2 BULLETIN 1408, U. S, DEPARTMENT OE AGRICULTURE 
STRUCTURES FOR USE WITH UNDERDRAINS 
MANHOLES 
On some tile drains more sand or silt finds its way into the drain 
than the water can carry. If the drain is to be kept open, it is 
necessary that this material be intercepted at suitable intervals and 
later removed. This can be done by building a manhole of suitable 
size. It should be so proportioned that there will be an appreciable 
loss of velocity as the water passes through it and consequent deposi- 
tion of the sand or other material that the water has in suspension. 
Manholes are not required on all drainage systems ; and, unless it is 
clear that their use is necessary, they should not be installed as they 
increase the cost of the drainage and there is always danger that 
debris may get into the manholes and obstruct the drains. 
When their use is necessary they should be located at points where 
a reduction in gradient occurs, at sharp angles, at connections with 
main laterals, and at special crossings. Usually it is not best to 
construct them at frequent intervals on tangents. The cost of 
installing them near enough to each other for efficient use of sewer 
rods is prohibitive, and they are of but little use except in the special 
circumstances mentioned. 
The depth of the silt basin below the bottom of the outlet tile 
should be not less than 18 inches; 2 feet is preferable. For farm 
drains not over 8 inches in diameter, in firm ground, a 30-inch man- 
hole with a depth of 2 feet for the silt basin should be the minimum 
size. Where the size of the drain is the only governing factor, the 
diameter of the manhole should be not less than 18 inches plus 
the diameter of the outlet, and 36 inches should be the minimum 
diameter for district systems. High velocities, existence of a long 
tile line above the manhole, junctions with laterals, and the proba- 
bility of infrequent cleaning, require larger sizes. 
Loss of head occurs at manholes, and for this reason a drop 
should always be provided between the inlet and outlet. Ordi- 
narily a drop equal to the velocity head in the pipe will be enough, 
but the loss may amount to 50 per cent more than this in manholes at 
sharp angle points. The minimum drop should be 1 inch. Surface 
inlets should not be connected with manholes, as the falling water 
may prevent the deposition of silt and defeat the purpose of the 
manhole. 
Where the lower part of the manhole is located in gravel, hard- 
pan, or stiff clay formation, a floor is not necessary ; otherwise timber 
or concrete must be used. In soft ground provision against settle- 
ment must be made by the use of gravel or piling. In fine silty 
material the joints in the lower part of the structure must be made 
tight. 
Elevation of the top depends upon the land of material used in 
the cover and the location of the manhole. When it is located on a 
fence line the elevation makes little difference, but if in a field the 
top must be strong enough to support farm machinery and stock or 
the structure must rise above ground. The latter is sometimes 
objectionable to the farmer. A cast-iron top and frame similar to 
but lighter than that used in sewer practice is an excellent type 
(fig. 1). Heavy reinforced concrete covers are satisfactory, pro- 
