In Wet Ground

IN WET GROUND. The difficulty in soils of this class is in disposing of the water, or in preventing the semi-liquid soil from running into the excavation. The difficulties are similar to those met with in constructing foundations under water—see Chapter XVI. Three general methods of laying a foundation in this kind of soil will be briefly described.

If the soil is only moderately we.

not satu rated,—it is sufficient to inclose the area to be excavated with sheet piles (boards driven vertically into the ground in contact with each other). Ordinary planks 8 to 12 inches wide and 11 or 2 inches thick are used. This curbing is a simple form of a coffer-dam (Art. 1, Chap. XVI). The boards should be sharpened wholly from one side; this point being placed next to the last pile driven causes them to crowd together and make tighter joints. The sheeting may be driven by hand, by a heavy weight raised by a tackle and then dropped, or by an ordinary pile-driver (§ 751-52). Unless the amount of water is quite small, it will be necessary to drive a double row of sheeting, breaking joints. It will not be possible to entirely prevent leaking. The water that leaks in may be bailed out, or pumped—either by hand or by steam (see § 818-23).

To prevent the sheeting from being forced inward, it may be braced by shores placed horizontally from side to side and abutting against wales (horizontal timbers which rest against the sheet piles). The bracing is put in successively from the top as the excavation proceeds; and as the masonry is built up, short braces between the sheeting and the masonry are substituted for the long braces which previously extended from side to side. Iron screws, somewhat similar to jack-screws, are used, instead of timber shores, in exca vating trenches for the foundations of buildings, for sewers, etc.

If one length of sheeting will not reach deep enough, an addi tional section can be placed inside of the one already in position, when the excavation has reached a sufficient depth to require it.

For a more extended account of the use of cofferdams, see Chapter XVI—Foundations Under Water, Art. 1, Coffer-Dams.

In some cases the soil is more easily excavated if it is first drained. To do this, dig a hole—a sump—into which the water will

drain and from which it may be pumped. If necessary, several sumps may be sunk, and deepened as the excavation proceeds.

Quicksand or soft alluvium may sometimes be pumped out along with the water by a centrifugal or a mud pump (§ 823 and § 877). On large jobs, such material is sometimes taken out with a clam-shell or orange-peel dredge (§ 845-46).

Ooncrete.

Concrete can frequently be used advantageously in foundations in wet soils. If the water can be removed, the concrete should be deposited in layers (§ 342-44); but if the water can not be removed, the concrete may be deposited under the water (see § 347), although it is more difficult to insure good results by this method than when the concrete is deposited in the open air.

Piles.

If the semi-liquid soil extends to a considerable depth, or if the soft soil which overlies a solid substratum can not be removed readily, a substantial foundation may be constructed either by driving wood piles at uniform distances over the area and con structing a timber grillage (see § 793-95) on top of them, or by driv ing concrete piles (see § 736-45) and depositing a concrete cap (see § 796) on top of them. Of course, wood piles should be sawed off (§ 791) below low-water, which usually necessitates a coffer-dam (§ 718, and Art. 1 of Chapter XVI) and the excavation of the soil a little below the low-water line.

722. In excavating shallow pits in sand containing a small amount of water, dynamite cartridges have been successfully used to drive the water out. A hole is bored with an ordinary auger and the cartridge inserted and exploded. The explosion drives the water back into the soil so far that, by working rapidly, the hole can be excavated and a layer of concrete placed before the water returns.

It is hardly worth while here to discuss this subject further. It is one on which general instruction can not be given. Each case must be dealt with according to the attendant circumstances, and a knowledge of the method best adapted to any given conditions comes only by experience.