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. 
Range of 
Kind o: soil. \ *%%££? 
I square inch). 
Kind of soil. 
Range of 
bearing power 
(pounds per 
square inch). 
Drr clav 
50 to 80. 
6 to 50. 
25 to 85. 
10 to 20. 
Moist loam 
3 to 10. 
Moist clav 
Drv gumbo 
15 to 30. 
Wet gumbo 1 to 10. 
Marl Same as clav. 
Dry loam 
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. Xo 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 Avill 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 -easons. 
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. 
