Deep Foundations

DEEP FOUNDATIONS. In the preceding sections have been described the methods of supporting a structure upon soft or com pressible soil by increasing the area of the footing; but under the head of "deep foundations" will be described the methods of found ing upon a hard stratum or bed-rock underlying the soft soil.

Piles.

One of the most common methods of founding upon a soft soil is to drive piles; but this method has already been briefly referred to in 1 675, and will be discussed at length in the next chapter, and hence will not be considered here.

Concrete Piers.

Instead of trying to extend the footings sufficiently to support a heavy load upon a soft soil, wells are some times dug through the soft soil to a hard substratum, the structure being founded directly upon the latter. If the soil contains much water, then some of the methods described in Chapter XVI—Founda tions under Water—must be employed; but if the soil is fairly com pact clay, a method devised at Chicago may be used. This consists in sinking a shaft to hard pan or bed-rock, and filling the well with concrete. The shafts are sunk as open wells 3 to 8 feet in diameter, and are usually lined with 2- by 6-inch tongue-and-groove planks from 4 to 6 feet .long, which are supported by two and sometimes three interior iron sectional hoops. A section about 6 feet deep is excavated and then lined. The intention is to make the excavation only large enough to get the lagging into place, to prevent settlement of adjoining buildings; and if the excavation is accidentally made too large, clay is packed behind the lagging as the latter is put into position. If beds of quicksand or other soft material are encountered, steel sheet piles (1 748-49) or steel cylinders are used instead of wood lining. The bearing power of the concrete column may be increased, by belling out the lower end of the well. After the excavation is completed, the hole is filled with concrete. The rings are taken out as the concreting progresses, except in soft swelling clay; but the lagging is usually left in place.

This method is now employed in Chicago almost exclusively for all large buildings. The usual practice there is to mix the concrete rather dry and put it into wells 60 to 100 feet deep by shoveling it in at the top and allowing it to drop freely, an attempt being made to drop it from the shovel in such a manner that the shovelful will go down without being broken up. Such columns safely carry 20 to 25 tons per square foot of top area—usually the former.

At Chicago this method is usually called concrete caissons, but the term concrete piers is better, and is used to some extent.

A marked advantage of this method as employed in Chicago is that the wells are sunk without vacating any part of the old build ing except the basement. The wells are filled with concrete to within 40 to 50 feet of the sidewalk level, and then the steel columns for the new two- or three-story basement are put into place on top of the concrete columns before the basement is excavated; and finally when the old building is demolished, the work of construction may proceed upward and downward at the same time.

Hydraulic Caisson.

A few deep foundations of buildings in New York City have been sunk to bed-rock by the hydraulic caisson method. This consists of sinking steel cylinders without interior excavation by means of hydraulic jets. A riveted steel cylinder is attached at its lower end to an annular cast-iron cutting edge of hollow triangular cross section having numerous small per forations along its lower edge; and the hollow cast-iron cutting section is connected with a force pump by pipes and flexible hose. The cylinder is heavily loaded with cast iron, the pump is started, and the numerous jets of water issuing from the cutting edge scour away the soil and form an annular trench into which the cylinder de scends. As the sinking progresses, another section of the cylinder is added at the top. When a hard substratum is reached, the pump is stopped, the soil in the interior cylinder dug or dredged or "washed" out. If the cutting edge of the cylinder stops in clay, probably little or no water will leak into the cylinder; but if it stops upon bed-rock which is irregular or not level, it may be necessary to seal the cylin der by depositing concrete under water.

The objection to this method is that in some cases it does not permit an inspection and proper preparation of the bed of the founda tion. The pneumatic method—see Art. 4 of Chapter XVI—permits inspection and proper preparation of the underlying hard stratum, and in recent years has frequently been used, particularly in New York City, in sinking foundations for buildings. The only advantage of the hydraulic caisson over the pneumatic method is that the former is sometimes the cheaper.

ON Boas. To prepare a rock bed to receive a foundation it is generally only necessary to cut away the loose and decayed portions of the rock, and to dress it to a plane surface as nearly perpendicular to the direction of the pressure as is practicable. If there are any fissures, they should be filled with concrete. A rock that is very much broken can be made amply secure for a foundation by the liberal use of good concrete. The piers of the Niagara Canti lever Bridge are founded upon the top of a bank of bowlders, which were first cemented together with concrete.

Sometimes it is necessary that certain parts of a structure start from a lower level than the others. In this case care should be taken (1) to keep the mortar joints as thin as possible, (2) to lay the lower portions in cement, and (3) to proceed slowly with the work; other wise the greater quantity of mortar in the wall on the lower portions of the slope will cause greater settling there and a consequent break ing of the joints at the stepping places. The bonding over the off sets should receive particular attention.