There’s a stereotype that New York’s skyscrapers are all founded on shallow bedrock. There used to be a myth that skyscrapers were clustered around Wall Street and midtown because that’s where bedrock was high, making the foundations cheap. There are a lot of problems with this story, starting with the fact that if people only built skyscrapers where foundation construction is easy, there’d be none in Chicago. Over the years, the “valley” between the downtown skyscraper cluster and the midtown skyscraper cluster has had a lot of tall buildings constructed, so the myth is less visually compelling than it used to be.
The picture above, a section through the entire Singer building down to the bottom of the foundations, gives a sense of another problem with the myth: bedrock isn’t uniformly high downtown or midtown. The test borings showed, at the building site at Liberty Street and Broadway, some 70 feet of sand on top of a hardpan layer 20 to 30 feet thick, on top of bedrock. Semsch’s book makes the sand sound very dramatic, ready to flow out from under a building at the mere mention of tunneling nearby, but the unexaggerated fact was that the sand was strong enough for smaller buildings but not for the concentrated column loads of a 40-story building. The solution, as mentioned yesterday, was to use pneumatic caissons.
In the picture above, the two cylinders on the right and the one just left of center, inside the wood box below “The Singer Building” sign are the working tops of caissons. (I am again struck by the relentlessness of The Foundation Company’s branding exercise. I see the company name six times in this photo.) The caissons proper were wood and were sunk in the traditional manner, pumped full of compressed air to keep the interior dry and excavated from below as concrete piers were cast on top, so that the entire mass gradually lowered itself. They used a relatively new form of airlock at the top, which allowed the cranes seen in the picture above to maneuver buckets of spill through the lock from above:
Mr. Moran, who invented this lock, worked at The Foundation Company, so bragging was inevitable. You can see in the diagram how the cable to the bucket passes through the airlock doors. The most interesting description of the foundations in the book was the process of correcting an early decision that came to be seen as a mistake. One caisson was completed and filled with concrete before the decision was made that all caissons should rest directly on bedrock; that caisson was resting on hardpan. The solution, executed with some difficulty, was to underpin the brand new caisson by sinking an adjacent shaft and digging sideways:
That picture does have the unfortunate effect of making it look like those four men were buried down there, as well as using a description – “undermining” – open to some incorrect interpretation.