serving as supports for the sheeting piles, must be on the sides of the uprights towards the puddling, and their faces in the same Fig. 23-Represents a section of the coffer-dam used for the Potomac aqueduct. a, main exterior piles. b, strong square beams corresponding to a on which the wales n, n are. notched and bolt ed. sheeting piles. e, cross pieces.. vertical plane. Between each pair of opposite uprights, horizontal shores may be placed at the points opposite the position of the string pieces, to increase the resistance of the dam to the pressure of the water; the top shores extending entirely across the dam, and being notched on the top string pieces. The interior shores must be so arranged that they can be readily taken out as the masonry on the interior is built up, replacing them by other shores resting against the masonry itself. 397. When the bed of a river presents a rocky surface, or rock covered with but a few feet of mud, or loose soil, cases may occur in which it will be more economical and equally safe to lay a bed of beton without exhausting the water from the area to be built on; enclosing the area, before throwing in the beton, by a simple coffer work formed of a strong frame work of uprights and horizontal beams and sheeting piles. The frame work (Fig. 24) in this case is composed of uprights connected by string pieces in pairs; each pair is notched and bolted to the uprights, a sufficient interval being left between them for the insertion of the sheeting piles. To secure the frame work to the rock, it may be requisite to drill holes in the rock to receive the foot of each upright. The holes may be drilled by means of a long iron bar, termed a jumper, which is used for this purpose, or else the ordinary diving bell may be employed. This machine is very serviceable in all similar constructions where an examination of work under water is requisite, or in cases where it is necessary to lay masonry under water. The frame work is put together on land, floated to its position, and settled upon the rock; the sheeting piles are then driven into close contact with the surface of the rock. 398. The convenience and economy resulting from the use of beton for the beds of structures raised in water, have led General Treussart to propose a plan for laying beds of this material, and then to take advantage of their strength and impermeability to construct a coffer-dam upon them, in order to carry on the superstructure with more care. To effect this, the space to be occupied by the bed (Fig. 25) is first enclosed by square piles, driven in juxtaposition and secured at top by a string piece. The mud and loose soil are then scooped from the enclosed area to the firm soil on which the bed of beton is to be laid. The bed of beton is next laid with the usual precautions, and while it is still soft a second row of square piles is driven into it, also in juxtaposition, and at a suitable distance from the first for the thickness of the dam ; for detaching the sides, is effected in the following manner. Strong hooks of wrought iron are fixed to the bottom of the Fig. 26-Represents a cross sec- b, square uprights to sustain c, cross pieces resting on b. e, longitudinal beams to stay A, section of the masonry. caisson at the sides of the capping piece, corresponding to the points where the uprights of the sides are inserted into this piece. Pieces of strong scantling are laid across the top of the caisson, resting on the opposite uprights, upon which they are notched. These cross pieces project beyond the sides, and the projecting parts are perforated by an auger-hole, large enough to receive a bolt of two inches in diameter. The object of these cross pieces is twofold; the first is to buttress the sides of the caisson at top against the exterior pressure of the water; and the second is to serve as a point of support for a long bolt, or rod of iron, with an eye at the lower end, into which the hook on the capping piece is inserted, and a screw at top, to which a nut, or female screw is fitted, and which, resting on the cross pieces as a point of support, draws the bolt tight, and, in that way, attaches the sides and bottom of the caisson firmly together. A bed is prepared to receive the bottom of the caisson, by levelling the soil on which the structure is to rest, if it be of a suitable character to receive directly the foundation; or by driving large piles through the upper compressible strata of the soil to the firm stratum beneath. The heads of the piles are sawed off on a level to receive the bottom of the caisson. To settle the caisson on its bed, it is floated to and moored over it; and the masonry of the structure is commenced and carried up, until the weight grounds the caisson. The caisson should be so contrived, that it can be grounded, and afterwards raised, in case that the bed is found not to be accurately levelled. To effect this, a small sliding gate should be placed in the side of the caisson, for the purpose of filling it with water at pleasure. By means of this gate, the caisson can be filled and grounded, and, by closing the gate and pumping out the water, it can be set afloat. After the caisson is settled on its bed, and the masonry of the structure is raised above the surface of the water, the sides are detached, by first unscrewing the nuts and detaching the rods, and then taking off the top cross pieces. By first filling the caisson with water, this operation of detaching the sides can be more easily performed. 400. To adjust the piles before they are driven, and to prevent them from spreading outward by the operation of driving, a strong grating of heavy timber, formed by notching cross and longitudinal pieces on each other, and fastening them firmly together, may be resorted to. This grating is arranged in a similar manner to a grillage; only the square compartments, between the cross and string pieces, are larger, so that they may enclose an area for 4 or 9 piles; and, instead of a single row of cross pieces, the grating is made with a double row, one at top, the other at the bottom, embracing the string pieces on which they are notched. The grating may be fixed in its position at any depth under water, by a few provisional piles, to which it can be attached. 401. Where the area occupied by a structure is very considerable, and the depth of water great, the methods which have thus far been explained cannot be used. In such cases, a firm bed is made for the structure, by forming an artificial island of loose heavy blocks of stone, which are spread over the area, and receive a batter of from one perpendicular to one base, to one perpendicular and six base, according to the exposure of the bed to the effects of waves. This bed is raised several feet above the surface of the water, according to the nature of the structure, and the foundation is commenced upon it. 402. It is important to observe, that, where such heavy masses are laid upon an untried soil, the structure should not be commenced before the bed appears entirely to have settled; nor even then, if there be any danger of further settling taking place from the additional weight of the structure. Should any doubts arise on this point, the bed should be loaded with a provisional weight, somewhat greater than that of the contemplated structure, and this weight may be gradually removed, if composed of other materials than those required for the structure, as the work progresses. 403. To give perfect security to foundations in running water, the soil around the bed must be protected to some extent from the action of the current. The most ordinary method of effecting this, is by throwing in loose masses of broken stone of sufficient size to resist the force of the current. This method will |