8 BULLETIN" 373, U. S. DEPARTMENT OF AGEICULTUKB. 
It is also advisable to require a minimum as well as a maximum 
percentage of loss which any specified sample of the brick may sus- 
tain. This is done in order to insure against too much variation 
between the softest acceptable brick and the hardest brick which may 
be supplied. 
CONSTRUCTION. 
PREPARING THE ROADBED. 
In forming a roadbed upon which a brick pavement is to be con- 
structed, the essential features to be considered are (1) thorough 
drainage, (2) firmness, (3) uniformity in grade and cross section, 
and (-4) adequate shoulders. • 
Thorough drainage can be secured for any particular road only by 
means of a careful study of the local conditions which affect the 
accumulation and " run-off " of both the surface and ground water. 
These conditions vary considerably even in the same locality, and no 
set of rules can be given which would cover all cases. For example, 
the material composing the roadbed may be springy, and in this case 
tile underdrains will probably be necessary. On the other hand, 
extremely flat topography may make it necessary to elevate the grade 
considerably above the surrounding land. The nature of the soil, the 
topography, and the rainfall must all be considered if a system of 
drainage is to be planned properly. 
The second requirement, firmness, can be secured only after the 
road has been properly drained. Soils which readily absorb moisture 
can not be properly drained in wet weather and should not be per- 
mitted to form a part of the subgrade. In order that the subgrade 
may be unyielding, it is also necessary that the roadbed be thoroughly 
compacted. In forming embankments the material should be put 
down in layers not over 8 inches thick, and each layer should be 
thoroughly rolled. In excavation care should be exercised, if the 
material is earth, not to permit plows or scrapers to penetrate below 
the subgrade. The subgrade in both excavation and embankment 
should be brought to its final shape by means of fine grading with 
picks and shovels and rolling. 
When completed the subgrade should be uniform in grade and 
cross section; otherwise the foundation must be made unnecessarily 
thick where depressions occur, in order that its grade and cross 
section may be uniform and its thickness not less at any point than 
that required. The subgrade should be repeatedly rolled and re- 
shaped until the desired shape is secured. If curbs are constructed 
independent of the base they should be set before the final finishing, 
in order that they may be made to serve as a guide for this work. 
The shoulders should never be less than 4 feet wide and should 
consist of some material which compacts readily under the roller and 
