This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
Before beginning a structure, the character of the soil should be investigated. For ordinary work, this can be done by boring - using a 2" auger - at short intervals, around the site. The auger will bring up samples sufficient to determine the character of the soil. This is a useful precaution, but can be dispensed with for the usual run of buildings, the bearing power being judged by experience of loads in adjacent structures, or by examinations of near-by excavations.
Soils may be classed as rock, ordinary soil, and made ground. A level bed of rock makes the best possible foundation; sand and gravel rank next; clay is safe for moderate loads if kept dry; quicksand should be removed if possible; and soft, marshy ground should be piled, or the footings spread enough to reduce the pressure to safe limits. Made ground, usually formed of garbage, waste earth, etc., should not be built on for important structures, without tests as to its bearing power. For small buildings, however, good made ground is safe to build on. Table XI, page 74, gives the loads which different kinds of soils will carry.
If water is encountered in excavating foundations, careful provision must be made for its removal by means of suitable drains. The ground water level should be considered also. Thus a building may have a good foundation on wet ground, until the water is drained off, by excavation of deep trenches in the street, which causes settlement in the foundation. The frost line, or depth to which ground becomes frozen, must be taken into account, and foundations must be started below it; otherwise they may be cracked and heaved out of place. This depth varies from 3 ft. to 6 ft., according to the severity of the climate.
Foundations should not be laid on a sloping bed, owing to the lia-bility of slipping. Nor should the walls be built partly on rock and partly on earth, as shown in Fig. 1, for the weight causes the earth to settle, and the wall, being carried by the rock only, would be unstable; or water, flowing between the rock and masonry, and freezing, might force out the thin wall. When a level bed cannot be otherwise obtained, concrete may be advantageously used for this purpose. If the natural surface is rough, the better will concrete adhere to it. In fact, con-. crete should be used much more than it is for foundation work. For buildings of moderate weight, erected on soft, clayey soils, the bearing power of the latter may often be considerably improved by spreading layers of sand, gravel, or broken stone, and pounding it into the soil. Or, the soil may be compacted by driving short piles, say 6 ft. long and 6 in. in diameter, as close together as necessary; from 2 to 4 ft. apart is generally close enough. The results will be better if the piles are drawn out and the holes filled with sand, well compacted. Thus the soil is consolidated, and the sand, acting as so many small arches, transmits the load to the sides of the hole, as well as to the bottom.