Bad Soil

by Don Friedman on June 8, 2016

The landfill that makes up Battery Park City is roughly forty years old. Most of the apartment buildings along South End Avenue were built in the late 1980s or early 1990s; the sidewalks are about the same age.

That’s the granite veneer on the facade of an apartment house where it abuts the sidewalk. There’s a line of sealant that was installed in the gap between the two materials, and the old line of the sealant is about an inch higher, still adhered to the stone. If we rule out the possibility that the building is actually a very slow rocket taking off into the air, then it’s clear that the sidewalk has dropped an inch since the sealant was put in place.

The buildings in this area are not founded on the fill material: they are on deep foundations. Most likely, although I don’t know for certain, they have caisson or pile foundations that rest on bedrock or hardpan, some sixty feet below grade. The sidewalks are simply slabs on grade, and so they are resting on the fill. Any motion of the soil, and specifically compaction of the fill, would result in exactly what we’re seeing here: the sidewalk dropping relative to the building.

Why now? In other words, that sealant in the picture is almost certainly not thirty years old, so why has the sidewalk moved in the last ten to fifteen years? Possibly Hurricane Sandy. This area was not quite underwater, but adjacent areas were, so the soil in the area was generally wet more so than it had been before and then dried out. Or maybe the sidewalk has been moving gradually for thirty years and this bead of sealant simply shows the most recent portion of the movement. Or there might be another reason that’s less obvious.

Movement of the sidewalk does not create any structural hazard. It does create changes in elevation along the line of the building foundation and therefore creates trip hazards. There is plenty of evidence that this slowly-developing problem has been dealt with through ordinary maintenance: shaving some concrete to create miniature ramps, placing some concrete to create ramps, painting the edges of the new steps yellow as a warning.

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