62. When it is necessary to support very heavy buildings on compressible or filled soil, where piles or spread footings cannot be used, or are not considered desirable, wells of masonry, sunk to bed rock or hard pan, will generally prove the next cheapest method of securing an efficient foundation. The wells are arranged as isolated piers, and the walls of the superstructure carried on steel girders resting on these piers.
The manner in which such wells or piers should be used can probably be best explained by describing those under the City Hall of Kansas City, Mo., which was one of the first instances in which such wells were used in this country.*
* The following description is an abstract of a short paper presented by the architect of the building, Mr. S E. Chamberlain, of Kansas City, to the twenty-fourth annual convention of the American Institute of Architects. The illustrations were prepared in the office of the Engineering Record from the architect's drawings. Several more illustrations are given in the Engineering Record of April a and 16, 1892.
"The site of the City Hall was formerly a ravine between abrupt bluffs. These had been so cut away and leveled as to leave a 50-foot filling of rubbish under two-thirds of the building and a solid clay bank under the other third. The fill was made by a public dump. Pile foundations were objectionable on account of the dryness of the fill and the anticipated tendency of the piles to rot therein. Ordinary trenching was considered too expensive and dangerous, therefore a system of piers was chosen, and a cylindrical form was adopted, so that the excavation could be done by a large steam power auger, followed by a 3/16-inch caisson filled with vitrified brick. The caissons were made in 5-foot lengths of the same thickness throughout, the joints being made with 3"x ½" splice plates, riveted to the inside of the shell.
"The piers were of vitrified brick, 4 feet 6 inches in diameter, laid in hydraulic cement mortar, grouted solid in each course, and well bonded in all directions. The piers were sunk to bed rock of oolitic limestone, 8 feet thick, and capped with cast iron plates (Fig. 22) and steel I-beams, which supported the walls. To the top of the beams was riveted a ¼-inch plate of boiler iron, on which the brickwork of the walls was built, as shown in Fig. 23.
"Between the beams, and 1 foot on each side and underneath them, is a concrete filling, so that the beams are entirely encased in masonry.
"Piers having excessive loads are reinforced by 12-inch Z-bar columns resting on rock bottom (Fig. 24). These columns pass through the cast iron caps, so that the loads resting on the columns are separate from those on the brick piers (an essential provision). Essentially the whole system is intended to secure the direct transmission of the entire weight to the solid rock by so arranging the interior construction that each subdivision is carried by an adequate isolated pier. The piers are of uniform size, and their loads are equalized by spacing them at proportionate distances apart."
63. Another instance of the use of masonry wells or deep piers is in the foundation of the new Stock Exchange in Chicago.
"The foundation is generally upon piles about 50 feet long, driven into the hard clay which overlies the rock. Next to the Herald Building, however, which adjoins it, wells were substituted, lest the shock of the pile driver close to its walls should cause settlements and cracks. A short cylinder, 5 feet in diameter, made of steel plate, was first sunk by hand, reaching below the footings of the Herald Building. Then around and inside the base of the cylinder sheet piles, about 3½ feet long, were driven, and held in place by a ring of steel inside their upper ends. The material inside the sheeting was excavated and a similar steel ring was placed inside their lower ends. By means of wedges the lower ends of the sheeting were forced back into the soft clay until another course could be driven outside the lower ring. This operation was repeated until the excavation had reached the hard clay about 40 feet below the cellar. In this material the excavation was continued without sheeting in the form of a hollow truncated cone to a diameter of 7½ feet, and the entire excavation was filled with concrete. The wells are spaced about 12 feet. The loads upon them vary; some of them will carry about 200 tons.
"The material excavated was a soft, putty-like clay to a depth of 40 feet, where a firm clay was reached deemed capable of carrying the weight proposed." *
*"Foundations of High Buildings." By W. R Hutton, C. E., etc. Read before the Congress of Architects at Chicago, 189;.