32 
Mural Roads. 
Telford did not, as is popularly supposed, invariably use 
a paved foundation for macadam roads, but suited his methods 
to the special local conditions, and 920 miles of roads which 
he constructed in Scotland for the Government were formed 
without pitching or paving. The subsoil or road bed is the 
real foundation, and if this is dry, uniform, and well drained 
no special foundation courses are required, but only a moderate 
thickness of crust. 
Mr. Thomas Codrington, at one time Superintendent of 
Welsh Highways, carried out some most interesting experi- 
ments which showed that the load on a wheel is transmitted 
to the subsoil or foundation by a portion of the road crust 
roughly of the form of a truncated pyramid, the sides of which 
are inclined at an angle of about 45° with the horizontal plane. 
This simple fact explains why a traction engine wheel often 
breaks through a road, though the load per inch of width of the 
wheel is not so great as that on a cart wheel, and it also explains 
how increased strength is obtained by thickening the road 
crust. In the first case the area of the bottom of the truncated 
pyramid is not proportionally much greater than the area of its 
top ; in the latter case the bottom area considerably exceeds that 
on which the cart wheel bears. A road requires such a thickness 
of road crust and foundation courses as will lead to a distribu- 
tion of the load over an area of subsoil sufficient to carry it 
without sensible compression, and the actual area required for 
a given load depends in most cases not only on the nature 
of the subsoil, but also on the degree to which it is drained and 
protected from the access of water. The distribution of the 
load is evidently dependent not only on the thickness of the 
road, but also on the extent to which the crust is consolidated, 
as with a well-knit crust the angle of distribution is flattened. 
In the construction of modern roads a hard set pitching or 
pavement is not generally used as a foundation for macadam 
roads, though it may be adopted with great advantage if the 
subsoil is of a clayey nature and suitable stones are easily 
procurable. In Great Britain and on the Continent roads are 
now generally made with a rolled bottoming or foundation 
course of large but uniformly sized stones, about six inches 
deep, laid on the subsoil (after it has been well drained and 
rolled) and covered by a layer of broken stones, two inches in 
size, rolled to a finished thickness of six inches. A layer about 
three inches thick of gravel, clinkers, or ashes is sometimes first 
laid on the road bed to separate the bottoming from the subsoil 
if it is soft or retentive of water. Concrete makes an excellent 
foundation for a macadam road, but its cost is almost prohibi- 
tive except in cities and in connection with block paving and 
asphalt. 
