An existing retaining wall at PS79 was cracked and failing. The retaining wall supported the street sidewalk and formed an areaway alongside the school building. Temporary lateral braces were used to keep the wall from collapsing.
YU & Associates, Inc. (YU) conducted a subsurface investigation involving borings drilled at the toe of the wall inside the areaway and behind the wall on the sidewalk. The borings indicated 6 to 9 ft of fill existed behind the wall, and rock was encountered about 2 ft below grade at the toe of the wall. The wall appeared to bear directly on rock without a footing.
Replacement with a new retaining wall was required. Due to the proximity of the property line and the shallow rock, special wall types were evaluated, including 1) L-shaped wall with lightweight aggregate backfill, 2) L-shaped wall with rock anchors, and 3) Counter fort wall. A conceptual design of the counter fort wall, the least costly alternative, was prepared. Seismic design parameters were developed per the latest NYC Building Code.