Because of the great variation in the level of the river at low and high water—a matter of twenty feet—it was necessary to make the excavation, for building the lock, about fifty feet deep. In solid soil this would be a simple matter. But this ground has been made by the gradual deposit of Mississippi River silt upon what was originally the sandy bed of the ocean, and through these deposits run strata of water-bearing sand, or quicksand. This flows into a cut and causes the banks to cave and slide into the excavation. Underneath there is a pressure of marsh gas, which, with the pressure of the collapsing banks, squeezes the deeper layers of quicksand upwards, creating boils and blowing up the bottom.
New Orleans has had plenty of experiences with these flowing sands in its shallow sewerage excavations. How, then, expect to make an excavation fifty feet deep? asked the doubting Thomases. It couldn't be done. The quicksands would flow in too fast. The dredges would drain the surrounding subsoil, but that wouldn't get beyond a certain depth. Furthermore, what assurance was there that the soil that far down would supply sufficient friction to hold the piles necessary to sustain the enormous weight of the lock and the ships passing through it?
Undaunted by these croakings, the engineers, from test borings, calculated the sliding and flowing character of the soil, and estimated the various pressures that would have to be counteracted, balanced this with the holding power of pine and steel and concrete, evolved a plan, and began an excavation of a hole 350 feet wide by 1,500 feet long, gradually sloping the cut (1 to 4 ratio) to a center where the lock, 1,020 by 150 feet, outside dimensions, was to be built.
INNER HARBOR—NAVIGATION CANAL
Lock and Vicinity
The gentle slope of the cut was to prevent slides.
It had been ascertained that the first stratum of quicksand began twenty-eight feet below the ground surface (-3 Cairo datum) and was three feet thick; the second stratum, forty-eight feet below the surface (-23 Cairo datum) and ten feet thick. Coarser sand extended eleven feet below this, from -33 Cairo datum. The second stratum of flowing sand began just below where the lock floor had to be laid. The third layer was 80 feet below the surface (-55 Cairo datum); the tips of the piling would just miss it.
Excavation began in November, 1918. While the dredges were at work a wooden sheet piling cofferdam was driven completely around the lock, and about 125 feet from the edge of the bank, to cut off the first quicksand stratum. About 150 feet further in, when the excavation was well advanced, a second ring of sheet piling was driven, to cut off the second stratum, which carried a static pressure of 55 feet and was just a foot or so below where the floor of the lock would be. It was not thought necessary to cut off the third stratum.
The excavation was made in the wet. When it was finished the dredges moved back into the Canal, the entrance closed, and the work of unwatering the lock site began. This was in April, 1919.
There had never been such a deep cut made in this section. Consequently, the character of the soil, while it could be estimated, could not be known absolutely. And the exact pressure of the gas could not be known.