The foundation is the most critical part of a masonry structure. The failures of masonry work due to faulty workmanship or to an insufficient thickness of the walls are rare in comparison with those due to defective foundations. When it is necessary, as so frequently it is at the present day, to erect gigantic edifices—as high buildings or long-span bridges—on weak and treacherous soils, the highest constructive skill is required to supplement the weakness of the natural foundation by such artificial preparations as will enable it 0 sustain the load with safety.
Natural Foundations. The soils comprised under this head may be divided into two classes. (1) Those whose stability is not affected by water, and which are firm enough to support the structure, such as rock, compact gravels, and hard clay, and (2) soils which are firm enough to support the weight of the structure, but whose stability is affected by water, such as loose gravels, sand, clay and loam.
Foundations on Rock. To prepare a rock foundation, all that is generally necessary is to cut away the loose and decayed por tions and to dress the surface so exposed to a plane as nearly perpen dicular to the direction of the pressure as practicable; or, if fiat rock forms an inclined plane, to cut a series of plane surfaces, like those 3f steps, for the walls to rest upon. If there are any fissures in the rock they should be filled with concrete.
Foundations on Gravel, Etc. In dealing with soils of this kind usually nothing more is required than to cover them with a layer of concrete of width and depth sufficient to distribute the weight properly.
Foundations on Sand. Sand is almost incompressible so long as it is not allowed to spread out laterally, but as it has no cohesion, and acts like a fluid when exposed to running water, it must be treated with great caution.
Foundations on Clay. Clay is much affected by the action of water, and hence the ground should be well drained before the work is begun, and the trenches so arranged that water does not remain in them. In general, the less a soil of this kind is exposed to the action of the air, and the sooner it is protected from exposure, the better for the work. The top of the footings must be carried below the frost line to prevent heaving, and for the same reason the outside face of the wall should be built with a slight batter and per fectly smooth. The frost line attains a depth of six feet in some of
the northern states.
The bearing power of clay and loamy soils may be greatly in creased: (1) By increasing the depth. (2) By drainage. This may be accomplished by a cover ing of gravel or sand, the thick ness depending upon the plas ticity of the soil, and by surround ing the foundation walls with a tile drain as in Fig. 5. If springs are encountered the water may be excluded by sheet pilings, puddling or plugging the spring with concrete. (3) By consoli dating the soil. This may be done by driving short piles close together, or by driving piles, then withdrawing them and filling the space immediately with damp sand well rammed. If the soil is very loose and wet, sand will not be effective, and concrete will be found more satisfactory.
Artificial Foundations. When the ground in its natural state is too soft to bear the weight of the proposed structure, recourse must be had to artificial means of support, and, in doing this, what ever mode of construction is adopted, the principle must always be that of extending the bearing surface as much as possible.
Foundations on Mud, silt, marshy or compressible soils are generally formed in one of three ways: (1) By driving piles in which the footings are supported. (2) By spreading the footings either by layers of timber, steel beams, or concrete, or a combination of either. (3) By sinking caissons of iron or steel, excavating the soil from the interior, and filling with concrete.
Foundations in Water are formed in several ways: (1) Wholly of piles. (2) Solid foundations laid upon the surface of the ground by means of cribs, caissons, or piles, and grillage. (3) Solid foundations laid below the surface, the ground being made dry by cofferdams or caissons. (4) Where the site is perfectly firm, and there is no danger of the work being undermined by scour, foundations are started on the surface, the inequalities being first removed by blasting or dredging. The simplest foundation of this class is called "Random" work or Pierre perdue. It is formed by throwing large masses of stone upon the site until the mass reaches the surface of the water, above which the work can be carried on in the usual manner. Large rectangular blocks of stone or concrete are also used, the bottom being first simply leveled and the blocks carefully lowered into place.