PLAN 07 PROPOSED DISCUSSION. In a general way, soils may be divided into three classes: (1) ordinary soils, or those which are capable, either in their normal condition or after that condition has been modified by artificial means, of sustaining the load that is to be brought upon them; (2) compressible soils, or those that are incapable of directly supporting the given pressure with any reason able area of foundation; and (3) semi-liquid soils, or those in which . the fluidity is so great that they are incapable of supporting any considerable load. Each of the above classes gives rise to a special method of constructing a foundation.
1. With a soil of the first the bearing power may be in creased by compacting the surface or by drainage; or the area of the foundation may be increased by the use of masonry footing courses, inverted masonry arches, or one or more layers of timbers, railroad rails, iron beams, etc. Some one of these methods is or dinarily employed in constructing foundations on land; as, for ex ample, for buildings, bridge abutments, sewers, etc. Usually all of these methods are inapplicable to bridge piers, i.e., for founda
tions under water, owing to the scouring action of the current and also to the obstruction of the channel by the greatly extended base of the foundation.
2. With compressible soils, the area of contact may be increased by supporting the structure upon piles of wood or iron, which are sustained by the friction of the soil on their sides and by the direct pressure on the soil beneath their bases. This method is frequently employed for both buildings and bridges.
3. A semi-fluid soil must generally be removed entirely and the structure founded upon a lower and more stable stratum. This method is specially applicable to foundations for bridge piers.
There are many cases to which the above classification is not strictly applicable.
For convenience in study, the construction of foundations will be discussed, in the three .succeeding chapters, under the heads Ordinary Foundations, Pile Foundations, and Foundations under Water. However, the methods employed in each class are not entirely distinct from those used in the others.