Bearing Power or Soils

The stiff blue clay of London seems not to be able to support more than 5 tons per sq. ft., for three bridges across the Thames— the old Westminster, the Blackfriars, and the "new" London (built in 1831)—each gave a pressure of about 5 tons per sq. ft. upon the clay and each settled badly.

Sand.

The sandy soils vary from coarse gravel to fine sand. The former when of sufficient thickness forms one of the firmest and best foundations; and the latter when saturated with water is practically a liquid. Sand when dry, or wet sand when prevented from spreading laterally, forms one of the best beds for a foundation. Porous, sandy soils are, as a rule, unaffected by stagnant water, but are easily removed by running water; in the former case they present no difficulty, but in the latter they require extreme care at the hands of the constructor, as will be considered later.

Compact gravel or clean sand, in beds of considerable thickness, protected from being carried away by water, may be loaded with 8 to 10 tons per sq. ft. with safety. In an experi ment in France, clean river-sand compacted in a trench sup ported 100 tons per sq. ft. Fine sand well cemented with clay and compacted, if protected from water, will safely carry 4 to 6 tons per sq. ft.

The piers of the Cincinnati Suspension Bridge are founded on a bed of coarse gravel 12 feet below low-water, although solid lime stone was only 12 feet deeper; if the friction on the sides of the pier* be disregarded, the maximum pressure on the gravel is 4 tons per sq. ft. The New York pier of the Brooklyn Suspension Bridge is founded 44 feet below the bed of the river, upon a layer of sand 2 feet thick resting upon bed-rock, the maximum pressure being about 61 tons per sq. ft.

At Chicago sand and gravel about 15 feet below the surface are successfully loaded with 2 to 2i tons per sq. ft. At Berlin the safe load for sandy soil is generally taken at 2 to 2i tons per sq. ft. The Washington Monument, Washington, D. C., rests upon a bed of very fine sand two feet thick underlying a bed of gravel and bowlders, the ordinary pressure on certain parts of the foundation being not far from 11 tons per sq. ft., which the wind may increase to nearly 14 tons per sq. ft.

Corthell cites t ten examples of structures that give pressures on fine sand ranging from 2.25 to 5.8 tons per sq. ft., the average being 4.5 tons; thirty-three examples of pressures on coarse sand and gravel ranging from 2.40 to 7.75 tons with an average of 5.1 tons; and ten examples on sand and clay from 2.5 to 8.5 tons per sq. ft., the average being 4.9 tons—all without settlement. The same author gives three examples in which pressures of 1.8 to 7.0 tons per sq. ft. (average 5.2) on fine sand gave notable settlement; and three ex amples where pressures of 1.6 to 7.4 tons per sq. ft. (average 3.3) on sand and clay gave undesirable settlement.

Soil

s. With a semi-liquid soil, as mud, silt, or quicksand, it is customary (1) to remove it entirely, or (2) to sink piles, tubes, or caissons through it to a solid substratum, or (3) to consolidate the soil by adding earth, sand, stone, etc. The method of performing these operations will be described later. Soils of a soft or semi-liquid character should never be relied upon for a foundation when anything better can be obtained; but a heavy superstructure may be supported by the upward pressure of a semi liquid soil, in the same way that water bears up a floating body. According to Rankine,* a building will be supported when the expression w is the weight of a unit volume of the soil, h is the depth of immersion, and a is the angle of repose of the soil. If a = then according to the preceding relation the supporting power of the soil is 1.4 w h per unit of area; if a = it is 2.0 w h; and if a = 15°, it is 2.9 w h. The weight of soils of this class, i.e., mud, silt, and quicksand, varies from 100 to 130 lb. per cu. ft. Rankine gives this formula as being applicable to any soil; but since it takes no account of cohesion, for most soils it is only roughly approximate, and gives results too small. The following experiment seems to show that the error is considerable. "A 10-foot square base of concrete resting on mud, whose angle of repose was 5 to 1 [a = bore 700 lb. per sq. ft."f This is 21 times the result by the above formula, using the maximum value of w.