GUIDING THZ °LIaBON. Formerly it was the custom to control the descent of the caisson by suspension screws connected with a framework resting upon piles or pontoons. In a strong current or in deep water, it may necessary to support the caisson partially in order to govern its descent; but ordinarily, the sus pension is needed only until the caisson is well imbedded in the soil. The caisson may be protected from the current by constructing a breakwater above and producing dead water at the pier site.
After the soil has been reached, the caisson can be kept in its course by removing the soil from the cutting edge on one side or the other of the caisson. In case the caisson does not settle down after the soil has been removed from under the cutting edge, a reduction of a few pounds in the air pressure in the working chamber is usually sufficient to produce the desired result. At the Havre de Grace Bridge ($ 870), it was found that by allowing the discharged material to pile up against the outside of the caisson, the latter could be moved laterally almost at will. The top of the caisson was made 3 feet larger, all round, than the lower course of masonry, to allow for deviation in sinking. The deviation of the caisson, which was founded 90 feet below the water, was less than 18 inches, even though neither suspension screws nor guide piles were employed.
In sinking the foundations for the bridge over the Missouri River near Sibley, Mo., it was necessary to move the caisson considerably in a horizontal direction without sinking it much farther. This was accomplished by placing a number of posts—l2 inches square—in an inclined position between the roof of the working chamber and a temporary timber platform resting on the ground below. When these posts had been wedged up to a firm bearing, the air pressure was released. The water flowing into the caisson loosened the soil on the outside, and the weight of the caisson coming on the inclined posts caused them to rotate about their lower ends, which forced the caisson in the desired direction. In this way, a lateral movement of 3 or 4 feet was secured while sinking about the same distance.
A caisson is also sometimes moved laterally, while sinking, by attaching a cable which is anchored off to one side and kept taut.
A method of controlling the descent of the caisson has occasionally been used, which is specially valuable in swift cur rents or in rivers subject to sudden rises. It was used first in the construction of the piers for a bridge across the Yazoo River near Vicksburg, Miss. A group of 72 piles, each 40 feet long, was driven into the river bed, and sawed off under the water; the caisson was then floated into place, and lowered until the heads of the piles rested against the roof of the working chamber. As the work pro ceeded, the piles were sawed off to allow the caisson to sink. One of the reasons for employing piles in this case, was that if the caisson did not finally rest upon bed-rock, they would assist in supporting the pier.
That such ponderous masses can be so certainly guided in their descent to bed-rock, is not the least valuable nor least interesting fact connected with this method of sinking foundations.
At the Havre de Grace Bridge (¢ 870), the normal frictional resistance on the timber sides of the pneumatic caisson was 280 to 350 lb. per sq. ft. for depths of 40 to 80 feet, the soil being silt, sand, and mud; when bowlders were encoun tered, the resistance, was greater, and when the air escaped in large quantities the resistance was less. At the bridge over the Missouri River near Blair, Neb. (¢ 868-69), the frictional resistance usually ranged between 350 and 450 lb. per sq. ft., the soil being mostly fine sand with some coarse sand and gravel and a little clay. At the Brooklyn Bridge (¢ 890), the frictional resistance at times was 600 lb. per sq. ft. At Cairo, in sand and gravel, the normal friction was about 600 lb. per sq. ft. At Memphis (¢ 891), in sand, the friction was 400 lb. per sq. ft.
For data on the friction of iron cylinders and masonry shafts, see ¢ 853-54; and for data on the friction of ordinary piles, see ¢ 781-84.