In this course the foundations likely to be required for ordinary buildings will alone be described, foundations under water, cofferdams, caissons, etc., being excluded, as appertaining more to engineering works than to ordinary building construction.
The great importance of a stable foundation will be apparent to every one, and need not be dilated upon.
A good foundation should fulfil the following conditions: -
1. It must either be incompressible, or at least equally yielding throughout.
2. It should be perpendicular to the pressure upon it.
3. It should be of sufficient area to bear that pressure.
4. It should be unalterable in nature, either by atmospheric or other influences that it can possibly be subjected to.
Some natural soils fulfil these conditions, requiring only to be excavated to the proper levels; in other soils artificial means must be adopted in order to form a stable foundation.
The different kinds of soils have been arranged as followsl: -
2. Soils that are incompressible but require to be laterally confined to prevent them from spreading, such as loose gravel and sand.
3. Compressible soils, such as ordinary clay, common earth, and marshy soils, some of which, such as clay and earth, are only compressible to a certain extent, while others are in an almost fluid state.
1 Mahan's Civil Engineering.
Before commencing a building, trial pits should be dug or borings made at different points on the site, in order to ascertain the nature of the ground, the thickness and inclination or "dip" of the strata; to find out whether water exists, and if so, at what level; also whether the ground has been mined or is full of dangerous fissures. If there be any springs on the site, their source should be ascertained and the water diverted.
The description of foundation having been decided upon, trenches must be dug to the widths and depths necessary, the bottoms of these carefully examined, sounded with a crowbar to ascertain any local defects, and then levelled throughout, in one plane if convenient, if not, in horizontal " benches " or terraces.
The ground should be well drained before digging the foundation, to increase its firmness; all bad parts should be cut out and made good with concrete, and loose portions rammed.
Surface water must be carried off by a catchwater drain, and " grips " or small trenches must be formed to carry off all water that may collect in the foundation during its formation.
As fast as walls are built up they should be " punned " (that is, filled in and rammed) on each side.
On benched foundations care must be taken in rising from the lower to the higher level (A to B, Fig. 398) to have large stones with well-dressed beds and the joints as few and as thin as possible, otherwise the unequal settlement caused by the number of joints in A C being greater than that in E F will lead to fracture.
In such a case the masonry should be left thoroughly to set before being built upon.
The foundations of the angles of a building should be laid with large and heavy stones, and great care should be taken that the foundations under the higher walls are not liable to consolidate more than those under the lower walls, otherwise where these are connected fractures will appear.
All foundations should be at such a depth as to be out of danger from the effects of frost.
In England a depth of 3 feet will generally be sufficient to ensure this, but in clay soils a depth of 4 feet will be necessary, as they are frequently laid open to injury by fissures formed by heat.
Permanent drains should be laid so as to keep all foundations constantly dry; water is their great enemy, causing swelling, removal and subsidence of the soil, expansion from frost, perpetual damp, etc. - even in some kinds of rock it percolates between the masses and loosens them, so that when pressure comes upon them they sink.