Foundations for piers or abutments of bridges or for the main columns of high build ings, where the individual piece of work is of lesser magnitude than those already cited, are usually constructed by the use of cylinders of iron as illustrated by Fig. 10. The arrange ment consists of a cylinder of metal divided into two unequal parts by a horizontal partition — the upper and larger part, essentially a cof ferdam, being the caisson proper, and the lower part within which the excavating operations are carried on being the working chamber. It is provided with one or two shafts made of boiler plate, which are connected with the air cham ber on top adjoining the equilibrium chamber or airlock through which the workmen and materials must enter. A pipe from the air compressor furnishes the air chamber with compressed air which is subsequently intro duced by a system of locks to the working chamber below.
Dredging in Wells.— Beyond the limit of the effective application of pneumatic methods, about 100 feet below the water surface, all excavating must be accomplished by dredging, and special care has to be exercised in plan ning the method of operations to prevent any contingency arising at the bottom that would require to be dealt with by the use of human labor, as diving operations are impossible. It is also important to observe that the skin fric tion, quite unimportant in a cylinder of mod erate depth, becomes so great at lower levels that special means have to be employed to over come it. In the Benares bridge the principal piers are sunk to the depth of 140 feet below the level of the water and consist of oval brick wells each 28 feet by 65 feet in diameter, the bottom lengths being cased in iron, as it was necessary to begin operations in the water.
Each well is divided into three vertical com partments in which the dredging was carried on. In constructing the Poughkeepsie bridge, the caisson is of timber 60 X 160 with a depth of 125 feet. It is divided into 40 vertical cells and was sunk to a depth of 130 feet by filling in some of the cells and excavating in others. In the case of the Hawkesbury bridge, the caisson is of steel and iron, of oval form 20 X 48 feet, and splayed out at the bottom an additional two feet all around. It is divided into three dredging wells set on the centre line and paral lel to its length. They splay out at the bottom so as to meet each other and the outer skin, thus forming a cutting edge. It was sunk to a depth of 161 feet below the water level by dredging in the wells, and then the space be tween the wells and the outer skin was filled with concrete. This is probably the greatest depth ever reached in the construction of a bridge foundation.
Pneumatic Forcing This is an improved method of constructing subaqueous foundations in sand or gravel substrata by con verting it into a solid in the form of sand or gravel concrete. This is effected in place, with
out excavating operations, by forcing cement in the form of the dry powder in which it is furnished commercially, through a pipe by air pressure into the substrata. The charging pipe, called the Iance pipe, may have an internal di ameter of one and one-half inches, and be drawn to a point at the lower end, and perforated with three or more holes three eighths of an inch in diameter. The upper end of the pipe is connected by means of a bend and rubber tubing with the air pressure supply pipe, suitable arrangements being pro vided to raise, lower or move it while in oper ation. The air pressure supply pipe is pro vided with suitable branches fitted with stop cocks to permit of its being connected to an injector device by which any desired quantity of cement powder may be fed into the current of air. In operation, the air pressure forces the cement powder through the small openings at the lower end of the lance pipe and drives it into the substrata of wet sand or gravel, with which it combines and forms sand or gravel concrete as the case may be. The lance tube may be sunk to depths of 16 or 19 feet in a comparatively short time and in order to insure a uniform mixture in the foundation pit, the foundation area is divided into small fields 8 to 12 inches square, into each of which the proper quantity of cement is blown. The proper amount of the cement charge is determined by dividing the cubic contents of the field by the required proportion of the mixture. This method of converting a sand-bed into concrete was employed in the construction of the subway under Brooklyn, to prevent the undermining of some of the large and heavy buildings which were passed in tunneling.
Another method by which bodies of cement or concrete may be placed in quicksand sub strata employs pipes in the following manner: Two or more iron pipes are stink through the quicksands to the desired depth and water pumped into one of the pipes, thus converting the substrata into a condition of fluidity suf ficient for the purpose of pumping it out through the other pipe. The cement is then in troduced through the forcing pipe and filled into the agitated area at the bottom of the pipes.
Bibliography.— For further information consult Baker, 'A Treatise on Masonry Con struction' (2d ed., New York 1906) ; Buell and Hall, Concrete' (New York 1904); Fowler, The Cofferdam Process for. Pier (New York 1898) ; id., Treatise on Sub-Aqueous Foundations' (3d ed., ib. 1914) ; Jacoby and Davis, (Foundations of Bridges and Buildings' (ib. 1914) ; Kidder, and Builders' Pocket Book' (16th ed., ib. 1914) ; Patton, 'A Practical Treatise on Foundations' (New York 1893) ; Taylor and Thompson, (Concrete, Plain and Reinforced' (New York 1905) ; and special articles on the subject in the various engineering magazines and periodicals.