As construction on the anchorages proceeded, two watertight caissons were sunk into the channel close to either shoreline, so that the bridge's suspension towers could be built atop each caisson. The bases of each caisson consisted of sixty-six circular openings each in diameter, arranged in a six-by-eleven grid. Shafts of reinforced concrete would be built along the inner rim of each opening, and once each section of shaft reached above water level, cranes with clamshell buckets would dig the sand and mud inside each shaft before sinking the shafts deeper into the water. The Staten Island side's caisson was sunk into the water, and necessitated the dredging of of sand and assorted muck. This caisson required of concrete, and in March 1961, it became the first of the two caissons to be sunk. The Brooklyn side's caisson required even more work, since it was deep, displaced of muck, and used of concrete. Once the caissons were sunk completely, the shafts inside each caisson were filled with water, and the bases of the caissons were covered by a sheet of reinforced concrete. The process of constructing the anchorages and caissons took just over two years, and it was complete by the end of 1961.