This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
6. Nearly all structures, from the four or five room cottage to the massive and towering mercantile building, rest on masonry foundations. If the foundations are insufficient or defective, all the subsequent stonework or brickwork is likely to settle and crack; the failure of the initial or primary part of the work thus jeopardizes whatever follows, by reason of its inability to support the load placed upon it.
7. Before putting in foundations, the ground must be excavated, either for the cellar or for trenches to place the masonry below frost line; but before the excavation is begun, especially if the structure is an important one, the nature of the soil which is to support the foundations should be determined. For this purpose, a 2-inch auger is used - similar to an auger for wood boring - and the boring tests are made from 5 to 10 feet apart, over the entire area of the foundation. The auger brings up samples of the materials, and the character of the substrata is thus determined.
When the importance of the proposed structure requires it, trial pits are sometimes dug from 10 to 20 feet apart, especially where a shelving bed of rock or gravel exists at a comparatively short distance below the surface.