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 ...

CULVERTS AND DRAINS. 179
be constructed with four ranges of slabs; Fig. 98.
grooves being cut in the top and bottom [-
slabs, to receive the upright ones which
form the sides.
A cheap culvert may be built of brick,
with a semicircular arch, of three feet
span and 4 inches thick. Fig. 99.
One thousand bricks will T
build 26 running feet. If
the flow of water be small,
the bottom may be merely
covered with gravel, over
which is then poured grout of hydraulic cement, forming
a superficial concrete.
To obtain greater strength, the Fig. 100.
arch may rest on abutments, sloping inward, and the bottom of the
culvert be a flat inverted arch.
When a road is in excavation,
the ditches on either side of it
will sometimes require to be cov-
ered, to prevent their being filled
up by washings fiom the sides.
They may then be formed as in
Fig. 97; but spaces of half an inch in width should be
left between the covering stones. A layer of brushwood
should be placed over these, and the remainder of the
ditch filled up to the surface with broken stones, through
which the water can filter.*
Similar but smaller drains may be formed at intervals
under the road, diverging from its centre like the two
* Parnell,-p 95