FIXED DAMS. 201 



in more than one case serious damage has resulted from their use. It should always 

 be remembered that the vulnerable part of a fixed dam or of a weir is its down-stream 

 side, and as much care must be taken in protecting it as in securing the up-stream side. 



Where the dam is to be upon a pile foundation, in whole or in part, a pile is pro- 

 vided at each of the intersections of the crib timbers, and one row close together along 

 the down-stream face, unless an apron crib is used. 



Details of Construction. The crib dam of modern design is usually built of 

 sawn timbers, 10 inches to 12 inches square, laid crosswise so as to form pens 8 to 12 

 feet apart, and filled with riprap. The stone is usually of small sizes and irregular 

 shapes, called "one-man stone," but sometimes these are taken out in large blocks as 

 blasted in the quarry, and placed with a derrick. Limestone and sandstone are both 

 utilized, but the latter wears rapidly through the action of water in .dams which have 

 leaks through the upper face or spaces in decking. The cost or inaccessibility of stone 

 may sometimes render it necessary to use gravel for filling the cribs, but this usually 

 contains so many small particles that will be washed away that its use is not desirable. 



The timber generally used is white oak or yellow pine, but any of the heavier 

 woods may be employed, if under the low-water line, or where they will always be moist. ' 

 It may be either sawn, hewn, or left in its natural state so far as the cribs themselves 

 are concerned, but the deck or floor covering the whole structure should be either 

 sawn or hewn. At the present time hewn and round timbers are very rarely 

 used, as sawn timbers can usually be obtained for a little extra price, and are much 

 preferable for rapidity of construction. The pieces may be laid directly upon each 

 other, or daps may be cut at their intersections, at which points they are drift-bolted. 

 The former method, however, is as satisfactory in practice as the latter, and less 

 expensive. 



The timbers which lie in a direction across the stream are called stringers; those 

 which have a direction parallel to the current are called ties. Both sets of timbers 

 should have a length as great as practicable, economy being considered, and the 

 joints may either butt or be spliced. If the former, a block of similar section to the 

 stringer or ties should support the joint and lap onto each timber from i' 6" to 2 feet, 

 drift-bolts being used to connect the block with the stick below as well as with the one 

 above. These joints should not appear immediately over each other in two suc- 

 ceeding layers of timber. If the joint is spliced, it may come at the intersection of 

 the timbers with those lying in the opposite direction; if it does not, then it should 

 also be supported by a block, although this is rarely done. The drift-bolts used are 

 usually |" or f" square, depending on the size of the timbers, with heads and with 

 wedge points. Round or nail points should not be used, as they split the timbers, 

 and square drift-bolts are preferable to round ones as they hold better. 



Both the up-stream and down-stream sides, known as the back and face of the 

 dam respectively, are carried up vertically, the former being covered with a single or 

 double row of sheet-piling, driven as deep as possible, and which should extend to the 



