The foundations of the Hawkesbury bridge in Australia is a notable example of this method. The caissons were of wrought iron, 4S feet long and 20 feet wide, with semicircular ends. Three circular dredging wells were used, S feet in diameter and 14 feet between centers. The pockets around and between the wells were filled with concrete to aid in sinking the caisson, and the sides and filling were continually built up as the sinking progressed. The caissons were bedded upon sand, the maximum depth reached being 162 feet below high water, through 108 feet of mud and silt.
The bottoms of the caissons were made flaring for the lower 20 feet, making the bottom 2 feet wider all around, this arrangement being intended to reduce friction on the sides, but it was found to increase seriously the difficulty of guiding the caisson. When the soil is not uniform over the base, there is a tendency to travel toward the firmer material, which was obviated by making the surfaces of the wider bottom sections vertical instead of flaring, with an offset about 20 feet above the cutting edge.
Concrete open caissons are rapidly coming into use and possess many advantages where they may be built in place and started in the open air. When the wells arc filled with concrete, it makes a mono lithic structure, with no parts subject to decay or corrosion, and when heavy walls are used, the weight is of advantage in sinking. Reinforcement is usually employed, although in a few instances heavy shells have been sunk without reinforcement. Light reinforcement seems desirable in nearly all cases as having additional security against cracking.
The method of dredging through wells was used in sinking con crete caissons for the foundations of the Pittsburgh & Lake Erie Railroad bridge at Beaver, Pa. The caissons were 80 feet long and 28 feet wide with semicircular ends. The shell was 7 feet thick with two cross-walls each 5 feet thick, and was tapered in the lower 9 feet to the cutting edge of steel. Rectangular cofferdams were constructed
around the site in about 7 feet of water and pumped out. The cais sons were then built inside the cofferdams and sunk through about 38 feet of sand and gravel to the rock. When they had been sunk nearly to the rock by dredging through the open walls, they were transformed into pneumatic caissons and bedded upon the rock by the pneumatic process.
Fig. 122 shows a reinforced-concrete caisson used in the foundation of a pier of the American River bridge of the Southern Pacific Rail road.' It was 76 feet long, 28 feet wide, and 22 feet high, with a shell 3 feet thick and three cross-walls each 4 feet thick, and was built when the stream was dry in a pit dug to the level of ground water, being sunk by dredging through the four wells. Mien the top of the caisson reached the ground level, a timber cofferdam was constructed on top. The sinking was then continued until the stratum of cobbles and boulders upon which it was to rest was reached. The wells were then filled with concrete and the pier built up in the cofferdam. This caisson was very light in weight and the sinking so slow that it was found more economical to construct the other piers by excavating inside of sheet iron cofferdams.
In constructing the channel pier of the North Side Point bridge over Allegheny River at Pittsburgh, a concrete caisson S3 X23 feet was used, with 4 wells 10XO feet spaced 19 feet between centers.' When the caisson reached a height of 31 feet, with the cutting edge 17 feet below the bed of the river, a transverse crack extending from top of caisson to below the river bed occurred near the mid length, probably due to tension in the top of the caisson caused by unequal dredging. This caisson was unrcinforced. It was blasted out and replaced by one reinforced by longitudinal bars.