350 LOGGING 



Timber dams on small streams usually have a sluiceway 

 through which logs are run and waste water passed, while on 

 large streams several waste gates are required to take care of 

 surplus water. ''Roll dams" which have no gates or sluice- 

 ways are also built to raise the stream level. The water and 

 logs pass over the crest of the dam. 



Crih Dams. — The crib dam is a common form and is so- 

 called because the buttresses and wings are built of cribs usually 

 filled with stone to hold them down. The necessity for the 

 use of stone is determined by the head of water carried and 

 by the size of timber used in construction. Crib dams are 

 made from round timber hewed on two sides, or from squared 

 timber. 



The foundation of a crib dam must be solid and, whenever 

 possible, it is built on bedrock; but if this cannot be done the 

 foundation may rest on piles driven into hard clay or to bed- 

 rock. If this is impossible, a row of 3 -inch plank or small hewed 

 poles sharpened on one end are driven in a row across the stream 

 channel just above the upstream mud-sill. These planks and 

 timbers are called toe-spiling. 



If there is much water on the stream bed it is diverted to one 

 side by temporary dams made of sand bags or by the construc- 

 tion of sluices made from logs or lumber. 



In constructing a dam whose sills are to rest on bedrock, the 

 first work done after the water is diverted is to excavate trenches 

 from 4 to 5 feet wide in which the logs forming the cribwork are 

 to rest. The foundation may be made slightly convex on the up- 

 stream side in order that the force of the water will tend to tighten 

 the joints of the dam. Three parallel lines of large logs called 

 "mud-sills" are placed across the stream from bank to bank, 

 each row being spaced 6 or 8 feet from the adjoining one. If 

 the dam is to be of greater height than 12 feet, additional 

 sills must be used. The mud-sills must lie flat on the bottom 

 and if possible should be fastened to bedrock with f-inch drift 

 bolts. A row of cross-skids from 12 to 16 inches in diameter 

 is laid 8 feet apart across the mud-sills in a direction parallel 

 with the stream bed. They extend from the front to the rear 



