FOUNDATIONS 169. It would be impossible to over-emphasize the importance of foundations, because the very fact that the foundations are under ground and out of sight detracts from the consideration that many will give to the subject. It is probably true that a yielding of the subsoil is responsible for a very large proportion of the structural failures which have occurred. It is also true that many failures of masonry, especially those of arches, are considered as failures of the superstructure, because of breaks occurring in the masonry of the superstructure, which have really been due, however, to a settlement of the foundations, resulting in unexpected stresses in the super structure. It is also true that the design of foundations is one which calls for the exercise of experience and broad judgment, to be able to interpret correctly such indications as are obtainable as to the real character of the subsoil and its probable resistance to concentrated pressure.
170. Classification of Subsoil. 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 sub soils 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 (e) 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 disintegfated 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 de scribed later.
(e) 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.
171. Testing the Bearing Power. The first step is to excavate the surface soil to the depth at which it would be convenient to place the foundation and at which the soil appears, from mere inspection, to be sufficiently firm for the purpose. Au examination of the trenches or foundation pits with a post-auger or steel bar will generally be sufficient to determine the nature of the soil for any ordinary building. The depth to which such an examination can be made with a post-auger or steel bar will depend on the nature of the soil. In ordinary soils there will not be much difficulty in extending such an examination 3 to 6 feet below the bottom of the foundation pits. In common soils or clay, borings 40 feet deep (or even deeper) can readily be made with a common wood-auger, turned by men. From the samples brought up by the auger, the nature of the soil can be deter mined; but nothing of the compactness of the soil can be determined in this manner.