EARTH, SAND-CLAY, AND GRAVEL ROADS. 49 
While there are many exceptions to this rule, it is sufficiently general 
to warrant treating as special cases those deposits in which the biuder 
is not clay. 
The suitability of clay for use as a binder for a gravel road sur- 
face depends on exactly the same characteristics as its suitability 
for use in sand-clay construction (see pp. 39 et seq.). That is, a 
quality of clay which could be used satisfactorily in a sand-clay 
surface also should make a satisfactory binder for a gravel surface, 
provided it is used in proper proportion and is properly mixed with 
sand. 
Where the binder consists of some material other than clay its 
efficiency usually may be tested by observing an exposed bank of 
the gravel deposit. If the binder is. of satisfactory quality, the 
gravel should be capable of standing on a practically vertical slope 
and nearly always will contain many lumps composed of numerous 
pebbles cemented together with the binder. These characteristics 
are especially noticeable in the case of gravel deposits in which the 
binder consists essentially of iron oxide, as in some deposits found in 
New Jersey, Mississippi, and the lake basins of the West. 
GRADING AND PROPORTIONS. 
For gravel to make a satisfactory road surface, the stone particles 
should be graded in size so* that the amount of binder required will be 
reduced to a minimum. The reason for this is that the binder usually 
is much less resistant to wear than the stone particles, and therefore 
it is desirable that the latter form as large a part of the wearing sur- 
face as is practicable. Most gravel deposits as thej occur in nature 
satisfy this requirement in so far as grading of the pebbles is con- 
cerned, but they nearly always contain pebbles of a size larger than it 
is desirable to incorporate in a road surface. Natural deposits also 
not infrequently contain too large a proportion of sand or clay to 
produce satisfactory results. It is desirable, therefore, that specifica- 
tions covering gravel for use in road construction should limit the 
proportions in which the fine and coarse materials shall be present 
and a maximum limiting size for the pebbles, as well as definite 
requirements regarding the quality of the pebbles and of the binder. 
Table 9 shows the results of a number of tests made by the Office 
of Public Eoads and Rural Engineering on gravel obtained from 
various parts of the country, with comments as to the suitability of 
the different samples for use in road surfacing. The comments are 
based on observation of existing gravel roads constructed of similar 
materials and are believed to be well grounded. 
61531°— Bull. 463—17 1 
