Bearing Power or Soils

BEARING POWER or SOILS. It is scarcely necessary to say that soils vary greatly in their bearing power, ranging as they do from the condition of hardest rock, through all intermediate stages, to a soft or semi-liquid condition, as mud, silt, or marsh. The best method of determining the load which a specific soil will bear is by direct experiment (§ 654-55); but good judgment and experience, aided by a careful study of the nature of the soil—its compactness and the amount of water contained in it—will enable one to determine, with reasonable accuracy, its probable supporting power. The following data are given to assist in forming an estimate of the load which may safely be imposed upon different soils.

Rock.

The ultimate crushing strength of stone, as deter mined by crushing small cubes, ranges from 150 tons per square foot for the softest stone—such as are easily worn by running water or exposure to the weather—to 2,000 tons per square foot for the hardest stones (see Table 2, page 11). The crushing strength of slabs, i.e., of prisms of a less height than width, increases as the height decreases. A prism one half as high as wide is about twice as strong as a cube of the same material. If a slab be conceived as being made up of a number of cubes placed side by side, it is easy to see why the slab is stronger than a cube. The exterior cubes prevent the detachment of the disk-like pieces (Fig. 1, page 10) from the sides of the interior cubes; and hence the latter are greatly strengthened, which materially increases the strength of the slab. In testing cubes and slabs the pressure is applied uniformly over the entire upper surface of the test specimen; and, reasoning from analogy, it seems probable that when the pressure is applied to only a small part of the surface, as in the case of foundations on rock, the strength will be much greater than that of cubes of the same material.

Table 58 contains the results of experiments made by the author, and shows conclusively that a unit of material has a much greater power of resistance when it forms a portion of a larger mass than when isolated in the manner customary in making experiments on crushing strength.

The ordinary "crushing strength" given in next to the last column of Table 58 was obtained by crushing cubes of the identical materials employed in the other experiments. The concentrated pressure was applied by means of a hardened steel die thirty-eight sixty-fourths of an inch in diameter (area = 0.277 sq. in.). All the tests were made between self-adjusting parallel plates. No cushions were used in either series of experiments; that is, the pressed sur faces were the same in both series. However, the block of limestone 7 inches thick (Experiments No. 8 and 13) is an exception in this respect. This block had been sawed out and was slightly hollow, and it was thought not to be worth while to dress it down to a plane. As predicted before making the test, the block split each time in the direction of the hollow. If the bed had been flat, the block would doubtless have shown a greater strength. The concentrated pressure was generally applied near the corner of a large block, and the distance from the center of the die to the edge of the block as given in the table is to the nearest edge. Frequently the block had a ragged edge, and therefore these distances are only approximate. The quantity in the last column—"Ratio"—is the unit crushing • load for concentrated pressures divided by the unit crushing load for uniform pressure.

The experiments are tabulated in an order intended to show that the strength under concentrated pressure varies (1) with the thick ness of the block and (2) with the distance between the die and the edge of the material being tested. It is clear that the strength increases very rapidly with both the thickness and the distance from the edge to the point where the pressure is applied. Therefore we conclude that the compressive strength of cubes of a stone gives little or no idea of the ultimate resistance of the same material when in thick and extensive layers in its native bed.