Large buildings have been securely founded on quicksand by making the base of the immersed part as large and at the same time as light as possible. Timber in successive layers (§ 705) is generally used in such cases. This class of foundations is frequently required in constructing sewers in water-bearing sands, and though apparently presenting no difficulties, such foundations often demand great skill and ability.
It is difficult to give results of the safe bearing power of soils of this class. A considerable part of the supporting power is derived from the friction on the vertical sides of the foundation, and hence the bearing power depends to a considerable degree upon the area of the side surface in contact with the soil; and with this class of soils it is particularly important that the area tested should be as large as possible. Furthermore, it is difficult to determine the exact supporting power of a plastic soil, since a considerable settle ment is certain to take place with the lapse of time.
Some careful and extensive experiments on the alluvial soil of Calcutta showed that loads not exceeding 2,700 Ib. per sq. ft. caused no greater settlement than 0.19 to 0.31 inch.$ Corthell¶ gives seven examples of structures founded on alluvium and silt in which the pressure ranged from 1.5 to 6.2 tons per sq. ft., the average being 2.9 tons, in which there was no settlement; and two examples in which there was notable settlement under pressures varying from 1.60 to 7.60 tons per sq. ft. The experience at New Orleans with alluvial soil and a few experiments * that have been made on quicksand seem to indicate that with a load of to 1 ton . per square foot the settlement will not be excessive.
It is well to notice that there are some practical considerations that modify the pressure which may safely be put upon a soil. For example, the pressure on the foundation of a tall chimney should be considerably less than that of the low massive foundation of a fire-proof vault. •In the former case a slight in equality of bearing power, and consequent unequal settling, might endanger the stability of the structure; while in the latter no serious harm would result. The pressure per unit of area should be less for
a light structure subject to the passage of heavy loads—as, for example, a railroad viaduct—than for a heavy structure subject only to a quiescent load, since the shock and jar of the moving load are far more serious than the heavier quiescent load.
The determination of the safe bearing power of soils, particularly when dealing with those of a semi-liquid character, is not the only question that must receive careful attention. In the foundations for buildings, it may be necessary to provide a safeguard against the soil's escaping by being pressed out laterally into excavations in the vicinity. For example, in Chicago some of the largest and finest buildings have settled owing to the flow of the plastic clay into foundations opened across the street. In New York City one of the largest buildings settled because of the pumping of fine sand from an artesian well on the site in getting water for the boilers of the build ing. A still more remarkable case occurred in London where 700 feet of the walls of the East India Dock settled in consequence of the sinking of a foundation at the Midland Dock 1,500 feet away, and the source of the trouble was not discovered until a "sand blow" at the latter place revealed the connection.
In the foundations for bridge abutments, it may be necessary to consider what the effect will be if the soil around the abutment becomes thoroughly saturated with water, as it may during a flood; or what the effect will be if the soil is deprived of its lateral support by the washing away of the soil adjacent to the abutment. The provision to prevent the wash and undermining action of the stream is often a very considerable part of the cost of the structure. The prevention of either of these liabilities is a problem by itself, to the solution of which any general discussion will contribute but little.