Then another ring of sheet piling was driven inside the other two. This one was of steel, and the walls were braced apart by wooden beams ten inches square and fifteen feet apart in both directions. This is one of the largest cofferdams of steel ever driven. As an added precaution against the danger of a blowout by the third stratum of quicksand, which had a static head of 75 feet, 130 ten-inch artesian wells were driven inside the steel cofferdam. Fifty-six similar wells were driven between the steel and the wooden cofferdams to dry out the second stratum of quicksand, as much as possible, and lessen its flowing character. In November, 1919, the work of unwatering the lock site again began. Only one foot every other day was taken off. Engineers watched every timber. It was not until January 4, 1920, that the unwatering was complete. The plan had worked. Only in one place had there been any movement--a section of the wooden sheet piling about 300 feet long bulged forward a maximum distance of three inches, when the bracing caught and stopped it. Then began the work of driving the 24,000 piles on which the lock was to be floated. They are 60 feet long and their tips are 100 feet below the surface of the ground. In March, 1920, the work of laying the concrete began. The work was done in 15-foot sections, for only a few of the braces could be moved at one time. When it was finished in April, 1921, the lock was in one piece, a solid mass of steel and stone, 1,020 feet long, 150 feet wide, and 68 feet high, weighing, with its gates and machinery, 225,000 tons, and filled with water, 350,000 tons. The concrete floor of the lock is 9 to 12 feet thick, the walls 13 feet wide at the bottom, decreasing to a two foot width at the top. Six thousand tons of reinforcing steel were used in the construction, and 125,000 barrels of cement. There are 90,000 cubic yards of concrete in the structure. Two and a half million feet of lumber were used in building the forms. Usable dimensions of the lock are 640 feet long, 75 feet wide, and 30 feet (at minimum low water of the river) deep. The top of the lock is 20 feet above the natural ground surface and 6 feet above the highest stage of the Mississippi River on record. To the top the ground will be sloped on a 150-foot series of terraces. This will brace the walls against the pressure of water within the monolith. It will be developed to a beautiful park. Heavy anchor-columns of concrete will hold the walls against the pressure of these artificial hills when the lock is empty. Traffic crosses the canal here by a steel bascule bridge 65 feet wide, with two railroad and two street car tracks, two vehicle roadways, and two ways for pedestrians. Concrete viaducts lead to the bridge. Gas and water mains, sewer pipes and telephone, telegraph and electric wires pass under the lock in conduits cast in the living concrete.