Bearing Power or Soils

The safe bearing power of rock is certainly not less than one tenth of the ultimate crushing strength of cubes; that is to say, the safe bearing power of solid rock is not less than 18 tons per s4. ft. for the softest rock and 180 for the strongest. It is safe to say that almost any rock, from the hardness of granite to that of a soft crumbling stone easily worn by exposure to the weather •or to run ning water, when well bedded will bear the heaviest load that can be brought upon it by any masonry construction.

It scarcely ever occurs in practice that rock is loaded with the full amount of weight which it is capable of sustaining, as the extent of base necessary for the stability of the structure is generally suffi cient to prevent any undue pressure coming on the rock beneath.

Corthell cites* five examples of structures that have stood without settlement in which the pressure on "hard pan" ranged from 3.0 to 12.0 tons per sq. ft., the average being 8.7 tons.

Clay.

The clay soils vary from slate or shale, which will support any load that can come upon it, to a soft, wet clay which will squeeze out in every direction when a moderately heavy pressure is brought upon it. Foundations on clay should be laid at such depths as to be unaffected by the weather; since clay, at even con siderable depths, will gain and lose considerable water as the seasons change. The bearing power of clayey soils can be very much im proved by drainage (˘ 671), or by preventing the penetration of water. If the foundation is laid upon undrained clay, care must be taken that excavations made in the immediate vicinity do not allow the clay under pressure to escape by oozing away from under the building. When the clay occurs in strata not horizontal, great care is necessary to prevent this flow of the soil. When coarse sand or gravel is mixed with the clay, its supporting power is greatly in creased, being greater in proportion as the quantity of these materials is greater. When they are present to such an extent that the clay is just sufficient to bind them together, the combination will bear nearly as heavy loads as the softer rocks.

The following data on the bearing power of clay will be of assistance in deciding upon the load that may safely be imposed upon any particular clayey soil.

Experiments made on the clay under the piers of the bridge across the Missouri River at Bismarck, with surfaces 14 inches square, gave an average ultimate bearing power of 15 tons per sq. ft. Clay in thick compact beds, without any admixture of loam or vegetable matter, has carried 10 tons per sq. ft. without appreciable settlement. In the case of the Congressional Library (1 654), the ultimate sup porting power of "yellow clay mixed with sand" was 131 tons per sq. ft.; and the safe load was assumed to be 27i tons per sq. ft.

The stiffer varieties of clay, when kept dry, will safely bear from 4 to 6 tons per sq. ft.; but the same clay, if allowed to become saturated with water, can not be trusted to bear more than 2 tons per sq. ft. From the experiments made in connection with the con struction of the capitol at Albany, N. Y., as described in $ 654, the conclusion was drawn that the extreme supporting power of that soil was less than 6 tons per sq. ft., and that the load which might be safely imposed upon it was 2 tons per sq. ft. "The soil was blue clay containing from 60 to 90 per cent of alumina, the remainder being fine siliceous sand. The soil contains from 27 to 43, usually about 40, per cent of water; and various samples of it weighed from 81 to 101 lb. per cu. ft." At Chicago it was formerly the custom to found upon the clay, and the load ordinarily put on a thin layer of clay (hard above and soft below, resting on a thick stratum of quicksand) was 1/ to 2 tons per sq. ft.; and the settlement, which usually reached a maximum in a year, was about 2 to 2/ inches per ton of load. Experience in central Illinois shows that, if the foundation is carried down below the action of frost, the clay subsoil will bear 1/ to 2 tons per sq. ft. without appreciable settling: Corthell cites* sixteen examples of structures that have stood without material settlement in which the pressure on clay ranged from 2.0 to 8.0 tons per sq. ft., the mean being 5.2 tons; and gives five other examples in which there was notable settlement with pressures on " hard clay " between 4.5 and 5.6 tons per sq. ft. with an average of 5.08. For corresponding data for " hard pan " see § 659.