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 4·00 to 6·00 water Solid blue clay, marl and indurated marl, and firm boulder 5·00 to 8·00 gravel and sand 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 6·00 to 7·00 depth, not less than 20 feet, and compact sandy gravel 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.