FOUNDATION MATERIALS Examination of Soil.—The stability of any structure requires that it be adequately supported by the ground upon which it rests, hence the nature of the soil upon which the structure is to be placed is the first subject for consideration in designing a foundation, and the local conditions under the surface of the ground must be determined. Numerous instances might be cited of the failure of structures due to lack of adequate investigation of soil conditions, and every effort should be made to obtain an accurate knowledge of the underlying strata.
For shallow foundations, open excavations may be made to a depth somewhat greater than that of the substructure, which will give the advantage of permitting the examination of the soil through and into which the substructure must be built and observing its condition. When the excavation is in wet material, pumping may be required to keep down the water and perhaps sheeting to prevent the sides caving in—an expensive procedure if excavating is carried to con considerable depth.
Soundings are sometimes made with a rod, or small pipe about an inch in diameter, which is driven into the ground with a maul. When the material near the surface is soft, the depth to rock or other hard material may usually be determined in this manner if it is not more than 20 or 30 feet. Soundings serve to indicate whether resist ance increases or decreases, and the depth at which hard material stops further progress. A number of soundings are usually neces sary. A sunken log or boulder may stop the rod, and mistakes in interpreting the results of such soundings are easily made.
Borings with earth augers may be easily made for small depths with good results. Ordinary wood augers about 2 inches in diameter have also been used for this purpose, borings 100 feet deep having been made in this manner, though for ordinary work to more moderate depths, the use of earth augers of larger diameters give better deter Initiations. An auger ti inches in diameter may readily be driven to a depth of 25 or,30 feet by two men with levers. It is held in vertical
position by pipes or rods in sections, which may be coupled together as the hole becomes deeper, and is turned by hand with handles at the top 2 to 4 feet long, which are adjustable in position on the rods. The auger is screwed into the soil sufficiently to fill it with earth, and is then brought to the surface and the material examined, giving a good determination of the character of the soil at any depth, but not show ing its degree of compactness. When the hole passes through material which will not, retain its shape, a casing somewhat larger than the auger is driven, through which the boring may be done. When the depth to which the boring must extend is considerable, a block and fall, supported by a tripod, may be used to draw the auger from the hole.
11"a$h borings may be rapidly driven through soft, soil or clay by sinking a casing, with a small pipe or hollow rod inside which carries a jet of water at its lower encl. The jet cuts the soil at the bottom and brings up the excavated material through the annular space between the jet pipe and casing. It is usual also to have the bottom of the inside pipe fitted with a bit or chisel, which may aid in cutting into hard material. Both jet pipe and casing are rotated as they descend. When hard material is met, it may be necessary to cut it with the bit by churning the inside pipe. The bottom of the casing is also sometimes flared slightly and fitted with teeth for cutting.
When the depth is not great and only a small amount of work is to be clone, ordinary water pipe about 2 inches in diameter is sunk as a casing, a smaller pipe ;-inch in diameter being used inside. Hand appliances may be used in handling these pipes, a tripod with block and fall, levers for turning the pipes, and a hand pump for applying pressure to the jet. On more important work hollow rods for holding the jet and bits, special casings, and pipes with flush joints are necessary. These may be controlled by hand, or machine outfits similar to those used in drilling wells may he employed.