGS and the New York State Office of Parks, Moretrench proposed a multitiered, ground-up approach for installation of the selected soldier beam and concrete panel retaining wall and performed the work as the general contractor.
Given the highly unstable condition of the disturbed ground and the precarious nature of the remaining original wall, ensuring personnel safety was a critical consideration prior to the start of any work. Moretrench retained GZA of New York, NY to install instrumentation along the wall which provided round-the-clock real-time data on any movement beyond the set 0.1-inch threshold. The project manager, on site safety supervisor, and superintendent were linked directly to the monitoring system via cell phone. Instrumentation was followed by installation of soldier beam and lagging temporary support of excavation for construction of a 25-ft wide work bench at the bottom of the slope to allow safe access to the toe of the new retaining wall alignment. Given the presence of a high concentration of boulders, cobbles and gravel throughout the slope, Moretrench selected overburden drilling to install temporary casing and place and concrete the soldier beams.
The permanent retaining wall, located 4 ft in front of the original wall, was designed as galvanized, double W12x35 soldier beams spaced at 8 ft on center, with precast concrete intermediate panels. The precast panels and covers for the soldier beams were finished with a stone effect in keeping with the original wall and the scenic location.
Moretrench elected to install the beams within permanent 24-inch diameter casings to ensure that tolerances would be met and that concrete backfill would be placed to the proper elevation. Drilling and casing installation was accomplished using the same methodology and tooling as the for the temporary SOE. As soldier beam installation progressed, timber lagging was placed behind the back flanges and the annulus between the lagging and the original wall was filled with crushed stone. Two levels of 3-strand double corrosion protected tieback anchors were installed through each soldier beam, extending through the original wall and with sufficient bond length to develop the required design loads in the boulderly soils. Laboratory testing of anchor grout was performed to confirm that the unconfined compressive strength requirement of 3,000 psi was met.
Proof testing was conducted on all anchors, with six anchors performance tested to 133% of design capacity. Concurrent with tieback installation, the slope above and behind the new wall was backfilled, preliminarily graded, and a drainage system installed. The bench and temporary support of excavation were also removed at the same time and grading performed immediately behind the building.
The precast panels were lifted into position utilizing a telescopic boom hydraulic crane and guided into place by workers on a man lift. The panels were designed to bear against the front flange of the soldier beams, with 3/8-inch stone placed between them and the temporary timber lagging. Final grading above the wall, drainage tie-in, landscaping, fencing and site restoration by Moretrench subcontractor Northbrook completed this challenging project.