6 
BULLETIN 1408, U. S. DEPARTMENT OF AGRICULTURE 
top stirs up the silt and much of it passes through the connection. 
The silt traps, of 18 or 24-inch sewer pipe, have a concrete cover 
provided with openings (pi. 1, A). 
A type more expensive but better adapted to exclude silt consists 
of a concrete box which permits the water to enter on grade with 
the outlet (fig. 3, A). This is an important part of the design, since 
the disturbance of the water in the box must be reduced to a 
minimum. 
Inlets with openings at the top have sometimes washed out. This 
is usually caused by swirling water. To prevent this motion, the 
openings in the grate must be of good 
size and ribs must be provided either 
on the grate or the concrete rim sur- 
rounding it, as shown in Figure 3. 
Preferably, the silt trap should be 
located at one side of the drain and not 
directly over it. The connection should 
be made with sewer pipe and the 
joints filled with mortar. Figure 
3, A, shows this pipe leading hori- 
zontally to a point over the drain and 
then straight down, an arrangement 
often necessary at connections with 
drains constructed in soft ground 
where sheeting is employed ; otherwise 
the better method is to slope the pipe 
directly to the drain. Connection with 
the drain should be made with a 
T-junction and the tile at this point 
surrounded with gravel or concrete. 
For those types shown in Figures 3, A, 
and 3, B, the covers should not be 
locked on, so that the farmer can clean 
the traps, and the outlet pipe should 
not be put lower than necessary to 
have it out of the way of cultivation. 
A concrete bottom should be provided in silt traps, and in every case 
the earth should be well puddled around the entire structure. 
LAMP HOLES 
For observation purposes on underdrains, lamp holes are some- 
times installed at angle points or curves where manholes are not 
constructed or at intervals on long tangents (fig. 4). The same 
objections apply to their use as to manholes but in a lesser degree. 
The connection with the drain is best made with a T-junction, but 
may be made by cutting a hole in the top of the tile and extending 
a 6-inch pipe from this to a point about 1 foot above ground. The 
earth around the riser should be puddled and a bank thrown up 
around that part above ground. If possible they should be out of 
the way of farm machinery. If necessarily installed in fields a 
protecting post should be set by each. 
A lock-fastened lid is likely to be unsatisfactory, but a wire screen 
across the first joint below the top will keep out rubbish and small 
