Subsoil. As the subsoil has a great deal to do with the foundation, it is important to know the nature of the ground upon which the building rests, though it is usually the architect who decides the depth and form of the foundation, and the width of the footing course. If the subsoil is not known, and a large building is to be erected, the architect generally will sink shafts some distance below the bottom of the proposed foundation, in order to ascertain the nature of the soil which is to support the building.

Rock makes the most solid foundation bed, but is apt to cause trouble in damp-proofing. The architect and builders should be careful not to mistake a large bowlder for bed rock.

Clay, if hard, will usually be found sufficiently firm for any ordinary building, as it will support from 1 ½ to 6 tons per square foot according to its nature. It should, however, be well drained for, if clay lies upon a slanting substratum, it is apt to slip when wet. (See Fig. 5.)

Gravel is the ideal soil upon which to build, providing it is compact, as it is not affected by water and very little by frost. It has a resistance of from 4 to 8 tons per square foot and, if the footing courses are properly estimated and the foundation well put in, will stand any load with safety.

Sand is useless as a foundation bed, unless it is confined laterally, in which case it is about as satisfactory as gravel in its resistance to frost and water, and will safely support from 2 to 4 tons per square foot.

If a building is to be erected upon mud or silt, piles should be driven. These are round, straight logs, usually of oak, hard pine, or cypress, though elm and spruce are often used. They should be driven into the ground to the desired depth by means of a pile-driving machine and, if not exposed to alternations of wet and dry, they are practically everlasting. Sometimes foundations are laid upon mud by extending the footing course to give a very broad bearing. One of the methods of doing this is to lay plank in several layers, crossing each alternate layer, and building sufficiently thick to insure rigidity. This method has been used successfully in building upon marsh lands. Another method is to build a form which is then filled with concrete.