A rod, r, connects the tops of the opposite main piles to prevent spreading when the puddle is put in. The timber, t, is put on primarily to carry the footway, f, and is sometimes notched over, or otherwise fastened to, the pieces w, w, to prevent the puddle space from spreading. b and b are braces extending from one side of the coffer-dam to the other. These braces are put in position successively from the top as the water is pumped out; and as the masonry is built up, they are removed and the sides of the dam braced by short struts resting against the pier.
The resistance to overturning is derived principally from the main piles, m, m. The distance apart and also the depth to which they should be driven depend upon the kind of bottom, the depth of water, and the danger from floating ice, logs, etc. Rules and formulas are here of but little use, judgment and experience being the only guides. The distance between the piles in a row is usually from 6 to 10 feet.
The dimensions of the sheet piles (1 807) employed will depend upon the depth and the number of longitudinal waling pieces used. Two thicknesses of ordinary 2-inch plank are generally em ployed. Sometimes, for the deeper dams, the sheet piles are timbers 10 or 12 inches square.
The thickness of the dam will depend upon (1) the width of gang way required for the workmen and machinery, (2) the thickness required to prevent overturning, and (3) the thickness of puddle necessary to prevent leakage through the wall. The thickness of shallow dams will usually be determined by the first consideration; but for deep dams the thickness will be governed by the second or third requirement. If the braces, b, b, are omitted, as is sometimes done for greater convenience in working in the coffer-dam, then the main piles, m, m, must be stronger and the dam wider in order to resist the lateral pressure of the water.
Thickness of Puddle Wall. An old rule for the thickness of the puddle wall, which is frequently quoted, is: "For depths of less than 10 feet make the width 10 feet, and for depths over 10 feet give an additional thickness of 1 foot for each additional 3 feet of wall." Another rule, also frequently quoted, is: "Make the thickness of the puddle wall three fourths of its height, but in no case is the wall to be less than 4 feet thick." Judged by ordinary experience both of the above rules are extravagant, for numerous coffer-dams from 20 to 30 feet deep have been built in which the thickness of the puddle varied from 3 to 5 feet, or say one sixth of the depth. Of course, the
thickness of the puddle wall should vary with the tightness of the sheet piling and the imperviousness of the puddle 812); and under ordinary conditions a thickness of one sixth to one quarter of the depth is sufficient.
The Puddle. The puddle should consist of impervious soil, of which gravelly clay is best. It is a common idea that clay alone, or clay and fine sand, is best. With pure clay, if a thread of water ever so small finds a passage under or through the puddle, it will steadily wear a larger opening. On the other hand, with gravelly clay, if the water should wash out the clay or fine sand, the larger particles will fall into the space and intercept first the coarser sand, and next the particles of loam which are drifting in the current of water; and thus the whole mass puddles itself better than the engineer could do it with his own hands. An embankment of gravel is comparatively safe, and becomes tighter every day; while a clay embankment may be tighter at first than a gravelly one, it is always liable to breakage.
Before putting in the puddling, all soft mud and loose soil should be removed from between the rows of sheet piles, for the most com mon cause of trouble with puddle-wall coffer-dams is a leak between the natural surface and the puddle.
The puddling should be deposited in layers, and compacted as much as is possible without causing the sheet piles to bulge so much as to open the joints.