190. Piles

The term pile is generally understood to be a stick of timber driven in the ground to support a structure. This stick of timber is generally thought of as the body of a small tree; but timber in many shapes is used for piling. Sheet piling, for example, is generally much wider than thick. Cast iron and wrought iron have also been used for all forms of piling. Structural steel has also been used for this purpose. Within the last few years, concrete and reinforced concrete piles have been used quite extensively in place of wood piles.

191. Cast-Iron Piles

Cast-iron piles have been used to some extent. The advantages claimed for these piles are that they are not subject to decay; they are more readily driven than wooden piles in stiff clays or stony ground; and they have a greater crushing strength than wooden piles. The latter quality will apply only when the pile acts as a column. The greatest objection to these piles is that they are deficient in transverse strength to resist sudden blows. This objection applies only in handling them before they are driven, and to those which, after being driven, are exposed to blows from ice and logs, etc. When driving cast-iron piles, a block of wood is placed on top of the pile to receive the blow; and, after being driven, a cap with a socket in its lower side is placed upon the pile to receive the load. Generally lugs or flanges are cast on the sides of the piles, to which bracing may be attached for fastening them in place.

192. Steel Sections

Structural steel sections, as well as many special sections, are being used for piling. This form of piling is generally used for dams, cofferdams, or locks, and seldom or never used as bearing piles. Fig. 44 illustrates some of these sections of piling.

193. Screw Piles

This term refers to a type of metal pile whose use is limited, but which is apparently very effective where it has been used. It consists essentially of a steel shaft, 3 to 8 inches in diameter, strong enough to act as a column, and also to withstand the twisting to which it is subjected while the pile is being sunk (see Fig. 45). At the lower end of the shaft is a helicoidal surface having a diameter of perhaps five feet. Such piles can be used only in comparatively soft soil, and their use is practically confined to foundations in sandbanks on the shore of the ocean. To sink such piles, they are screwed into place by turning the vertical shaft with a long lever. Such a sinking is usually assisted by a water-jet, as will be described later. 194. Disc Piles. A variation of the screw pile is the disc pile (Fig. 46), which, as its name implies, has a circular disc in place of a helicoidal surface. Such a pile can be sunk only by use of a water-jet, the pile being heavily loaded so that it shall be forced down.

195. Sheet Piles

Ordinary planks, two or more inches thick, and wider than they are thick, are, when driven close together, known as sheet piling. The leakage between the piles may be very materially diminished by using piles which interlock with each other instead of making merely a butt joint. (See Fig. 47.) The simplest form is the ordinary tongue-and-groove joint similar to that of matched boarding. A development of this in timber sheet piling is a combination of three planks which are so bolted together as to make a largescale tongue and groove on each side. The increasing cost of timber, and the large percentage of deterioration and destruction during its use for a single cofferdam, have developed the manufacture of steel sheet piling, which can be redrawn and used many times. The forms of steel for sheet piling are nearly all patented. The cross-sections of a few of them are shown in Fig. 44. One feature of some of the designs is the possible flexibility secured in the outline of the dam without interfering with the water-tightness.

Fig. 41. Types of Sheet-Steel Piling.

Fig. 45. Screw Pile.

Fig. 46. Disc Pile.

Sheet piling is usually driven in close contact (as shown in Fig. 48), either to prevent leakage, or to confine puddle in cofferdams, to prevent the materials of a foundation from spreading, or to guard a foundation from being undermined by a stream of water. To make wooden piles drive with their parts close against each other, they are cut obliquely at the bottom, as shown in Fig. 49. They are kept in place while being driven, by means of two longitudinal stringers or wales. These wales are supported by gauge-piles previously driven, which are several feet apart. Sheet piling is seldom used as bearing piles.

Fig. 47. Lapped Sheet-Piling.

Fig. 48. Single and Sheet Piling Plan View.