STRUCTURES USED IN DRAINING AGRICULTURAL LAND 
9 
^i Bolls for 
holding gate 
I or screen 
destroyed where weeds on the banks of the main drain are burned, 
and in such places heavy corrugated pipe must be used. The outlet 
end of such conduits should protrude a short distance from the bank 
so that the water in the channel will make a cushion for the outflow 
of the drain. 
For large drains, concrete bulkheads, wing walls, and an apron for 
the falling water should be provided (fig. 6). Modification of the 
wing walls is required where the drain enters the channel at a 
sharp angle. Reinforcing must be used for large structures, and 
for all structures if in very soft soils. The main considerations 
should be stability and prevention of undermining or cutting around 
walls. To prevent damage from floods in the main channel, the 
structure should be set well into the bank. Failures may be caused 
by surface water flowing over or around the outlet, to prevent which 
surface-water inlets should be provided with the lower end of the 
pipe passing through the head 
wall of the concrete outlet struc- 
ture at one side of or just above the 
drain, as shown in Plate 1, E. This 
type is particularly desirable where 
the outlet is just below a road. If 
frequent heavy floods are antici- 
pated the structure must be de- sj£§=p 
signed as a check or drop similar ^^^^ 
to that shown in Plate 1, G, with W^Sr 
the underdrain outlet in the lower 
part of the breast wall. 
Where outlets of underdrains are 
at times completely submerged, it 
is advisable to provide flap gates 
to prevent water from backing up 
the drain. These valves may be 
made of lumber, but under many 
conditions an automatic iron gate 
will prove very satisfactory, as 
shown in Plate 1, F. For small 
drains they may be attached to 
the end of a corrugated iron pipe, and for large drains they should 
be installed on the head wall of the concrete outlet structure. Drains 
not submerged may require a flap gate made of rods to keep out 
small animals. 
SECTION A-A 
Concrete 8" thick up to D=24" 
10" " 24" to 36" 
4 • 
Fig. 
-Outlet structure for closed drains 
CROSSINGS UNDER RAILROADS AND CANALS 
Where underdrains cross under railways and canals, pipe with 
bell-and-socket joints should be used and the joints made tight 
with mortar. Pipe of extra strength should be used immediately un- 
der the fill or canal. A concrete foundation usually envelops the 
lower half as shown in Figure 7, and the pipe is laid before this con- 
crete has set. This foundation should extend not less than 6 inches 
below the bottom of the pipe and its total width should exceed 
the outside diameter of the pipe by not less than 8 inches. In 
trenches with exceptionally soft bottoms, gravel should be tamped 
in before the concrete is placed. 
83435°— 26 2 
