A manual of the principles and practice of road-making: comprising the location, construction, and improvement of roads, (common, macadam, paved, plank, etc.) and rail-roads. By W. M. Gillespie ...

178 THE CONSTRUCTION OF ROADS.
CULVERTS AND DRAINS.
These structures are necessary for carrying under a
road the streams which it intersects. They are also needed to carry the waters of the ditches, from the upper side
of a road, to that side on which lie the natural water
courses into which they must finally be discharged Theii
simplest form consists of two walls of stone or brick
covered with slabs, and having a foundation, either of
wood (if always wet) or of stone, laid in the form of an
inverted arch, as shown in Fig. 97.
cross-section in Figure 97.
Their size must be propor- //
tioned to the greatest quantity
of water which they can ever /
be required to pass, and should
be at least 18 inches square,
or large enough to admit a boy
to enter to clean them out. Their bottoms should be inclined 1 in 120, or 1 inch in 10 feet. When the road
slopes, the inclination of the culvert may be increased, if
necessary, by making it cross the road obliquely. At
each end flat stones should be sunk vertically, or sheetpiling driven, to guard against the undermining effects of
the water. The length of a culvert under an embankment will be equal to the width of the road, increased by
the distance on each side, to which the slopes run out, at
the depth at which the culvert is placed. At each end of
it should be built wing-walls, their tops having an outward
and downward slope corresponding to that of the embankment. Their ground plan may be rectangular, trapezoidal, or curved.
In districts where stone is scarce, a small culvert may