STRUCTURES USED IX DRAINING AGRICULTURAL LAND 
MANHOLE COVER 
Partial Plan 
vided the diameter of the top of the manhole is not over 24 inches 
and it is not located in a road. Locks on covers prevent unauthor- 
ized persons from opening them but are a source of trouble where 
the top is flush with the 
ground surface, especially in 
freezing weather. Probably 
it is better to make the cov- 
ers very heavy or otherwise 
hard to lift. 
Manholes are made in 
various shapes and of dif- 
ferent materials. A jug- 
shaped structure is the most 
desirable, but some mate- 
rials can not be made into 
this shape. 'Wooden man- 
holes have the lowest first 
cost, but their life is short. 
Figure 2 shows a wooden 
manhole so designed that 
earth pressure will hold it 
in place after the failure of 
the nails. 
A very good type of man- 
hole for small drains can be 
made from clay or concrete 
pipe. Large sizes are heavy 
and very difficult to trans- 
port and handle. Plate 
I, A, shows a concrete- 
pipe manhole made up of 
1-foot sections 36 inches in 
diameter, reinforced with 
heavy wire mesh. When 
used on large tile lines the 
lower part of the structure 
is made of brick. 
Galvanized corrugated 
pipe has been used to some 
extent. It is light in weight 
and very easy to install. 
Customarily the top ex- 
tends above ground and has 
a cone-shaped cover of the 
same material. (See pi. 1, 
B.) The metal should not 
be lighter than 14 gauge. 
Excellent manholes are 
made of concrete with 6-inch 
walls. They are usually rectangular in form. Concrete blocks have 
not been found very satisfactory. Figure 1 shows a brick manhole 
with cast-iron top. This type is considered good. It is easier to 
construct than the monolithic concrete manhole. 
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Fig. 1. — Brick manhole withi cast-iron cover 
