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VI.

FOUNDATIONS OF STRUCTURES IN WATER.

444. In laying foundations in water, two difficulties have to be overcome, both of which require great resources and care on the part of the engineer. The first is found in the means to be used in preparing the bed of the foundation; and the second in securing the bed from the action of water, to insure the safety of the foundations. The last is generally the more difficult problem of the two; for a current of water will gradually wear away, not only every variety of loose soils, but also the more tender rocks, such as most varieties of sandstone, and the calcareous and argillaceous rocks, particularly when they are stratified, or are of a loose texture.

445. To prepare the bed of a foundation in stagnant water the only difficulty that presents itself is to exclude the water from the area on which the structure is to rest. If the depth of water is not over 4 feet, this is done by surrounding the area with an ordinary water-tight dam of clay, or of some other binding earth. For this purpose, a shallow trench is formed around the area, by removing the soft or loose stratum on the bottom; the foundation of the dam is commenced by filling this trench with the clay, and the dam is made by spreading successive layers of clay about one foot thick, and pressing each layer as it is spread to render it more compact. When the dam is completed, the water is pumped out from the enclosed area, and the bed for the foundation is prepared as on dry land.

446. When the depth of stagnant water is over 4 feet, and in running water of any depth, the ordinary dam must be replaced by the coffer-dam. This construction consists of two rows of plank, termed sheeting piles, driven into the soil vertically, forming thus a coffer-work, between which clay or binding earth, termed the puddling, is filled in, to form a water-tight dam to exclude the water from the area enclosed.

The arrangement, construction, and dimensions of cofferdams depend on their specific object, the depth of water, and the nature of the subsoil on which the coffer-dam rests.

With regard to the first point, the width of the dam between the sheeting piles should be so regulated as to serve as a scaffolding for the machinery and materials required about the work. This is peculiarly requisite where the coffer-dam en

closes an isolated position removed from the shore. The interior space enclosed by the dam should have the requisite capacity for receiving the bed of the foundations, and such materials and machinery as may be required within the dam.

The width or thickness of the coffer-dam, by which is understood the distance between the sheeting piles, should be sufficient not only to be impermeable to water, but to form, by the weight of the puddling, in combination with the resistance of the timber-work, a wall of sufficient strength to resist the horizontal pressure of the water on the exterior, when the interior space is pumped dry. The resistance offered by the weight of the puddling to the pressure of the water can be easily calculated; that offered by the timber-work will depend upon the manner in which the framing is arranged, and the means taken to stay or buttress the dam from the enclosed space.

The most simple and the usual construction of a coffer-dam

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(Fig. 47) consists in driving a row of ordinary straight piles around the area to be enclosed, placing their centre lines about 4 feet asunder. A second row is driven parallel to the first, the respective piles being the same distance apart; the distance between the centre lines of the two rows being so regulated as to leave the requisite thickness between the sheeting piles for the dam. The piles of each row are connected by a horizontal beam of square timber, termed a string or wale piece, placed a foot or two above the highest water line, and

notched and bolted to each pile. The string pieces of the inner row of piles are placed on the side next to the area enclosed, and those of the outer row on the outside. Cross beams of square timber connect the string pieces of the two rows upon which they are notched, serving both to prevent the rows of piles from spreading from the pressure that may be thrown on them and as a joisting for the scaffolding. On the opposite sides of the rows interior string pieces are placed, about the same level with the exterior, for the purpose of serving both as guides and supports for the sheeting piles. The sheeting piles being well jointed are driven in juxtaposition, and against the interior string pieces. A third course of string or ribbon pieces of smaller scantling confine, by means of large spikes, the sheeting piles against the interior string pieces.

