DEAINAGE METHODS FOR COUNTY ROADS. 

 BEARING POWER OF SOILS. 



41 



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 subgracle 

 Avould. 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 hearing 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 square 



inch) . 



3 to 10. 

 15 to 30. 



1 to 10. 

 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 subgTacle is liable to become at least slightly 

 moist, due to capillarity, even when the road surface is perfectly 

 water-tight, and the effect of moisture in lowering the safe bearing 

 power of the soil is an important consideration in determining 

 w^hether 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 



