pins, or tree-nails driven into an auger-hole, made through the cap into the head of each pile. After the cap is fitted, longitudinal beams, termed string pieces, are laid lengthwise on the heads of each row, and rest at each extremity on the cap, to which they are fastened by a dove-tail joint and a wooden pin. Another series of beams, termed cross pieces, are laid crosswise on the string pieces, over the heads of each row of piles. The cross and string pieces are connected by a notch cut into each, so that, when put together, their upper surfaces may be on the same level, and they are fastened to the heads of the piles in the same manner as the capping. The extremities of the cross pieces rest on the capping, and are connected with it, like the string pieces.

The platform is of thick planks laid over the grillage, with the extremity of each plank resting on the capping, to which, and to the string and cross pieces, the planks are fastened by nails.

The capping is usually thicker than the cross and string pieces by the thickness of the plank; when this is the case, a rabate, about four inches wide, must be made on the inner edge of the capping, to receive the ends of the planks.

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388. An objection is made to the platform as a bed for the foundation, owing to the want of adhesion between wood and mortar; from which, if any unequal settling should take place, the foundations would be exposed to slide off the platform. obviate this, it has been proposed to replace the grillage and platform by a layer of beton resting partly on the heads of the piles, and partly on the soil between them. This means would furnish a firm bed for the masonry of the foundations, devoid of the objections made to the one of timber.

To counteract any tendency to sliding, the platform may be inclined if there is a lateral pressure, as in the case, for example, of the abutments of an arch.

389. In soils of alluvial formation, it is common to meet with a stratum of clay on the surface, underlaid with soft mud, in which case, the driving of short piles would be injurious, as the tenacity of the stratum of clay would be destroyed by the operation. It would be better not to disturb the upper stratum in this case, but to give it as much firmness as possible, by ramming it with a heavy beetle, or by submitting it to a heavy

pressure.

390. Piles and sheeting piles of cast iron have been used with complete success in England, both for the ordinary purposes of cofferdams, and for permanent structures for wharfing. The piles have been cast of a variety of forms; in some cases they have been cast hollow for the purpose of excavating the soil within the pile as it was driven, and thus facilitate its penetration

into the subsoil. Fig. 15 represents a cross section of one of the more recent arrangements of iron piles and sheeting piles.

Fig. 15-Represents a horizontal section of an arrangement of piles and sheeting piles of cast iron.

a, sheeting pile with a lap e to cover the joint between it and the next sheeting pile.

b, piles with a lap on each side.

c, sheeting pile lapped by pile and sheeting pile next it.

d, ribs of piles and sheeting piles.

391. Sand has also been used with advantage to form a bed for foundations in a very compressible soil. For this purpose a trench is (Fig. 16) excavated, and filled with sand; the sand being spread in layers of about 9 inches, and each layer being firmly settled by a heavy beetle, before laying the next. If water

should make rapidly in the trench, it would not be practicable to pack the sand in layers. Instead, therefore, of opening a trench,

should be made, by means of a short pile, as close together as practicable; when the pile is withdrawn from the hole, it is immediately filled with sand. To cause the sand to pack firmly, it should be slightly moistened before placing it in the holes, or trench.

Sand, when used in this way, possesses the valuable property of assuming a new position of equilibrium and stability, should the soil on which it is laid yield at any of its points. Not only does this take place along the base of the sand bed, but also along the edges, or sides, when these are enclosed by the sides of the trench made to receive the bed. This last point offers also some additional security against yielding in a lateral direction. The bed of sand must, in all cases, receive sufficient thickness to cause the pressure on its upper surface to be distributed over the entire

base.

392. When, from the fluidity of the soil, the vertical pressure of the structure causes the soil to rise around the bed, this action may be counteracted, either by scooping out the soil to some depth around the bed and replacing it by another of a more compact nature, well rammed in layers, or with any rubbish of a solid character; or else a mass of loose stone may be placed over the surface exterior to the bed, whenever the character of the structure will warrant the expense.

393. Precautions against Lateral Yielding. The soils which have been termed compressible, strictly speaking, yield only by the displacement of their particles either in a lateral direction, or upward around the structure laid upon them. Where this action arises from the effect of a vertical weight, uniformly distributed over the base of the bed, the preceding methods for giving permanent stability to structure, present all requisite security. But when the structure is subjected also to a lateral pressure, as for example, that which would arise from the action of a bank of earth resting against the back of a wall, additional means of security are demanded.

One of the most obvious expedients in this case, is to drive a row of strong square piles in juxtaposition immediately in contact with the exterior edges of the bed. This expedient is, however, only of service where the piles attain either an incompressible soil, or one at least firmer than that on which the bed immediately rests. For otherwise, as is obvious, the piles only serve to transmit the pressure to the yielding soil in contact with them. But where they are driven into a firm soil below, they gain a fixed point of resistance, and the only insecurity they offer is either by the rupture of the piles, from the cross strain upon them, or from the yielding of the firm subsoil, from the same

cause.

In case the piles reach a firm subsoil, it will be best to scoop out the upper yielding soil before driving the piles, and to fill in between and around them with loose broken stone, (Fig. 18.) This will give the piles greater stiffness, and effectually prevent them from spreading at top.

When the piles cannot be secured by attaining a firm subsoil, it will be better to drive them around the area at some distance from the bed, and, as a farther precaution, to place horizontal buttresses of masonry at regular intervals from the bed to the piles. By this arrangement, some additional security is gained from the counter-pressure of the soil enclosed between the bed and the wall of piles. But it is obvious that unless the piles in this case are driven into a firmer soil than that on which the structure rests, there will still be danger of yielding.

In using horizontal buttresses, the stone of which they are constructed should be dressed with care; their extremities near the wall of piles should be connected by horizontal arches, (Fig. 19,) to distribute the pressure more uniformly; and where there is an upward pressure of the soil around the structure, arising from its weight, the buttresses ought to be in the form of reversed arches.

In buttresses of this kind, as likewise in broad areas resting on a very yielding soil, since as much danger is to be apprehended from their breaking by their own weight as from any other cause, it must be carefully guarded against. Something may be done for this purpose by ramming the earth around the structure with a heavy beetle, when it can be made more compact by this means; or else a part of the upper soil may be removed, and be replaced by one of a more compact nature which may be rammed in layers

The following methods, where they can be resorted to, and where the character of the structure will justify the expense, have been found to offer the best security in the case in question.

When the bed can be buttressed in front with an embankment, a low counter-wall (Fig. 20) may be built parallel to the edge of the bed, and some 10 or 12 feet from it; between this wall and the bed a reversed arch connecting the two may be built, and a surcharge of earth of a compact character and well rammed, may be placed against the counter wall to act by its counter pressure against the lateral pressure upon the bed.

When the bed cannot be buttressed in front, as in quay walls, a grillage and platform supported on piles (Fig. 21) may be built to the rear from the back of the wall, for the purpose of supporting the embankment against the back of the wall, and preventing the effect which its pressure on the subsoil might have in thrusting forward the bed of the foundation.

In addition to these means, land ties of iron will give great ad