8 BULLETIN 220, U. S. DEPARTMENT OF AGRICULTURE. 
requires it. All hollows and depressions which develop during the 
rolling should be filled with good material. If soft or spongy places 
develop during the rolling, the soft or spongy material should be 
taken out and replaced with good material and the subgrade again 
rolled. This process of filling and rolling is repeated until no depres- 
sions develop and the subgrade has been brought to its proper ele- 
vation, which is as much below the established grade as the thickness 
of the surfacing course to be used. The shoulders ought also to be 
rolled in the same manner, but in places where the character of the 
material makes the use of a heavy roller impracticable, a lighter 
roller may be used. The shoulders are built at the same time as the 
subgrade and with it form a trench to hold the surfacing material. 
They may be built either by excavation or by piling up earth as the 
work progresses. The shoulders also serve to widen the road for the 
passing of vehicles. They are usually given a sharper slope than 
the paved portion of the road. 
Plate V, figure 1, shows a model illustrating several methods of 
road drainage. It will be noted that the roadbed is located partly in 
a cut and partly on a sidehill. 
Section A shows a surface ditch (h) at the top of the slope, a side 
drain (c) opening into a culvert (a) with a drop inlet (b), a telford 
base, a section of guard rail (g) , and a shoulder drain (d) . 
The surface water falling on a large area sloping toward the road is 
often kept from the road and out of the road gutters, where it usually 
does considerable damage, by a surface ditch such as (h). The side 
ditch (e) is used to gather the water originating on or along a roadway 
and to carry it to points of outlet. As a general rule the bottom of 
the ditch is lower than the low point on the subgrade of the road. 
The size of the ditch varies with the amount of water it is necessary to 
accommodate. The shape of the ditch varies with climatic, soil, and 
topographic conditions. The side drain (c) is constructed to carry off 
the ground water which is often found in hilly sections. In the model 
the trench is represented to be 2\ feet deep, 12 inches wide at the 
bottom, and 18 inches wide at the top. On the bottom of the trench 
is placed tile pipe, usually 6 inches in diameter. The pipe is laid 
without cement, and loose stone is carefully filled around the sides 
and top of the pipe to the top of the trench. The side drain empties 
in some suitable outlet. For example, the one in this model empties 
in a drop inlet (b), which also serves as an outlet to the surface and 
side ditches. The drop inlet empties into a culvert (a), which carries 
the water under the road. This type of inlet prevents the bank from 
sliding and closing the culvert, which sometimes happens in hilly 
sections. 
The purpose of the shoulder drain (d) is to permit water to escape 
from the subgrade during or immediately after the construction of a 
