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Cofferdams 197

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COFFERDAMS 197. Types of Cofferdams.—A cofferdam is a structure intended to exclude water and soft materials from an inclosed area, in order to permit the water to be pumped out and the work of placing a foundation to be done in the open air. This method is applicable only to rather shallow' foundations, and for depths greater than about. 30 feet other methods are more economical. Cofferdams can be used only where the soil at the bottom is fairly impervious, so that an excessive flow of water under the dam does not occur.

The type of structure for this purpose varies with the depth of the foundation and the character of the soil upon which it is to he built. Earth, sheet piling, timber cribs, or combinations of these arranged to meet special conditions, are the materials employed.

Earth cofferdams are banks of earth surrounding the area of the foundation, and are made thick enough to sustain the pressure of the water and to prevent excessive leakage into the inclosed space. The use of plain earth clams for this purpose is limited to shallow water without currents; where danger of washing from a light current exist:, a wall of bags filled with clay and gravel or ati revetment of such bags upon the exposed face of the embankment may be em ployed. The top of the dam should he at least 2 feet above the water surface, and the top width not less than 3 feet. A row of sheet piling is sometimes driven and inclosed in an earth dam for the purpose of reducing the size of embankment needed, or of cutting off a flow of water through the soil under the dam.

Sheet-pile cofferdams are constructed either of timber or steel piles in single or double rows, and are supported by guide piles, timber frames, or cribs. Where a double row of sheet piling is used, a filling of earth between the rows is necessary.

A crib cofferdam consists of a timber crib built so as to be water tight and is floated into place and sunk around the site of the foun dation.

Movable cofferdams which may be removed after using and sunk again have been employed in a number of instances. These may be cribs with watertight compartments, or framework supporting sheet piling.

198. Sheet-Pile Cofferdams.—When timber sheet-piling is used the most common form of cofferdam consists of two rows of piles with a filling of puddled earth between them—a system of tion shown in Fig. 118. Two rows

of guide piles are first driven. Horizontal timbers known as walls are attached to these, and the sheet-piling driven inside against the wales, the tops of the guide piles being tied together to prevent spreading when the puddle is put in. The guide piles should be driven to a firm bearing in order to develop the transverse strength of the pile in resisting the water pressure. Horizontal braces across the area to be drained may sometimes be used to assist the cofferdam against lateral pressure, and when this is not feasible, the width of cofferdam must be made sufficient to provide lateral strength.

The sheet-piling must be driven into a fairly impervious stratum to prevent leakage under the dam, and pervious material overlying such stratum between the rows should be excavated sufficiently to give the puddle contact with the impervious material below. The puddle needs to be both impervious and stable, and a mixture of gravel and clay is desirable for the purpose. Clay is impervious but washes easily if the water finds an opening through it, while gravel or coarse sand mixed with the clay tends to prevent such washing. The thick ness of puddle required depends upon its quality and upon the pres sure to be resisted, a thickness of one-fourth to one-sixth of the depth being usually sufficient. For best results, the puddle should be placed in thin layers and well tamped in damp condition.

A single wall of sheet-piling is often used supported by guide piles or by an interior framework—a method which require.; less space than the puddle wall type and is preferable where it is important not to restrict the water way. Plan of a cofferdam of this type for use in constructing a bridge pier is shown in Fig. 119. The guide piles are first driven, wales attached, and the sheet-piles driven against the outside waling. Braces from wall to wall across the opening are used to assist in resisting the lateral pressure. Such bracing when needed may be placed at lower levels as the water is pumped out and excavation proceeds.

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