Ordinarily, buildings are underpinned one or two at a time, when a new building’s foundations are deep enough to undermine its neighbors. If you think about it, the construction of cut-and-cover subways on narrow streets was inevitably going to require underpinning of a lot of buildings – dozens at a time, or more. I’m sure that somewhere there’s a record of how many buildings were underpinned during the construction of the New York subways, but I don’t know where: the construction drawings for the subways themselves don’t show that work. The illustration above, Building the New Rapid Transit System of New York City, shows some of the generic details used during the 1910s dual-system expansion. Unlike the tightly-controlled details for the tunnels themselves, these details had to be adaptable to the wide variety of foundations present.
I think the illustrator was having some fun here, because a quick look suggests that these are five alternate versions of underpinning, while it’s actually more complicated than that. So here’s a summary of what they were planning:
- “A” is a section showing a masonry wall supported on “needle beams,” which are simply closely-spaced steel beams perpendicular to the wall plane and supported on temporary foundations. The masonry between the beams stays up by arching action. Note also the temporary wood-plank roadway while the open cut for the tunnel was present. In this section, the wall is being supported for underpinning only a couple of feet high.
- “B” shows a wall on needle beams, with the needles supported on girders and piles. This was, in the 1910s, an incredibly labor-intensive method of work, but in very soft ground might have been the only realistic option.
- “C” shows two large beams – 30I200s were at that time about as big as rolled beams got – spanning a long distance and supported on concrete piers. This would be overkill for an ordinary underpinning job, but would make sense for a concentrated load, like a column or pier footing.
- “D” shows the needle beams supporting the street-side wall as cantilevers, held down by the dead weight of the back wall. This is a good example of a design that can be made perfectly safe, but looks scary because of a hundred years of movies showing cars, trains, and occasionally houses teetering on the edge of a cliff.
- “E” shows the final stage of pier underpinning, with the wall resupported on a pair of beams directly below it (and the space between the beams filled with concrete) and the needle beams removed. The white part of the wall at its bottom is where the needles would have been during the process.
Note that all of these techniques were actually used in the 1910s and 20s as the system was built out. The IRT company did a pretty good job with its underpinning: I can’t tell if a building along an IRT line was underpinned without looking in the cellar. The BMT’s work quality was…variable. At a lot of buildings, they underpinned only the street facade immediately adjacent to their tunnel, and there was ground movement a little further back. There are many buildings along Fourth Avenue in Brooklyn that are overall tilted, with the street facade at the original elevation and the rest of the building sloping down to the rear. And One Union Square West has a rather prominent curve in its south facade between the east wall that was underpinned and the remainder of the south wall that subsided.
