This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The foundation of a building is the horizontal platform, either natural or artificial, prepared for carrying the walls and superstructure. It must not bo confounded with "footings," which are the bases of walls made broader to distribute the weight more equally over the foundation; nor with piers, although it is not always easy to define where a foundation ends and where a pier begins : in general, all those parts of a structure which are sunk in the natural soil, the conditions of which are therefore different from those parts above ground, are foundations. There are 3 important points which should be considered in all foundations: - (1) That the weight to a unit of area imposed upon it should not be more than it and the subsoil below it can bear.
(2) That it should be as nearly as possible homogeneous and equally strong throughout.
(3) That the upper surface should be horizontal: if not in one, then in several plains.
It is generally supposed that rock is a dangerous substratum to make a foundation platform from; for it is rarely that rock is found so homogeneous as to provide a large horizontal surface without artificial filling in; and it is difficult to make the filling in as hard as the rock itself, which it should be, that the settlement, if any, may be uniform. Also in many cases of inclined strata there is the danger of one part of the strata slipping over the other from the additional pressure of the building. A foundation in rock should never be less than 1 ft. in depth, for security against slipping and detrusion.
Many consider a sound thick stratum of gravel to be the most secure foundation possible. In such cases it is only necessary to sink a little into the stratum. rather more than into rock, and to take care that the area of foundation is proportional to the weight per square unit the gravel is calculated to bear. When the gravel is not sound, besides the latter precaution, it is advisable to sink deeper and fill in with an artificial foundation of concrete or large stones or hard durable timber.
When in thick strata, and not liable to be moved by water or other disturbing cause, sand forms a very good foundation; it is desirable to sink deeper into sand than into gravel, and to fill in with an artificial foundation to counteract any irregular settlement of the sand. When exposed to the action of water or any other moving action, however slight, sand is a dangerous foundation to trust to, on account of its great mobility.
Clay appears to be considered an uncertain and troublesome substratum for a foundation, on account of the irregularity of its strata, and its action on being disturbed; for there is a tide in the land as well as in the sea. In consequence of clay'.- plasticity and its retention of water, it is liable to yield unequally to the pressure of a building, and to move irregularly when exposed or cut into: consequently, care must be taken both to spread the structure over a large area of foundation and to load the foundation uniformly in the course of the construction. A bed of clay can be sometimes made firmer by piling or by making holes in it and filling them with stones or gravel: the elasticity of clay is sometimes so great that piles are often forced up again by the action of driving the neighbouring piles.
It frequently happens, especially in the alluvial banks of rivers, that below the soft ground of the immediate surface lies a hard stratum, and when the thickness of the soft superstratum is not great (30 ft. may be considered a maximum for ordinary cases), a secure foundation may be obtained by carrying piles or piers down to the hard ground below, and supporting a horizontal platform on their tops. These may be wooden or iron piles driven till they enter the hard bottom; or piers formed by sinking well-holes through the soft ground and filling them up with masonry, loose stones, or even sand, though this last should only be used when the superstratum is sufficiently firm to resist the lateral pressure of the sand. The tops of these, if piles, may be connected by beams and planks forming a horizontal platform; or if piers, by arches filled in at the spandrils to a horizontal surface. These piles or piers must be considered as columns fixed at the bottom and calculated accordingly, without trusting to the lateral support of the intermediate strata.
In some cases of alluvial foundations, a comparatively firm stratum of gravel or clay is found at the surface or near it, the substrata below that being much softer. In such cases, if the weight of the structure is not very great, it is frequently desirable to leave the hard crust unbroken; but then the area of foundation should be enlarged, beyond what would be used for the same stratum, if of considerable thickness; and special care should be taken to distribute the pressure equally. Also in these cases the hard crust should be cut into as little as possible for any purpose; if it is clay, there is danger of it yielding by exposure to air and wet; if the substratum is sand, there is danger of its being moved by the action caused, by drainage or any operations of that kind, consequent on the building.