64. Although caissons have been extensively used in constructing the foundations of bridge piers, they have as yet been used for the foundations of but few buildings in this country, the first instance being the building for the Manhattan Life Insurance Company, near the foot of Broadway, New York City - Messrs. Kimball & Thompson, Architects; Charles O. Brown, Consulting Engineer.
As it is claimed that the method there employed proved perfectly satisfactory, and cost only about 8 or 9 per cent, of the estimated cost of the building, it is deemed of sufficient importance to merit a short description of the manner in which the foundations were constructed and the superstructure supported therefrom.*
"The building occupies an area of about 8,200 square feet, and is seventeen stories high on Broadway and eighteen on New Street. The height from the Broadway curb to the parapet of the main roof is 242 feet, and the dome and tower rises 108 feet above the parapet. All the walls, together with the iron floors and roof (which are very heavy), are directly supported by thirty-four cast iron columns, which sustain an estimated weight of about 30,000 tons.
"The great height and massive metal and masonry construction impose enormous loads on the foundations, amounting to as much as 200 tons for some single columns, and giving about 7,300 pounds per square foot over the whole area of the lot. This enormous weight could not be safely carried on the natural soil, which is essentially of mud and quicksand to the bed rock, which has a fairly level surface about 54 feet below the Broadway street level. Above this rock the water percolates very freely, standing at a level of about 22 feet below the Broadway street line, and therefore making excavations below this plane difficult and costly. If piles had been driven as close together as the city regulations permit - i. e., 30 inches centre to centre over the whole area, about 1,323 might have been placed, and would have carried an average load of 45,300 pounds each, which was inadmissible, the statute laws of New York allowing only 40,000 pounds each on piles 2 feet 6 inches apart and with a smallest diameter of 5 inches.
"Special foundations were therefore necessary, and it was imperative that their construction and duty should not jeopardize nor disturb the existing adjacent heavy buildings which stand close to the lot lines. On the south side the six-story Consolidated Exchange Building is founded on piles which are supposed to extend to the rock. On the north the foundations of a four-story brick building rest on the earth about 28 feet above the rock, and were especially liable to injury from disturbances of the adjoining soil, which was so wet and soft as to be likely to flow if the pressure was much increased by heavy loading or diminished by the excavation of pits or trenches.
"In view of these conditions it was determined to carry the foundations on solid masonry piers down to bed rock. The construction of the piers by the pneumatic caisson process was, after careful consideration by the architects, backed by opinions from prominent bridge engineers as to its feasibility, adopted.
"The smaller caissons were received complete and the larger ones in convenient sections, bolted together when necessary, and located in their exact horizontal positions, calked and roofed with heavy beams to form a platform, on which the brick masonry was started and built up for a few feet before the workmen entered the excavating chamber and began digging out the soil. The removal of the soil allowed the caissons to gradually sink to the rock below without disturbing the adjacent earth, which was kept from flowing in by maintaining an interior pneumatic pressure slightly in excess of the outside hydrostatic pressure due to the distance of the bottom of the caisson below the water line.
* The following is an abstract from a very full description, with ten illustrations, published in the Engineering Record of January 20, 1894.
Fig. 25. - The Manhattan Life Insurance Building, New York City. - Plan of Piers.
"The adjacent buildings were shored up at the outset and scrupulously watched, observations being made to determine any possible displacement or injury of their walls, which were not seriously damaged, though the pressure they exerted on the yielding soil tended to deflect the caissons which were sunk within a foot of them. They were kept in position by excess of loading and excavating on the edges that tended to be highest. The caissons encountered boulders and other obstructions, and were sunk through the fine soil and mud at an average rate of 4 feet per day. No blasting was required until the bed rock was reached and leveled off under the edges and stepped into horizontal surfaces throughout the extent of the excavating chamber. Usually one caisson was being sunk while another was being prepared, there being only one time when air pressure was simultaneously maintained in two caissons. Generally about eight days were required to sink each caisson."
Fig. a6. - The Manhattan Life Insurance Building, New York City. - Transverse Section.*
* Published by consent of the Engineering Record.
The first caisson was delivered at the site April 13, 1893, and the last pier was completed August 13, 1893.
"After the caissons were sunk to bed rock, and the surface cleared and dressed, the excavating chambers and shafts were rammed full of concrete, made of 1 part Alsen Portland cement, 2 parts sand and 4 parts of stone, broken to pass through a 2½-inch ring. The superimposed piers were built of hard-burned Hudson River brick, laid in mortar composed of 1 part Little Giant cement to 2 parts sand."
Fig. 25 is a plan showing the piers (all of which, except P, which is built on twenty-five piles, are founded on caissons of the same size) and the bolsters on top of them, together with the girders and the columns, which are indicated by solid block cross sections.
"Cylindrical caissons are the most convenient and economical, and would have been used throughout if the conditions had permitted, but the positions of the columns and the necessity of distributing the load along the building lines and other considerations determined the use of rectangular ones, except in four cases." All the caissons were 11 feet high, made of ½-inch and 3/8-inch plates and 6x6-inch angle framework, stiffened with 7-inch bulb angles, vertical brackets and reinforced cutting edges.
The columns supporting the outer side walls of the building were located so near the building line as to be near or beyond the outer edge of the foundation piers, as shown in Fig. 25, so that if they had been directly supported therefrom they would have loaded it eccentrically and produced undesirable irregularities of pressure. This condition was avoided and the weights transmitted to the centres of the piers by the intervention of heavy plate girders, which supported the columns in the required positions and transferred their weights to the proper bearings above the piers. From these bearings the load was distributed over the whole area of the masonry by special steel bolsters.
Fig. 26 is a transverse section at D-H-M, Fig. 25, showing the quadruple girder C, 17-18-19, and the manner in which it supports columns 23 and 33. The cantilever is made continuous across the building, with intermediate supports under columns 21 and 22.
Pneumatic caisson foundations were also used in the foundation construction of the American Surety Building, New York, a full description of which is given in the Engineering Record of July 14, 1894. Caisson foundations, whether in the shape of wells or of the pneumatic form, should only be used under the advice or direction of a competent engineer.
In preparing the foundations of high buildings the same principles apply as for other buildings, except that the loads on the foundations being so much greater the footings must be proportioned with the utmost care.
When building on firm soils it is only necessary to carefully observe all the precautions given in Chapter I (Foundations On Firm Soils. Staking Out The Building)., and on compressible soils one of the methods described in this chapter should be employed, always, however, under the advice of an experienced engineer.