Foundation Materials

The depth of the foundation below the surface of the ground is important in plastic material, the weight of the earth being relied upon to confine the material, and prevent it squeezing out and lifting the surrounding area. Corthell in his "Allowable Pressure on Deep Foundations" has cited a large number of instances showing working loads upon foundations under varying conditions.

The pressures allowed upon foundations by the specifications of various authorities differ quite widely. The values in Table XXXV represent the range of maximum pressures commonly given.

The character of the structure to be carried by a foundation may frequently have an influence upon the choice of a limiting value for the hearing capacity. Where a slight settlement in the foundation may be serious in its effect upon the structure, very conservative pressures should be adopted.

187.

Tests for Bearing Capacity.—Direct tests of the capacity of the soil to support the loads corning upon a foundation are frequently desirable, they 'should be supplemental to the examination of the site and cannot replace such examination. They are intended to give a more accurate idea of the actual bearing capacity than can be derived from observation of the material upon which the foundation is to be placed and its underlying strata, and should therefore be made in the excavation at the level upon which it is proposed to place the base of the foundation.

The methods of making these tests vary considerably. Some times a small area is loaded and observations made of the settlement under varying loads, from which the probable safe bearing capacity may be deduced. In other instances, a load of about twice that pro

posed for the foundation is placed upon a small area and settlement for different periods of time observed, with a view to judging the safety of the proposed loading. Usually a platform is employed to carry the load. The platform is customarily supported on a pier of about 1 square foot area, or sometimes upon four legs at its corners. The soil to be tested is leveled to receive the piers and provision made for observing the settlement of the base of the pier under the loads. The platform must be so arranged as to bring uniform pressure upon the area under test.

The time element is frequently a matter of importance, settle ment in some soils occurring gradually during a period of twenty four or forty-eight hours, until a stable position is reached. Some soils are elastic under working loads, and the settlement diminishes as the load is decreased after a test.

The resistance offered by the soil to pressure upon a small area is not necessarily the same a-s that which may exist over a large area, and the results of such tests must be used very conservatively in the design of foundations. These results, however, when combined with careful observations of the character of the materials underlying the foundation. give a basis upon which to form a judgment of safe bearing capacity.