The straightest-grained timber should be selected, the bark removed, and any rough projections smoothed off; all large knots should be avoided, and diagonal knots especially are a source of danger, as a pile is very likely to be broken off at the point where they occur.
Piles should, if possible, be of whole timbers, and driven with the butt, or natural lower end, downwards.
The lower end should be pointed, and if it has to encounter stony or hard ground should be shod with iron.
Fig. 399 shows an ordinary form of wrought-iron shoe, and Fig. 400 an improved form, in which the lower portion C is of cast-iron, forming a good wide abutment for the timber, which tends to prevent the danger of its being crushed as the pile is driven. The wrought-iron pins, a a, are cast into the portion C, and their heads hammered out like a rivet to secure the straps s s.
Bearing Piles are driven down either until they reach a hard stratum, or until the friction on their sides prevents them from sinking, upon which they are used as pillars to support a platform of timber.
Such piles, if of wood, should be whole timbers from 9 to 18 inches in diameter, and if they are in soft soil their length should never be more than about twenty times their diameter, or there will be danger of their bending when driven.
These piles are only from 6 to 12 feet long - of round timber about 6 inches in diameter. They should be driven as close together as is possible without the driving of one pile causing the others to rise; to prevent this, it is found necessary to place them at intervals of about 2 feet 6 inches from centre to centre.
Sheeting Piles are used to enclose the areas of a foundation, and thus prevent the soil from spreading laterally, or to protect it from the action of water.
Sheet piles are flat planks, varying in width, and from 3 to 10 inches thick. They are sometimes grooved and tongued down their edges so as to form a tight joint, and sharpened to an edge at the lower end which may be shod with iron.
In using sheet piling to enclose soft ground long "guide piles," about 6 to 10 feet apart, are first driven in the direction required.
On opposite sides of these are fixed beams ("string pieces" or "wales") at a horizontal distance apart just equal to the thickness of the sheet piles, which are driven down between them, commencing at the guide piles, and working inwards in each bay, so that the last sheet pile driven acts as a wedge and tightens up the whole.
After the piles are driven, and their heads sawn off level, a timber platform is generally laid upon them.
This consists of heavy square balks, called string pieces and cross pieces, notched into one another so as to form a grating or "grillage." The string pieces are notched over the heads of the piles, and secured to them by trenails.
The ground between the piles is often taken out to a depth of 3 or 4 feet, and the space filled with concrete. The intervals between the timbers of the platform are sometimes similarly filled in, and in some cases a bed of concrete is substituted for the platform altogether.
Fig. 401 is an illustration of a portion of pile foundation for a thick wall. P P P are the piles (shod in different ways), S S the string pieces, and C C the cross pieces. The platform Y Y is composed of Yorkshire landings 6 inches thick.
A portion of the foundation is secured by sheet piling, S P, driven between the waling, W, and the outer cross piece of the grillage.
A disadvantage in the string pieces and cross pieces is that the heads of the piles, bearing upon their sides, bend and crush into the longitudinal fibres, indenting the timber, and causing it to sink down upon the pile heads. Where there is a really good strong bed of concrete the string and cross pieces can, with advantage, be omitted; in fact, in many cases a good broad and deep bed of solid concrete enables the use of the piles themselves to be dispensed with altogether.
Pile foundations are liable to fail, from the softness of the ground being such that it does not offer sufficient resistance to a lateral movement, in consequence of which the piles lose their original position, and the wall has a tendency to upset.
Wooden piles are sure to be destroyed by rot in any position where they are alternately wet and dry.
If used in sea water they are liable to attacks from worms, by which they are soon destroyed. These attacks can best be delayed by completely charging the pores of the timber with creosote (see Part III.), or they may be prevented for a time by covering the surface of the timber with scupper nails driven close together, or, at a great cost, by sheathing the pile with copper. After a time, however, the worm manages to get under the nails or sheathing, and to eat the wood completely away, leaving an apparently sound but entirely hollowed pile. (For an account of the worms, etc., see Part III.)
Partial piling provided under a portion only of a wall is most dangerous, as it leads to unequal settlement, by which the wall may be fractured.
In a wall with buttresses the unequal weight on the piles has led to failure.
Irox Piles have been introduced to avoid the natural defects of those made of timber.
Cast-iron Piles have been used of various cross sections, such as square, round, hollow, and cross shaped.
In driving them a block of wood or "dolly" must be interposed between them and the monkey, for fear of breaking the pile by the sudden shock.
They have a disadvantage for the foundations of buildings, inasmuch as they cannot be cut off to a level at the top.
Cast-iron sheet piling has been extensively used; it consists generally of flat plates, stiffened by vertical ribs, and furnished with overlapping edges. The guide piles may be of the same construction, square or semicircular in cross section.
In these the pile itself may be of timber, or a cylinder of cast or wrought iron.
It is furnished at the lower end with a short and broad cast-iron screw blade, which is twisted (Fig. 402) round under pressure so that it enters the ground, from which a great force would be required to withdraw it.
The best way of driving these piles is by attaching long radiating levers to the upper end, and turning them round by means of animals moving on a temporary platform.