As has been stated, the thickness of the dam and the dimensions of the timber of which the coffer-work is made will depend upon the pressure due to the head of water, when the interior space is pumped dry. For extraordinary depths, the engineer would not act prudently were he to neglect to verify by calculation the equilibrium between the pressure and resistance; but for ordinary depths under 10 feet, a rule followed is to make the thickness of the dam 10 feet; and for depths over 10 feet to give an additional thickness of one foot for every additional depth of three feet. This rule will give every security against filtrations through the body of the dam, but it might not give sufficient strength unless the scantling of the coffer-work were suitably increased in dimensions.

In very deep tidal water, coffer-dams have been made in offsets, by using three rows of sheeting piles for the purpose of giving greater thickness to the dam below the low-water level. In such cases strong square piles closely jointed and tongued and grooved, should be used in place of the ordinary sheeting piles.

Besides providing against the pressure of the head of water, suitable dimensions must be given to the sheeting piles, in order that they may sustain the pressure arising from the puddling when the interior space is emptied of water. This pressure against the interior sheeting piles may be further increased by that of the exterior water upon the exterior sheeting piles, should the pressure of the latter be greater than the former. To provide more securely against the effect of these pressures, intermediate string pieces may be placed against the interior row of piles before the sheeting piles are driven; and the opposite sides of the dam on the interior may

be buttressed by cross pieces reaching across the top string pieces, and by horizontal beams placed at intermediate points between the top and bottom of the dam.

The main inconvenience met with in coffer-dams arises from the difficulty of preventing leakage under the dam. In all cases the piles must be driven into a firm stratum, and the sheeting piles should equally have a firm footing in a tena cious compact substratum. When an excavation is requisite on the interior, to uncover the subsoil on which the bed of the foundation is to be laid, the sheeting piles should be driven at least as deep as this point, and somewhat below it if the resistance offered to the driving does not prevent it.

The puddling should be formed of a mixture of tenacious clay and sand, as this mixture settles better than pure clay alone. Before placing the puddling, all the soft mud and loose soil between the sheeting piles should be carefully extracted; the puddling should be placed in and compressed in layers, care being taken to agitate the water as little as practicable.

With requisite care coffer-dams may be used for foundations in any depth of water, provided a water-tight bottoming can be found for the puddling. Sandy bottoms offer the greatest difficulty in this respect, and when the depth of water is over 5 feet, extraordinary precautions are requisite to prevent leakage under the puddling.

When the depth of water is over 10 feet, particularly where the bottom is composed of several feet of soft mud, or of loose soil, below which it will be necessary to excavate to obtain a firm stratum for the bed of the foundation, additional precautions will be requisite to give sufficient support to the interior sheeting piles against the pressure of the puddling, to provide against leakage under the puddling, and to strengthen the dam against the pressure of the exterior water, when the interior space is pumped dry and excavated. The best means for these ends, when the locality will admit of their application, is to form the exterior of the dam, as has already been described, by using piles and sheeting piles, giving to the latter additional points of support, by intermediate string pieces between the one at top and the bottom of the water; and to form a strong framing of timber for a support to the interior sheeting piles, giving to it the dimensions of the area to be enclosed. The framework (Fig. 48) may be composed of upright square beams, placed at suitable distances apart, depending on the strength required, upon which square string pieces are bolted at suitable distances from the top to the

bottom, the bottom string resting on the surface of the mud. The string pieces, serving as supports for the sheeting piles, must be on the sides of the uprights towards the puddling, and their faces in the same vertical plane. Between each

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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.

447. Caisson and Cribwork Coffer-dams. In the construction of the foundations for the piers and abutments of the Victoria tubular iron railroad bridge over the river Saint Lawrence, at Montreal, the engineers had to contend against unusual difficulties; in a rocky bottom covered with boulders, which prevented the use of piles; and in a swift current, bringing down in the spring of the year enormous fields of ice, the effects of which none of the ordinary methods of caisson or coffer-dams could have withstood.

These difficulties were successfully met, in some cases by the use of a large water-tight caisson, shown in plan (Fig. 49), and in cross-section (Fig. 51), of such a form and dimensions as to leave a sufficient interior area, between its interior sides,

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