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.
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.