DRAIN" AGE METHODS FOR COUNTY ROADS. 
41 
BEARING POWER OF SOILS. 
On account of the uncertainty regarding the manner in which 
pressure due to concentrated loads on a road surface is distributed 
by means of the road crust and foundation, very exact knowledge 
concerning the bearing power of the soil composing the subgrade 
would.be of small advantage in designing foundations. Such infor- 
mation would become of value only if a minimum bearing power 
were known, taking into consideration the effects of changing soil 
moisture. The most that can be done is to compare, in a general 
way, the soil conditions anticipated in the design with similar condi- 
tions on successful projects already completed and to consider known 
failures, the causes of which have been ascertained. 
Such tests as have -made for the purpose of determining the bear- 
ing powers of different soils have given more or less contradictory 
results, but all have shown a very wide variation for soils of prac- 
tically the same character tested under identical circumstances. The 
following table gives averages deduced from available test results 
and may prove helpful in applying knowledge gained by experience 
in designing and constructing road foundations to similar work 
where the soil conditions differ from those already encountered: 
Table 10. — Safe bearing powers for different soils. 
Kind of soil. 
Range of 
bearing power 
(pounds 
per square 
inch) . 
Kind of soil. 
Range of 
bearing power 
(pounds 
per square 
inch). 
50 to 80. 
6 to 50. 
25 to 85. 
10 to 20. 
3 to 10. 
15 to 30. 
1 to 10. 
Marl 
Same as clay. 
This table indicates the extremely wide range of the available 
data and gives some idea of the value of judgment by the engineer 
in designing road foundations. It also demonstrates that figures in 
the matter are of little use with the very limited knowledge at our 
disposal. 
Under the conditions ordinarily encountered in road work, the 
soil composing the subgrade is liable to become at least slightly 
moist, due to capillarity, even when the road, surface is perfectty 
water-tight, and the effect of moisture in lowering the safe bearing 
power of the soil is an important consideration in determining 
whether a foundation is necessary and, if so, how it shall be designed. 
The effect of capillarity can not be stated definitely, but that it 
has an influence on the condition and physical characteristics of the 
surface of the natural soil is certain. The amount of influence de- 
pends on meteorological conditions without and on various properties 
