FOUNDATIONS.
The choice of the method to be employed in laying a foundation depends chiefly on the nature of the ground, and the distinction may be made between good ground (such as rock, gravel, sand, and firm clay), medium ground (such as loam, dry peat, light clayey and meadow soils), and bad f.,Trolind (such as wet clay, marshy and boggy soils, quicksand, etc.). The less compressible the soil and the less it is affected by the action of water, the better it is suited for foundations. To avoid injury from frost, the base of the foundation should bc carried below the frost-line.
Foundalion.—Where the foundation is not reached by water and may be laid dry, the work resolves itself simply into the excavation of a suitable pit or trench or a series of pits and trenches, according to the nature of the work, to the depth of 3 or 5 feet (for ordinary work) where it has been ascertained that the surface soil is reliable. The bottom of the excavation must be properly levelled, and in disposing the blocks of masonry constituting the foundation the largest stones should be placed at the bottom. In the case of foundations made in soil the base should be broadened, to insure the proper distribution of the pressure. In such cases, also, care must be taken that the stones of the bottom course are solidly bedded in the g-round, so that they shall not exhibit any tendency to dis placement when loaded with the weight of the masonry laid upon them.
Founder/ions Unslable Ground.—If a firm bottom can be found only at a considerable depth, it will be necessary, in the case of large and lieaYy constructions, to excavate the overlying soil until solid ground is reached; where the weight of the intended structure is not very great, it will often suffice to found it upon separate pillars or piles of stone or iron united above by arches or girders and reaching down to the solid bottom. Should it prove impossible to reach solid ground at a reasonable depth, the only resource is, on the one hand, to endeavor to make the available soil more dense, and, on the other hand, to broaden the base of the masonry founda tion to such an extent that the pressure upon the unit of ground-surface will be reduced to the point where, without lateral displacement, it will transmit the weight of the intended structure downward. With this object in view, the excavation should be filled throughout its entire length to the depth of 3 or 6 feet—more or less, according to the nature of the soil and the weight of the intended structure—with broken stone, which must be well rammed down; or the stone packing may be made deep enough and broad enough to answer the purpose without ramming- or rolling. Layers of sharp, dry sand or gravel introduced into the excavation and thoroughly rolled or rammed are well adapted for distributing an equable pressure over a large surface. If it is intended simply to build foundation-walls at the margins of the excavation, the load they will be called upon to bear may be distributed over the entire floor of the excavation by joining the walls with inverted arches. Should the access of water complicate the work, it
will be possible in many cases to prevent this, while the work is in prog ress, by the application of one or the other of the various devices for drainage.
Subaqueous Foundalions.—Work is rendered decidedly more difficult in cases where water has access to the location of the intended excavation. To get rid of it and keep the base of the foundation dry while the work of construction is going on, mechanical appliances of various kinds, such as bucket-wheels, Archimedean screws, ordinary pumps, centrifugal pumps, etc., are resorted to. It is not always practicable, however, to remove the water by these means, and in such cases it will be necessary to devise methods for sinking the foundation beneath the water.
Excluding TUater: the ground in which the foundation is to be stink lies below the surface of the water, the area to be excavated must be isolated and made impenetrable to water by interposing some form of dant, the walls of which must project above the water-surface. Where the water is still and shallow, an embankment of earth—which is the sim plest form of dani—may suffice; or where there is some current, bags filled with earth, to avoid the washing away of the material, may be employed. Stich simple earth embankments are in use in all countries along rivers for protecting adjacent lowlands from overflow. In the United States, earth embankments (called " levees") are built Tip for hundreds of miles along the course of the Mississippi River. • backed with an earth filling- is more effective. To con struct a dam of this description, g-auge- or guide-piles are driven about 6 to io feet apart and joined by stringers. These serve to keep the piles in line while being- driven. They consist of squared timbers—sometimes thinner than they are wide—cut obliquely or to an edge at the foot to allow them to be driven close together. Sometimes, also, the foot is furnished with a sharp-edged or pointed iron shoe. Figure 17a,b(fil. 41) shows in cross-section the form of these sheet-piles and the way in which they are fitted together. One or several rows of sheet-piling may be driven, accord ing- to the nature of the ground, their tops rising sufficiently above the water-surface to prevent danger to the excavation from overflow, and being protected on the outside by an embankment of well-packed earth. The piling surrounding the excavation may at times be required to protect the finished foundation from the undermining, action of the water, in which case it will be driven with this object in view and made to serve as a per manent portion of the structure. Where the water gains admission chiefly throng-1i the floor of the excavation the difficulty may be overcome by the use of a layer of concrete, a mixture of broken stone and hydraulic cement which attains great hardness under water and becomes impermeable.