8 BULLETIN 23. U. S. DEPARTMENT OF AGRICULTURE. 
Good paving brick will ordinarily lose from 17 per cent to 22 per 
cent of their original weight in the rattler test, and specifications 
concerning this loss should be prepared with a view to the character 
of the traffic for which the pavement is designed. Some reasonable 
requirement as to the loss sustained by any individual brick should 
also be made. This loss should ordinarily not exceed 25 per cent. 
and under severe traffic conditions a smaller percentage should be 
required. 
CONSTRUCTION. 
PREPARING THE SUBGRADE. 
In forming a roadbed upon which a brick pavement is to be con- 
structed, the essential features to be considered are (1 1 thorough 
drainage. (2) firmness. (3-) uniformity in grade and cross section, and 
(4 I adequate shoulders. 
Thorough drainage can be secured for anv particular road onlv bv 
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 aU be considered if a sys- 
tem 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 finish grading 
with picks and shovels and rolling. 
When completed the subgrade should be uniform in grade and 
cross section, or otherwise the foundation must be made unneces- 
sarily thick where depressions occur, in order that its gi ade and 
cross section may be uniform and its thickness not less at any point 
than that required. The subgrade should be repeatedly rolled and 
reshaped until the desired shape is secured. The curbs, which 
should be set before the final finishing, may be made to serve as a 
guide for this work. 
