52 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 
In general, it has been found that satisfactory gravel will conform 
to the following limits as to percentages of fine and coarse material : 
(1) Material retained on a ^-inch sieve, 55 to 75 per cent. 
(2) Material retained on a f-inch sieve, not less than 15 per cent. 
(3) Material (clay) passing a 200-mesh sieve for the surface 
course. 8 to 15 per cent. 
(4) Material (clay) passing a 200-mesh sieve for the foundation 
course. 10 to 15 per cent. 
The sand content should be at least twice as great as the clay con- 
tent ; and the sand and clay, when thoroughly mixed, should be 
sufficient to fill the voids between the larger gravel particles. The 
percentages given above usually will conform to this requirement. 
The maximum limiting size for the pebbles ordinarily should be from 
2J to 3 inches, because where larger particles are permitted in the 
surface the rate of wear is made unequal, and it is more difficult to 
maintain a satisfactory bond between the different particles. 
A satisfactory mechanical analysis of a gravel sample along the 
lines indicated above frequently may be made in the field by first 
separating the coarse material from the fine by means of a J-inch 
mesh screen and then washing the clay out of the fine material, as 
described in the discussion of sand-clay roads. Both the coarse and 
fine materials obtained from the sample may be further separated 
into different sizes by means of suitable screens if. for any reason, this 
is desirable in making the test. 
It happens very often that a mechanical analysis will indicate the 
necessit}^ of screening the natural gravel to remove a portion of either 
the fine or coarse material so as to keep the proportions within the 
limits mentioned above. It is also necessary sometimes to wash 
the gravel in order to remove excessive clay, but this process is ex- 
pensive, and it is generally practicable to secure the desired results 
b}^ screening. When the analysis shows the desirability of adding 
clay or sand to the gravel, this should be done as the surface is con- 
structed by spreading a uniform layer of the needed material over 
each course of gravel and mixing it thoroughly into the mass by 
harrowing. 
CONSTRUCTION METHODS. 
Figure 22 shows a typical cross section for a gravel-road surface 
and indicates the customary steps involved in the construction of 
such a surface. The limiting thicknesses shown on this cross section 
are by no means followed universally, but are believed to represent 
the best current practice. The minimum thickness shown is supposed 
to be employed where the traffic is light and the subgrade is uni- 
formly stable, while the maximum thickness is adapted for opposite 
conditions. Perhaps the most usual compacted thickness of surface 
