Construction Next Door: Facing a Threat to the Building's Integrity

From an engineering standpoint, this would be a delicate, complex job. Like most tenements, this one was built on the cheap — a cellar hole was dug and rough-cut, and four- by eight-foot bluestone blocks were laid on the ground as a footing. A mortar and stone-rubble foundation was fashioned on top of them, and joists made of wooden beams were put in place to support the floors. The exterior walls are brick. In a touch common to such structures when they were first built, there was a communal latrine in the basement at the right rear corner of the building. In many 19th-century tenements, according to The Encyclopedia of New York City, "privies began to be placed in a subterranean vault where the effluvial stench became so powerful that it contributed to concerns about cholera."

Stench and cholera were not concerns when construction began next door in the summer of 2014. Underpinning the co-op's foundation was the issue. Since the new condo building's basement was to be several feet deeper than the tenement's, this presented the very real danger that excavating the condo site would compromise the integrity of the tenement's foundation on the lot line, possibly even leading to a building collapse.

To prevent shifting or collapse, the developer agreed to build an underpinning that would extend the tenement's foundation to the same depth as the condo's. Beginning in the middle of the wall on the property line, crews dug out three-foot sections to the required depth, built forms, and poured concrete. Meanwhile, the co-op set up four crack gauges in the building to measure any shifting of the structure. Under the city's building codes, a shift of more than a quarter of an inch can lead to a work stoppage and/or a re-evaluation of the construction technique.

The Building Moves

"We noticed movement — settlement of half an inch," says Christine Hobson, a structural engineer with RAND Engineering & Architecture, who joined the co-op's team just as construction was beginning. (A different RAND engineer helped negotiate the licensing agreement.) The developer's engineer agreed with Hobson's suggestion that work should be stopped.

The developer's engineer then proposed a "grout injection" system, in which holes were drilled into the dirt, and tubes were inserted in the holes and filled with grout, a substance similar to mortar. Once the grout hardened, new three-foot sections of trench were dug, forms were built, and the underpinning was poured. The co-op installed 16 more crack gauges.

One day, Allen Salkin, vice president of the tenement's board, heard shouts coming from the construction site. For reasons that have yet to be ascertained, one of the massive pieces of bluestone in the tenement's footing shifted, or was moved. "They kept breaking our building," Hobson says of the construction crew.

The building now settled by an inch and a half. The effects were alarming: the brick wall on the property line bowed out at the top; cracks appeared on interior and exterior walls; some walls separated from floors; doors and windows failed to open and close properly; and four joists in the basement cracked. Work stopped again.

Photo by Jennifer Wu