ouldn'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.

[Illustration: 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.

The sands proved to be more liquid and the gas pr