41. Actual Loads on Piles

The following examples of the actual loads which are carried by each pile under the buildings named will serve as a guide to architects erecting buildings in those localities :

Boston. - Under Trinity Church, 2 tons each.

Chicago. - New Public Library building, 30 tons.

Schiller Building, estimated load 55 tons per pile; building settled from 1 to 2 inches.

Passenger Station, Northern Pacific Railroad, Harrison Street: piles 50 feet long carry 25 tons each without perceptible settlement.

The enormous grain elevators in Chicago rest upon pile foundations. Mr. Adler states that the unequal and constantly shifting loads are a severer test upon the foundations than a static load of a twenty-story building.

New Orleans. - Piles driven from 25 to 40 feet in a soft, alluvial soil carry safely from 15 to 25 tons, with a factor of safety of 6 to 8. - Patton.

42. Spacing

Piles should not be spaced less than 2 feet on centres, nor more than 3 feet, unless iron or wooden grillage is used.

When long piles are driven nearer than 2 feet from centres there is danger that they may force each other up from their solid bed on the bearing stratum. Driving the piles close together also breaks up the ground and diminishes the bearing power.

When three rows of piles are used the most satisfactory spacing is 2 feet 6 inches from centres across the trench and 3 feet from centres longitudinally, provided this number of piles will carry the weight of the building. If they will not, then the piles must be spaced closer together longitudinally, or another row of piles driven, but in no case drive two piles less than 2 feet apart from centres.

* Trans. Am. Soc. of C. E., Vol. VII., p. 264.

In all cases, wherever buildings are supported, the number of piles under the different portions of the building should be carefully proportioned to the weight which they have to carry, so that every pile will support very nearly the same load. This precaution is of especial importance when part of the piles must be loaded to their full capacity.

43. Cutting Off and Capping

The tops of the piles should invariably be cut off below low water mark, otherwise they would soon commence to decay.

The cutting off of the piles in building foundations is generally done by means of a large cross-cut saw worked by two men. The tops of the piles should be left true and level and on a line with each other. A variation of an inch in the top of the piles may be allowed, but it should not exceed this limit.

Three methods of capping piles are commonly employed: 1. By granite blocks. 2. By concrete. 3. By timber grillage.