A soft, clayey subsoil may frequently be improved by covering it with gravel, which is rammed and pressed into the clay. Such a device is not very effective, but it may sometimes be sufficiently effective for its purpose.
A subsoil is often very soft because it is saturated with water which cannot readily escape. Frequently a system of deep drainage which will reduce the natural level of the ground-water considerably below the desired depth of the bottom of the foundation, will trans form the subsoil into a dry, firm soil which is amply strong for its purpose. Even when the subsoil is very soft, it will sustain a heavy load, provided that it can be confined. While excavating for the foundations of the tower of Trinity Church in New York City, a large pocket of quicksand was discovered directly under the proposed tower. Owing to the volume of the quicksand, it 14;as found to be impracticable to drain it all out; but it was also discovered that the quicksand was confined within a pocket of firm soil. A thick layer of concrete was then laid across the top, which effectively sealed up the pocket of quicksand, and the result has been perfectly sat isfactory.
17S. Preparing the Bed on Rock. The preparation of a rock bed on which a foundation is to be placed, is a simple matter com pared with that required for some soils on which foundations are placed. The bed-rock is prepared by cutting away the loose and `decayed portions of the rock and making the plane on which the foundation is placed perpendicular to the direction of the pressure. If the rock bed is an inclined plane, a series of steps can be made for the support of the foundation. Any fissures in the rock should be filled with concrete.
Whenever it is necessary to start the foundation of a structure at different levels, great care is required to prevent a break in the joints at the stepping places. The precautions to be taken arc that the mortar-joints must be kept as thin as possible; the lower part of the foundations should be laid in cement mortar; and the work should proceed slowly. By following these precautions, the settlement in the lower part will be reduced to a minimum. These precautions apply to foundations of all classes.
179. Preparing the Bed on Firm Earth. Under this heading is
included hard clay, gravel, and clean, dry sand. The bed is prepared by digging a trench deep enough so that the bottom of the foundation is below the frost line, which is usually 3 to 6 feet below the surface.
Some provision, similar to that shown in Fig. 3S, should be made for drainage.
Care should be taken to proportion the load per unit of area so that the settlement of the founda tion will be uniform.
1S0. Preparing the Bed on Wet Ground. The chief trouble in making an excavation in wet ground, is in disposing of the water and preventing the wet soil from flowing into the excavation. In moderately wet soils, the area to be excavated is enclosed with sheet piling (see Fig. 39). This piling usually consists of ordinary plank 2 inches thick and (1 to 10 inches wide, and is often driven by hand; or it may be diiven by methods that will be described later. The piling is driven in close contact, and in very wet soil it is necessary to drive a double row of the sheeting. To prevent the sheeting from being forced inwards, cross-braces are used between the longitudinal timbers. When one length of sheeting is not long enough, an additional length can be placed inside. A more extended discus sion of pile-driving will be given in the treatment of the subject of "Piles." The water can sometimes he bailed out, but it is generally necessary to use a hand or steam pump to free the excavation of water. Quicksand and very soft mud are often pumped out along with the water by a centrifugal or mud pump.
Sometimes areas arc excavated by draining the water into a hole the bottom of which is always kept lower than the general level of bottom of the excavation. A pump may be used to dispose of the water drained into the hole or holes.
When a very soft soil extends to a depth of several feet, piles are usually driven at uniform distances over the area, and a grillage is constructed on top of the piles. This method of constructing a foundation is discussed in the section on "Piles." 181. Footings. The three requirements of a footing course are: (1) That the area shall be such that the total load divided by the area shall not be greater than the allowable unit-pressure on the subsoil.