Continuing from yesterday’s dive, we have an 1870 drawing by Washington Roebling showing the relationship of the Brooklyn tower of the Brooklyn Bridge to the caisson that serves as its foundation. There’s a lot going on in this line drawing and it’s worth the time to describe it all. From the top down:
- The tower superstructure masonry, with the courses marked off and dimensions in red. The surface of the water is nicely shown by decreasing the spacing of the lines representing water.
- The tower substructure masonry, continued down from above, getting wider at each course to spread the load as much as possible. The bottom courses of stone are marked 1 through 11 on the right, next to the vertical blue line.
- The timber grillage that makes up the roof of the caisson, with closer-spaced lines than the masonry. The outward slope continues, to grab as much bearing area at the bottom of the caisson as possible.
- A layer of water- and air-proofing (I forget if it was sheet metal or something else) between the fourth (from the bottom) and fifth layers of timber, shown as a red line, bending down over the sides of the lower timbers.
- The timber sides of the caisson are the inverted triangles that have the same horizontal striping as the roof, end at the bottom at a metal “cutting edge.”
- The caisson floor was divided by a series of partitions running the short way across the rectangular caisson (we’re looking down the long axis) and we’re seeing one below the roof and between the sides, marked on the left by diagonal plank sheathing which is cut away on the right to show its heavy-timber support framing. Note the door in the planked area and the framing for the door in the bare area.
- The sides of the caissons are protected by plank (outboard of the waterproofing) at the bottom and what looks like mass concrete above.
- The vertically-oriented blob in the middle is a cast-iron air-lock above the caisson roof. There’s an open-top timber box around it (probably to keep it dry when the caisson was first set in place) and a ladder down to the caisson floor work area. The ladder is surrounded by an iron tube.
- That vertical blue line I mentioned earlier (and its twin on the left side) is an iron pipe used for communication between the caisson floor and the workers up top. Given the amount of time and effort it took to get through the airlock, how do you communicate quickly between the two sets of workers? A mechanical telegraph running through those pipes was the answer.
- Finally, the depression in the earth below the caisson is the airlock for the “water shaft,” a very large water-filled barometer that had a vertical shaft used to remove excavation spoil from within the caisson. The water shaft was in line with the airlock and out of the plane of the drawing, so it’s behind the page here.
Tomorrow: how this all worked.
