| Description of Earth. | Approximate Safe Maximum Load in Tons per Square Foot. |
|---|---|
| Bog, morass, quicksand, peat moss, marsh-land, silt | 0 to 0·20 |
| Slake and mud, hard peat turf | 0 to 0·25 |
| Soft wet pasty or muddy clay, and marsh clay | 0·25 to 0·33 |
| Alluvial deposits of moderate depths in river beds, &c. | 0·20 to 0·35 |
| Note.—When the river bed is rocky and the deposit firm they may safely support 0·75 ton, but not more. | |
| Diluvial clay beds of rivers | 0·35 to 1·00 |
| Alluvial earth, loams and loamy soil (clay and 40 to 70 per cent. of sand), and clay loams (clay and about 30 per cent. of sand) | 0·75 to 1·50 |
| Damp clay | 1·50 to 2·00 |
| Loose sand in shifting river bed, the safe load increasing with depth | 2·50 to 3·00 |
| Upheaved and intermixed beds of different sound clays | 3·00 |
| Silty sand of uniform and firm character in a river bed secure from scour, and at depths below 25 feet | 3·50 to 4·00 |
| Solid clay mixed with very fine sand | 4·00 |
| Note.—Equal drainage and condition is especially necessary in the case of clays, as moisture may reduce them from their greatest to their least bearing capacity. When found equally and thoroughly mixed with sand and gravel their supporting power is usually increased. | |
| Sound yellow clay containing only the normal quantity of water | 4·00 to 6·00 |
| Solid blue clay, marl and indurated marl, and firm boulder gravel and sand | 5·00 to 8·00 |
| Soft chalk, impure and argillaceous | 1·00 to 1·50 |
| Hard white chalk | 2·50 to 4·00 |
| Ordinary superficial sand beds | 2·50 to 4·00 |
| Firm sand in estuaries, bays, &c. | 4·50 to 5·00 |
| Note.—The Dutch engineers consider the safe load upon firm clean sand as 5½ tons per square foot | |
| Very firm, compact sand foundations at a considerable depth, not less than 20 feet, and compact sandy gravel | 6·00 to 7·00 |
| Note.—The sustaining power of sand increases as it approaches a homogeneous gravelly state. | |
| Firm shale, protected from the weather, and clean gravel | 6·00 to 8·00 |
| Compact gravel | 7·00 to 9·00 |
| Note.—The relative bearing powers of gravel may be thus described:— | |
| 1. Compact gravel. 2. Clean gravel. 3. Sandy gravel. 4. Clayey or loamy gravel. | |
| Sound, clean, homogeneous Thames gravel has been weighted with 14 tons per square foot at a depth of only 3 to 5 feet below the surface, and presented no indication of failure. This gravel was similar to that of a clean pebbly beach. |
In loose non-cohesive earths the load may be increased, when the depth is considerable, as the soil has been subject to a greater normal pressure due to the weight of the soil upon it at any depth; but it is not advisable to consider such increase of bearing power of the soil, unless at any depth it is found that the normal pressure augments the bearing power and makes the earth more dense, which may be approximately ascertained by experiment. In such event the load upon the base can be increased by the weight of the normal pressure removed. Supposing 5 tons per square foot was known to be the safe load upon the surface of the ground, and at any depth it was found that the normal pressure of the soil was 2 tons; 5 + 2 = 7 tons placed at that depth would equal 5 tons at the surface. In the worst case, when the loose earth is of great depth, and it is certain that it cannot be tapped or disturbed at the depth at which it is decided to place the foundations of a structure, provided the load is not more than the normal pressure, it is not probable that it will subside or slip, as no additional weight is imposed.
In foundation and general work, rocks are usually not loaded with a greater weight than from 8 to 18 tons per square foot, according to the character of the rock. As the crushing strength has been principally ascertained from cubes, and not from prisms, rectangular blocks, or irregularly shaped pieces, and as the resistance of rocks to transverse strain or breaking across is considerably less than the compressive strength, and varies greatly and not always according to the crushing resistance of the material, from 8 to 20 tons per square foot is a prudent limit for the safe load, and should not be exceeded unless under exceptional circumstances; as unequal bearing may greatly intensify the strain, and irregularity in the texture may reduce the resisting powers to that of the weakest part. Sandstone rock that can be crumbled in the hand should not be loaded with more than 1½ to 1¾ ton per square foot, or it will probably begin to flake and disintegrate. The strength of sandstone varies very greatly, and in experiments it has been found that when fine close grained, it supported before being crushed five times the weight that very coarse gritty sandstone, having a sandy appearance, would sustain; the respective crushing pressures per square foot being 362 and 67 tons.
Reference to authorities on the resistance of stones to crushing, tension and transverse strain, will give the approximate safe load per square foot; but in foundations, i.e., upon the rock in its natural location, it should not exceed one-tenth of the ultimate resistance, and the compressive strength should not alone be taken as a guide to the safe load, but the resistance of the rock to tensional and transverse strain should be considered. The value given for crumbling sandstone is for the softest material that can be called rock, and is merely stated to show that although some earths may be generally classed as rocks their bearing power may be limited. The safe load upon an artificial rubble mound foundation depends upon its character, and firmness and solidity when deposited, and upon that of the ground on which it is placed. No general values can be named, although it may be classed as clean or compact gravel.
In a cutting, by excavation, the normal pressure of the earth, which varies with the depth and weight of the soil, is removed.
Let
D = the depth in feet of a cutting from the original surface of the ground,
W = the weight of a cubic foot of earth in decimals of a ton,
P = the normal pressure in tons per square foot at any given depth,
Then P = D × W.