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 binder 

 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 satisfactorj^ 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 they 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 Roads 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 rpad 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 4 



