Foundation Materials

Examination of the materials brought up by the water shows the nature of the underlying strata. It does not, however, reveal the moisture or compactness of the material. It may therefore be desir able to obtain cores of the materials as they occur at certain points in the test holes, which may he done by substituting a cylinder for the jet and bit upon the end of the rod and pressing or screwing the cylin der into the soil at the bottom of the hole until it is filled with a sample of the material, which is then drawn to the surface and examined. This may sometimes prevent mistakes in judging of subsurface conditions where the wet method of excavation is em ployed.

Core drills are used in testing rock strata. These consist of hollow circular bits, which are rotated so as to cut an annular channel into the rock, leaving a circular core on the inside of the core barrel to which the bit is attached. This core is removed at intervals for examination, and furnishes definite information concerning the character of the material. The core barrel is attached to hollow rods through which water may he supplied to cool the bit.

Several types of bits are uscd for this purpose; in some the cutting edge is formed of black diamonds or bort; in others, chilled shot are used under a hollow soft steel bit; or steel bits with teeth may be employed. When diamond drills are used, the cores are commonly from 1 to 2 inches in diameter; the other types are usually somewhat larger, varying from 2 to 4 inches in diameter.

Chopping bits are often used in connection with core drills, cores being taken at intervals and the intermediate cutting being done by the chopping.drills. In any such work, complete drilling machines are necessary and they should be operated by men experienced in the work.

186. Bearing Capacity of Soils.

Definite values of bearing capacity for various soils cannot be stated with accuracy, because of the variations in character and condition of the same kind of soils and the consequent difficulty in classifying them. The ability of the soil to sustain loads depends not only upon its character, but also upon the amount of water it contains and the degree to which it is confined in position. The location and drainage of the foundation as well as the character of the soil must therefore be considered in determining its bearing capacity.

Solid rock makes the best and most substantial foundation, and usually is capable of carrying any load that the masonry may bring upon it. The loose and decayed portions of the rock upon its surface

need to be cut away, and the surface should be trimmed so that there will be no tendency for the structure to slip upon it.

Clay soils vary widely in character. They may be found in any condition from soft, wet clay, which will squeeze out laterally under light pressure, to hard, indurated clays capable of bearing heavy foundations without yielding. The supporting power is mainly dependent upon the amount of moisture contained in the clay. The tendency of clay to retain water which it may absorb and to soften as the amount of water increases is its most important property. Clays differ considerably in the readiness with which they absorb water. Compact, hard clays may by proper drainage be kept dry and capable of bearing heavy loads, frequently S to 10 tons per square foot, while wet clay may not safely carry more than 1 ton per square foot.

Sand or grarcl and sand makes a good foundation when confined laterally so that there is no danger of it being washed out, compact gravel and sand being capable of carrying heavy loads without sen sible settlement. Water will not soften it, and it is but slightly affected by frost. Loads of S or 10 tons per square foot seem to be conservative for such material under favorable conditions. Fine sand when saturated becomes soft and mushy and is easily dis placed; it must be confined laterally to form a good foundation. Dry clean sand may carry- loads of 2 to 4 tons per square foot, and when cemented with clay and protected from water it may safely carry loads of 4 to 6 tons per square foot.

When the top soil is loam or made land, foundations should go through such materials to natural subsoil beneath.

The thickness of the layer of material in which the foundation is placed and the nature of underlying strata are important factors in determining the supporting power, as well as the character of the foundation material itself. Foundations in hard clay which is soft underneath may sometimes safely carry 1 or 2 tons per square foot.

For the foundations of buildings, local conditions usually lead to a standard practice, and the building codes of the various cities are designed to insure safety under the particular circumstances of each place.