10 
BULLETIN 1408, U. S. DEPARTMENT OF AGRICULTURE 
For canal crossings three cut-off walls should be constructed as 
shown in Figure 7. The minimum height and width of these should 
be 4 feet. Under some canals it may be desirable to use, on the 
lower side, only one cut-off wall extending to the top of the embank- 
ment instead of two small ones as shown. Should the top of the 
pipe coincide closely with the bottom of the canal, the whole sec- 
tion between the two upper cut-off walls should be cased in concrete. 
Railway crossings are similar to canal crossings except that cut- 
off walls are omitted. 
LUMBER BOX DRAINS 
Box drains are chiefly used where cost of transportation prohibits 
use of tile. The life of a wooden structure is reasonably long if it 
is always wet ; but failure, especially of the top, may occur in a few 
years where alternate wetting and drying take place. The boxes 
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Fig. 7. — Closed drain crossing' under canal 
should be built to retain their form after failure of the nails, which 
will happen soon if alkali is present. Shoulders milled in the lum- 
ber effect this; they are cut cheaply by passing each end of the top 
and bottom boards over a circular saw set to cut a rabbet as deep 
as the thickness of the saw and as wide as the thickness of the side 
planks (fig. 8). 
The type shown in Figure 8, A may be used for small sizes where 
cheapness is desirable. The lumber runs with the box and long 
sections may be made. The bottom is held from the sides by short 
pieces of lath separated to admit the water. The widest boards 
should not exceed 8 inches for 1-inch lumber and 12 inches for 
2-inch lumber. Lumber for the top and bottom of larger drains 
should run crosswise, with the ends milled to provide shoulders 
(fig. 8, B). Decay is likely to be the most rapid at the top, which 
