This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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. An 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 determined; but nothing of the compactness of the soil can be determined in this manner.
In order to test a soil to find its compressive value, the bottom of the pit should be leveled for a considerable area, and stakes should be driven at short intervals in each direction. The elevations of the tops of all the stakes should be very accurately taken with a spirit level. For convenience, all stakes should be driven to the same level. A mast whose base has an area one foot square can support a platform which may be loaded with several tons of building material, such as stone, brick, steel, etc. This load can be balanced with sufficient closeness so that some very light guys will maintain the unstable equilibrium of the platform. As the load on the platform is greatly increased, at some stage it will be noted that the mast and platform have begun to sink slightly, and also that the soil in a circle around the base of the mast has begun to rise. This is indicated by the rising of the tops of the stakes. Even a very ordinary soil may require a load of five or six tons on a square foot before any yielding will be observable. One advantage of this method lies in the fact that the larger the area of the foundation, the greater will be the load per square foot which may be safely carried, and that the uncertainty of the result is on the safe side. A soil which might yield under a load concentrated on a mast one foot square, would probably be safe under that same unit-load on a continuous footing which was perhaps three feet wide; and if, in addition, a factor of safety of three or four was used, there would probably be no question as to the safety. Such a test need be applied only to an earthy soil. It would be practically impossible to produce a yielding by such a method on any kind of rock or even on a compacted gravel.