12. Gravel

This material gives less trouble than any other as a foundation bed. It does not settle under any ordinary loads, and will safely carry the heaviest of buildings if the footings, are properly proportioned. It is not affected by water, provided it is confined laterally, so that the sand and fine gravel cannot wash out. This soil is also not greatly affected by frost.

13. Sand

This material also makes an excellent foundation bed when confined laterally, and is practically incompressible, as clean river sand compacted in a trench has been known to support 100 tons to the square foot.

As long as the sand is confined on all sides, and the footings are all on the same level, no trouble whatever will be encountered, unless it be in the caving of the banks in making the excavations. Should the cellar be excavated to different levels, however, sufficient retaining walls must be erected where the depth changes to prevent the sand of the upper level from being forced out from under the footings, and precautions should be taken in such a case to keep water from penetrating under the upper footings.

13 Sand 1003

Fig. 3.

14. Loam and Made Land

No foundation should start on loam (soil containing vegetable matter), or on land that has been made or filled in, unless, indeed, the filling consist of clean beach sand, which, when settled with water, may be considered equal to the natural soil.

Loam should always be penetrated to the firm soil beneath, and when the made land or filling overlies a firm earth, the footings should be carried to the natural soil. When the filled land is always wet, as on the coast or the borders of a lake, piles may be used, extending into the firm earth, and the tops cut off below low water mark; but piles should never be used where it is not certain that they will be always wet.

15. Mud and Silt

Under this heading may be included all marshy or compressible soils which are usually saturated with water.

Foundations on such soils are generally laid in one of the three following ways: 1. By driving piles on which the footings are supported. 2. By spreading the footings either by wooden timbers or steel beams so as to distribute the weight over a large area. 3. By sinking caissons or steel wells, filled with masonry, to hard pan. As each of these methods are more or less complicated they will be described in Chapter II (Foundations On Compressible Soils).

16. Bearing Power of Soils

The best method of determining the load which a particular soil will bear is by direct experiment; but good judgment, aided by a careful examination of the soil - particularly of its compactness and the amount of water it contains - in conjunction with the following table, will enable one to determine with reasonable accuracy its probable supporting power. A mean of the values given below may be considered safe for good examples of the kinds of soils quoted:

Table I. - Bearing Power Of Soils

KIND OF MATERIAL.

BEARING POWER

IN TONS PER SQUARE FOOT.*

MIN.

MAX.

Rock, hard.....................

25

30

Rock, soft....................

5

10

Clay on thick beds, always dry..................

4

6

Clay on thick beds, moderately dry......................

2

4

Clay, soft.........................

1

2

Gravel and coarse sand, well cemented......................

8

10

Sand, compact and well cemented...........................

4

6

Sand, clean, dry...............................

2

4

Quicksand, alluvial soils, etc...............................

0.5

1

* Ira O. Baker, C. E., in " Treatise on Masonry Construction."

Should it be desirable to exceed the maximum loads here given, or should there be any doubt of the bearing capacity of the soil or lack of precedent, tests should be made on the bottom of the trenches in several places to determine the actual load required to produce settlement, as described in Section 18.

17. Examples of Actual Loads and Tests.

On Clay. - The Capitol at Albany, N. Y., rests on blue clay containing from 60 to 90 per cent, of alumina, the remainder being fine sand, and containing 40 per cent, of water on an average. The safe load was taken at 2 tons per square foot. A load of 5.9 tons applied on a surface 1 foot square produced an uplift of the surrounding earth.

The Congressional Library at Washington, D. C, rests on yellow clay mixed with sand. It was found that it required about 13 tons per square foot to produce settlement, and the footings were proportioned for a maximum pressure of 2 tons.

A hard indurated clay, containing lime, under the piers of a bridge across the Ohio River, at Point Pleasant, W. Va., carries approximately 2 tons per square foot.

Sand. - " In an experiment in France, clean river sand compacted in a trench supported 100 tons per square foot.

"The piers of the Cincinnati suspension bridge are founded on a bed of coarse gravel 12 feet below water; the maximum pressure is 4 tons per square foot.

"The piers of the Brooklyn suspension bridge are founded 44 feet below the bed of the river, upon a layer of sand 2 feet thick, resting upon bed rock; the maximum pressure is about 5 tons per square foot." *