Construction of the original IRT subway at the Brooklyn Bridge station, 1904:
Construction of the BMT*** main line, Broadway and 38th Street between 1913 and 1915:
As those photos show, a lot of early* subway construction in New York was built using the cut-and-cover method, where a street is excavated, tunnel structures built in the hole, and the tunnel roof used to support the restored street. If you look closely at the BMT photo, a lot of the “street” is temporary wood structure, including the trolley-track path. Similarly, in the IRT photo, most of the stuff that looks like modern art (in the brightly-lit portion of the shot) is timber shoring.
Part of the work was dewatering and underpinning. These two aspects of tunneling, particularly the latter, are unavoidable when working in a dense city like New York. And, like any other construction, quality varies. To paint with a broad brush, the IRT company was quite good at underpinning, and the BMT not so good.** In several locations, the BMT crews did not underpin as much as they should have, so that their dewatering broke the adjacent buildings.
An example can be seen at One Union Square West, the Lincoln Building. The east facade was underpinned during the adjacent BMT subway work, performed at roughly the same time as the picture above. That’s the facade on the right side of the picture at the link. The underpinning pretty clearly went about 25 feet west along the south facade. How do I know? The masonry is straight on the east side and the first two bays of the south side, and then abruptly sags. The storefront lintel in the third bay (with “TRAIN” in the window) drops off to the west, and the masonry in the spandrel panel between the third and fourth floor windows is cracked. There are not a lot of ways to get differential settlement in the middle of a building like that, given that the loads are fairly constant along the length of the building.
A similar effect can be seen along Fourth Avenue in Brooklyn, where the tunnel was constructed between 1909 and 1916. The facades facing the avenue were underpinned while the remainder of the buildings were not, resulting in a number of small buildings that appear to be tilted back from the street.
Part of the logic of preservation work is that it has to be locally based. Materials, systems, craftsmanship, and styles are intensely local, and it is difficult for anyone who does not know the area to properly analyze the condition of a building. In the case of New York, knowledge of the basics of subway history is part of understanding our local building conditions.
* As well as some current work. Portions of the soon-to-open Second Avenue line were built cut and cover.
** To some degree, I’m speaking ill of people who performed difficult work more than 100 years ago. On the other hand, I’ve had to deal with the consequences of some of their decisions, so my sympathy is limited.
*** At the time of construction the Brooklyn company was the BRT. It was renamed the BMT in 1923, and tat name continued in use after the city bought the private subway companies, so I’m using “BMT” here.