When it is required to build on a compressible soil that is constantly saturated with water and of considerable depth, the cheapest and generally the best foundation bed is obtained by driving piles.

33. Classes of Piles

A great many kinds of piles are used in engineering works, but for the foundations of buildings it is very seldom, if ever, that any other than wooden piles are used.

The different conditions under which piles are used for supporting buildings may be classed as follows:

1. When the compressible soil is not more than 40 feet deep and overlays a bed of rock, gravel, sand, or clay, long piles should be driven to the rock, or one or two feet into the clay or sand, in which case they may be considered as columns.

2. If the soft soil is more than 40 feet deep piles varying from 15 to 40 feet in length should be driven, according to the character of the soil, the sustaining power of the piles depending upon the friction between the pile and the surrounding soil.

3. Short piles, 10 to 15 feet long, are sometimes driven, particularly in Southern cities, to consolidate the soil and give it greater resisting power. As piles are seldom used in this way, this method of forming a foundation bed will be dismissed with the following quotation:

34. -

" In some sections of the country, especially in the Southern cities, the soil is of a soft alluvial material, and in its natural state is not capable of bearing heavy loads. In such cases trenches are dug as in firm material, and a single or double row of short piles are driven close together, and under towers or other unusually heavy portions of the structure the area thus covered is filled with these piles. The effect of this is to compress and compact the soil between the piles, and to a certain extent around and on the outside, thereby increasing its bearing power; whatever resistance the piles may offer to further settlement may be added, though not relied upon. These piles are then cut off close to the bottom of the trench, and generally a plank flooring is laid resting on the soil and piles, or a layer of sand or concrete is spread over the bottom of the trench to the depth of 6 inches or 1 foot, and the structure, whether of brick or stone, commenced on this. There is little or no danger of such structures settling, and if they do the chances are that they will settle uniformly if the number of piles are properly proportioned to the weight directly above them; but if the piles are not so proportioned, the same number being driven under a low wall as under a high wall, unequal settlement is liable to take place, causing ugly or dangerous cracks in the structure."*

4. Sheet piles, consisting of two or three-inch plank driven close together, edge to edge, are often used to sustain a bank during excavation, but are seldom depended upon for permanent effect.

35. Material

Piles are made from the trunks of trees, and should be as straight as possible, and not less than 5 inches in diameter for light buildings or 8 inches for heavy buildings. The woods generally used for piles in the Northern States are the spruce, hemlock, white pine, Norway pine, Georgia pine, and occasionally oak. In the Southern States Georgia or pitch pine, cypress and oak are used. Oak is considered as the most durable wood for piles, and is also the toughest, but it is too expensive for general use in the Northern States, besides being difficult to obtain in long, straight pieces. Next to oak come Georgia pine, Oregon pine, cypress and spruce, in the order named.

Of the 1,700 piles supporting the new Illinois Central Railway Station in Chicago, 32 per cent, were black gum, 22 per cent, pine, 7 per cent, basswood, 21 per cent, oak, 15 per cent, hickory, with a few maple and elm. A less proportion of the hickory piles were broken or crushed than of any other wood.