390 LOGGING 
sliding is prevented by anchoring the structure to bed rock or 
by placing the mud sills deep enough in the earth to hold them, 
and overthrow is overcome by increasing the weight of the 
structure by filling it with rock. 
Crib 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. It is the preferred type 
where a large head of water is to be carried and when bed rock 
or a solid foundation can be reached at a depth of a few feet. 
Crib dams are made from round timbers hewed on two sides, or 
from squared timbers. The foundation of a crib dam must be 
solid, and whenever possible built on bedrock, but if this cannot 
be done the foundation may rest on piles driven into hard clay 
or to bedrock. If this is impossible, a row of 3-inch plank or 
small hewed poles sharpened on one end, is driven across 
the stream channel just a1)ove the upstream mud-sill. These 
planks and timbers are called toe-spiling. 
If there is much water in 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. Parallel lines of 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. The width of the 
base should be approximately the same as the height of the dam. 
The mud-sills should be made from large timbers, preferably from 
16 to 20 inches in diameter. They should lie flat on the bottom 
and if possible be fastened to bedrock with |-inch drift bolts. A 
row of cross-skids from 12 to 16 inches in diameter is then laid 
from 6 to 8 feet apart across the mud-sills in a direction parallel 
with the stream bed thus forming cribs from 6 by 6 to 8 by 8 feet 
in size. They extend from the front to the rear row of mud-sills 
into which they are notched so as to rest firmly. Peeled logs 
are placed on top of the cross-skids to which they are drift bolted. 
These lie parallel to the mud-sills. Timbers on the upstream side 
of the dam are hewed on three faces and fitted to each other so that 
