16 



BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 



depends (1) on the quality of the soil composing the roadbed, (2) on 

 proper construction, and (3) on adequate maintenance. 1 



SOILS. 



The principal qualities of soils from the standpoint of road con- 

 struction depend upon texture and structure, permeability, and capil- 

 lary power. Soils usually are classified as clay, sand, loam, gravel, 

 marl, or gumbo, according to the material predominating in their 

 composition. Soils composed of two different materials mixed in 

 such proportions that the character of the mixture is decidedly inter- 

 mediate may be designated conveniently by naming both components, 

 as sand-clay, sand-gumbo, gravelly clay, etc. Reference to Table 4, 

 giving approximate bearing powers for different soils, will show 

 that this classification may be of considerable significance in road 

 construction. 



Table 4. — Approximate safe bearing powers for different soils. 



Kind of soil. 



Dry clay 



Moist clay 



Sand or gravel 

 Dry loam 



Range of 



bearing power 



(pounds per 



square inch). 



50 to 80. 

 6 to 50. 

 25 to 85. 

 10 to 20. 



Kind of soil. 



Moist loam. 

 Dry gumbo 

 Wet gumbo 

 Marl 



Range of 



bearing power 



(pounds per 



squaretnch). 



3 to 10. 

 15 to 30. 

 1 to 10. 

 Same as clay. 



Clay is a soil of .very fine texture which results from the complete 

 decomposition of rocks or minerals. Pure clay is very retentive of 

 moisture and usually becomes plastic and unstable when wet, but 

 when mixed with other materials, such as sand or gravel, its sta- 

 bility may be increased greatly. In general, a clay road, to be 

 passable, must be kept dry, and since clay is very difficult to drain, it 

 happens seldom that roads composed entirely of clay can be kept in 

 good condition at all seasons of the year. No matter how well such 

 a road may be graded and crowned the surface absorbs water in 

 wet seasons and subsequent traffic will produce mud. But when 

 the road is shaped and drained properly it will dry out quickly 

 when the weather becomes favorable and may soon be restored to its 

 original shape. On the other hand, clay roads, when very dry. 

 usually produce considerable dust under traffic, and for this reason 

 may become very insanitary in extended dry seasons. 



The extent to which clay roads are subject to the objections men- 

 tioned above varies generally according to the so-called " slaking " 



1 Since the discussion of maintenance, as treated in this bulletin, will apply to the three 

 road types under consideration, it has been placed after the chapter on gravel road 

 construction. 



