The character of soil on which it may be desired to place a structure, varies all the way from the most solid rock to that of semi-fluid soils whose density is but little greater than that of water. The gradation between these extremes is so uniform that it is practically impossible to draw a definite line between any two grades.' It is convenient, however, to group subsoils into three classes, the classification being based on the method used in making the foundation. These three classes of subsoils are: (a) Firm; (b) Compressible; and (c) Semi-fluid.

(a) Firm Subsoils. These comprise all soils which are so firm, at least at some reasonably convenient depth, that no treatment of the subsoil or any other special method needs to be adopted to obtain a sufficiently firm foundation. This, of course, practically means that the soil is so firm that it can safely withstand the desired unit-pressure. It also means that a soil which might be classed as firm soil for a light building should be classed as compressible soil for a much heavier building. It frequently happens that the top layers must be removed from rock because the surface rock has become disintegrated by exposure to the atmosphere. Nothing further needs to be done to a subsoil of this kind.

(b) Compressible Subsoils. These include soils which might be considered as firm soils for light buildings such as dwelling-houses, but which could not withstand the concentrated pressure that would be produced, for example, by the piers or abutments of a bridge. Such soils may be made sufficiently firm by methods described later.

(c) Semi-Fluid Subsoils. These are soils such as are frequently found on the banks or in the beds of rivers, which are so soft that they cannot sustain without settlement even the load of a house, to say nothing of a heavier structure. Nor can they be materially improved by any reasonable method of compression. The only possible method of placing a heavy structure in such a locality, consists in sinking some sort of a foundation through such soft soil until it reaches and is supported by a firm soil or by rock, which may be 50 or even 100 feet below the surface. The general methods of accomplishing these results will be detailed in the following sections.