The New York Building Law (1892) provides that
"Piles intended for a wall, pier or post to rest upon shall not be less than 5 inches in diameter at the smallest end, and shall be spaced not more than 30 inches on centres, or nearer, if required by the Superintendent of. Buildings, and they shall be driven to a solid bearing.
"No pile shall be weighted with a load exceeding 40,000 pounds.
"The tops of all piles shall be cut off below the lowest water line. When required, concrete shall be rammed down in the interspaces between the heads of the piles to a depth and thickness of not less than 12 inches and for 1 foot in width outside of the piles."
The Boston Building Law requires that
"Where the nature of the ground requires it all buildings shall be supported on foundation piles not more than 3 feet apart on centres in the direction of the wall. . . . . Buildings over 70 feet in height shall rest, where the nature of the ground permits, upon at least three rows of piles, or an equivalent number of piles arranged in less than three rows. All piles shall be capped with block granite levelers, each leveler having a firm bearing on the pile or piles it covers."
In Chicago it is required that sustaining power of piles in various localities gives a good idea of the manner of making such tests, as well as the loads which it required to sink them :
General William Sooy Smith, in an address delivered March 31, 1892, before the students of engineering of the University of Illinois, stated that "A pile at the bottom of a pit 30 feet deep and well into hard pan, or to the rock where this is within reach, can be safely relied upon to sustain from 30 to 40 gross tons."
The following description of several tests made to determine the actual.
Chicago Public Library. - To determine the actual resistance of the piles on which it was proposed to erect the Public Library building in Chicago, the following test was made: In order to make the experiment under the same conditions as would exist under the structure three rows of piles were driven into the trench, the piles in the middle row being then cut off below the level at which those in the outside row were cut off, so as to bring the bearing only on four piles, two in each outside row. This gave the benefit arising from the consolidation of the material by the other piles. The piles were of Norway pine, 54 feet long, and were driven about 52½ feet - about 27 feet in soft, plastic clay, 23 feet in tough, compact clay, and 2 feet in hard pan. They had an average diameter of 13 inches and area at small end of 80 square inches.
On top of the four outside piles, which were spaced 5 feet apart on centres, 15-inch steel I-beams were placed, and upon these a platform, 7x7 feet, composed of 12 x 12-inch yellow pine timbers. On this platform pig iron was piled up at irregular intervals. When 4 feet high the load was 45,200 pounds, and was then continued, until at the end of about four days it was 21 feet high, giving a load of 224,500 pounds. Levels were taken, but no settlement had occurred. By the end of about eleven days the pile of iron had reached the height of 38 feet, giving a load of 404,800 pounds upon the four piles, or about 50.7 tons per pile. Levels were then taken at intervals during a period of about two weeks, and, no settlement having been observed, a load of 30 tons was considered perfectly safe.
Perth Amboy, 1873. - Pretty fair mud, 30 feet deep. Four piles, 12, 14, 15 and 18 inches diameter at top, 6 to 8 inches at foot, were driven in a square to depths of from 33 to 35 feet. A platform was built upon the heads of the piles and loaded with 179,200 pounds, say 44,800 pounds per pile. After a few days the loads were removed. The 18-inch pile had not moved, the 12-inch pile had settled 3 inches, and the 14 and 15-inch piles had settled to a less extent.*
Buffalo, N. Y. - In the construction of a foundation for an elevator at Buffalo, N. Y., a pile 15 inches in diameter at the large end, driven 18 feet, bore 25 tons for twenty-seven hours without any ascertainable effect. The weight was then gradually increased until the total load on the pile was 37½ tons. Up to this weight there had been no depression of the pile, but with 37½ tons there was a gradual depression which aggregated 5/8 of an inch, beyond which there was no depression until the weight was increased to 50 tons. With 50 tons there was a further depression of 7/8 of an inch, making the total depression 1½ inches. Then the load was increased to 75 tons, under which the total depression reached 3 1/6 inches. The experiment was not carried beyond this point. The soil, in order from the top, was as follows : 2 feet of blue clay, 3 feet of gravel, 5 feet of stiff red clay, 2 feet of quicksand, 3 feet of red clay, 2 feet of gravel and sand and 3 feet of very stiff blue clay. All the time during this experiment there were three pile-drivers at work on the foundation, thus keeping up a tremor in the ground. The water from Lake Erie had free access to the pile through the gravel.†
*"A Practical Treatise on Foundations." † "Masonry Construction." Baker.
" Subsequent use shows that 74,000 pounds is a safe load." - Patton.
Philadelphia. - At Philadelphia in 1873 a pile was driven 15 feet into soft river mud, and five hours after 7.3 tons caused a sinking of a very small fraction of an inch; under 9 tons it sank ¾ of an inch, and under 15 tons it sank 5 feet.
"The South Street bridge approach, Philadelphia, fell by the sinking of the foundation piles under a load of 24 tons each. They were driven to an absolute stoppage by a 1-ton hammer falling 32 feet. Their length was from 24 to 41 feet. The piles were driven through mud, then tough clay, and into hard gravel."*
The failure in this case may have been caused by vibrations which allowed' the water to work its way down the sides of the piles and thus decrease the friction; or, what is more probable, the last blow was struck on a broomed head, which would greatly reduce the penetration and cause the bearing power to be overestimated.
When the penetration is very slight or unobservable, and the head much broomed, the broomed portion should be cut off and the blows repeated if the full load of the formula is to be put on the piles.