EARTH, SAND-CLAY, AND GRAVEL ROADS. 17 



qualities of the clay. JSome varieties of clay, when compressed into 

 a ball and placed in water, will continue in the original shape for 

 a considerable time, even if thoroughly saturated and softened, 

 while compressed balls made from other varieties will slake or break 

 down almost immediately upon being placed in water. Clays of the 

 first-mentioned variety are called "nonslaking clays," and of the 

 latter, " slaking clays." It has been observed that slaking clays pro- 

 duce more mud in wet weather and more dust in dry weather than 

 those of the nonslaking variety. This characteristic of clays is fur- 

 ther considered in the discussion of sand-clay roads appearing in 

 another part of this bulletin, where methods for testing clay are 

 suggested. 



Most clays, as they occur in nature, contain more or less sand or 

 gravel, which, as pointed out above, usually has a stabilizing effect by 

 making the clay more easily drained and increasing its bearing 

 power when wet. In fact, a good quality of clay mixed with coarse 

 sand in such proportions that the clay fills the voids in the sand and 

 cements the individual particles together produces a soil which pos- 

 sesses the good qualities of both materials and which overcomes, to 

 a great extent, the objections to each. 



SAND. 



Sand is composed of granular particles of mineral or stone which 

 occur in nature and which will pass a ^-inch mesh screen. The 

 ^-inch mesh screen is fixed arbitrarily as the dividing line between 

 sand and gravel and is generally, though not universally, accepted 

 as such. Nearly all sand consists essentially of quartz grains that 

 are very hard and durable. But there is no coherence between 

 the different grains, and therefore soils composed principally of 

 sand are unstable, except when confined in some way. If properly 

 confined and protected from undermining, sand foundations may, 

 under favorable climatic conditions, prove entirely satisfactory for 

 almost any type of road crust, but it is practically impossible, under 

 any circumstances, to make a satisfactory road surface of sand alone. 

 Dry sand offers almost as great resistance to traffic as mud, and except 

 in very wet seasons sandy roads are likely to dry out to a considerable 

 depth. The reason for this is that the capillary power of sand is 

 comparatively low and the ground-water elevation may be lowered 

 practically to the prevailing level of surface streams in extended dry 

 seasons. 



Sand roads are at their best when they are kept moist, and for 

 this reason they should be designed with a view to retaining moisture 

 in the sand rather than to effective drainage, as is the case with clay 

 61531°— Bull. 463—17 2 



