Roads are now generally made in a less careful manner, and the same
observations as to course and levelling will apply to them, that
are made on these subjects in the article “Railways,” only that less
cutting and filling up are required, as the gradients on common roads
are allowable of much greater steepness than on railways; one in
twenty-five is however about the greatest declivity that should be
made, although old roads exist with slopes much greater. The surface
of a road should be slightly arched and a drain made on each side,
that pools of water may not collect on its surface and soften it so
that it will work into holes. Ordinary roads are made by laying down
a stratum of broken granite or other hard stone a few inches deep,
each piece being of about half a pound weight, when this first layer
is worked in by traffic, a second is laid, and so on till the surface
has become quite firm and level, but in the streets of towns more
care is required. In the suburbs of London, the usual plan is, to lay
a deep stratum of rubbish, consisting of broken bricks, pottery and
oyster-shells (obtained from the dust-bins of London) nearly a foot
in thickness, and on this, broken stone or shingle (consisting of
pebble stones) is laid in repeated layers as it consolidates, but in
the streets having more traffic the roadway is made by a foundation
of “concrete,” consisting of coarse gravel and lime made into a sort
of paste, which sets into a hard cement in a day or two; on this
foundation (which is carefully smoothed on the surface) is laid
a paving of squared granite blocks, placed carefully side by side
either in lines at right angles to the road or else diagonally. When
these are all laid down and rammed to a perfectly even surface, a
mixture of quick lime and sand is made into a thin paste with water,
poured on and swept all over the surface with brooms, so as to fill
up the interstices and cement the blocks of granite firmly together.
In London the roadways are always made arched, so that the centre is
a few inches higher than the sides, and a gutter or drain is placed
on either side between it and the pavements which run on each side,
but in many towns in the country, and on the continent, the gutter is
placed in the middle of the road and each side made to slope slightly
towards it.