CONSTRUCTION OF THL D. The construction of coffer dams varies greatly. In still shallow water, a well-built bank of clay and gravel is sufficient. If there is a slow current, a wall of bags partly filled with clay and gravel does fairly well; and a row of cement barrels filled with gravel and banked up on the outside has been used. If the water is too deep for any of the above methods, a single or double row of plank may be driven and banked up on the outside with a deposit of impervious soil sufficient to prevent leaking. If there is much of a current, the puddle on the outside will be washed away; or, if the water is deep, a large quantity of material will be required to form the puddle-wall; and hence the preceding simple methods are inapplicable where there is much current or where the water is more than 3 or 4 feet deep.
The more elaborate coffer-dams may consist of a wall of either wood or steel sheet piles, or of two rows of sheet piles with a puddle wall between them, or of a timber crib.
Wood For shallow depths the sheet piles may be simply plank, and for greater depths either thick tongued and-grooved plank or Wakefield piles (* 747). Sheet piles should be sharpened wholly from one side, and the long edge should be placed next to the last pile driven to cause the piles to crowd together and make closer joints. In hard soil at small depths, or in soft soil at moderate depths, the sheeting may be driven by hand with a wooden maul or iron sledge; but for any considerable depth a power pile-driver must be employed, although sometimes where only a few piles are to be driven a hand pile-driver is rigged up with a block of wood for a hammer. The sheeting should be driven at least a foot or two below the lowest excavation inside of the dam; and in soft soil the sheet piles should be driven at least 3 or 4 feet below the proposed excavation to prevent leakage under the bottom.
If the sheet piles are to resist a head of more than 4 or 5 feet of water or a semi-fluid soil, their tops should be supported by wales (horizontal timbers on the inside of the dam against the top of the sheeting), which in turn are braced at their ends by the wales on the ad jacent sides of the dam or at intermediate points by horizontal timbers across the dam; and in deep dams similar wales and cross braces are inserted at vertical intervals as progresses. Some times, in comparatively shallow water and with a suitable bottom, the waling pieces are supported by ordinary bearing piles driven inside of the dam, thus eliminating the braces across the dam, and therefore facilitating the excavation and the laying of the masonry.
Sometimes the top and the bottom waling pieces are framed together respectively, and the upper and lower waling frames are separated from each other by small vertical posts placed between them and joined to them. This frame is sunk in the desired position, and the sheet piles are driven around it.
The thickness of the sheet piling required in any particular case is usually a matter of judgment based upon past experience; but the strength required can be closely approximated by regarding the sheet pile as a beam either fixed at one end and free at the other, or as supported at both ends. The amount of the lateral pressure against the pile is one half of the continued product of the weight of a cubic foot of water, the width of the pile in feet, and the depth of the water in feet; and the point of application of this pressure is two thirds of the depth of the water from the top. Of course, the weight of liquid mud is more than that of water; but extreme accuracy is impossible, and hence the above method is probably sufficient for the purpose.