Cost of Pneumatio Foundations

907.

next the cost of piers of masonry on wrought iron caissons of excavation, the foundations of the Lorient viaduct over the Scorff cost the large sum of $24.11 per cubic yard, owing to difficulties caused by the tides, the labor of removing the bowlders from underneath the caisson, and the large cost of plant for only two piers. The foundation of the Kehl Bridge cost still more, about $28.23 * per cubic yard; but this can not be regarded as a fair instance, being the first attempt [see § 861] of the kind.

"The foundations of the Nantes bridges, sunk 56 feet below low water level, cost about $14.84 per cubic yard. The average cost per pier was as follows: "One pier of the bridge over the Meuse at Rotterdam, with a caisson of 222 tons and a coffer-dam of 94 tons, and sunk 75 feet below high water, cost $70,858, or $13.97 per cubic yard.

"The Vichy Bridge has five piers built on caissons 34 feet by 13 feet, and two abutments on caissons 26 feet by 24 feet. The foun dations were sunk 23 feet in the ground, the upper portion consisting of shingle and conglomerated gravel, and the last 10 feet of marl. The cost of the bridge was as follows: The cost per cubic yard of the foundation below low water was $16.69, of which the sinking alone cost $3.50 in gravel, and $4.37 in marl.

"At St. Maurice, the cost per cubic yard of foundation was $15.94, exclusive of staging."

Except in very shallow or very deep water, the compressed-air process has almost entirely superseded all others. The following are some of the advantages of this method. 1. It is reliable, since there is no danger of the caisson's being stopped before reaching the desired depth, by sunken logs, bowlders, etc., or by excessive friction, as in dredging through tubes or shafts in cribs. 2. It can be used regardless of the kind of soil overlying the rock or ultimate foundation. 3. It is comparatively rapid, since the sinking of the caisson and the building up of the pier go on at the siame time. 4. It is comparatively economical, since the weight added in sinking is a part of the foundation and is permanent, and the removal of the material by blowing out or by pumping is as uniform and rapid at one depth as at another,—the cost only being increased somewhat by the greater depth. 5. This method allows ample opportunity to examine the ultimate foundation, to level the bottom, and to remove any disintegrated rock. 6. Since the rock can be laid bare and be thoroughly washed, the concrete can be com menced upon a perfectly clean surface; and hence there need be no question as to the stability of the foundation.