WEIGHT OF SOIL 



95 



Whitney has obtained the following specific gravities of 

 composite soil separates. 



There is a very small increase in the specific gravity 

 of the clay group, probably due to the greater concentra- 

 tion of the iron compounds here as a result of chemical 

 processes; but it is not sufficient to materially change 

 the result. The average specific gravity of soil material 

 is, therefore, usually taken as 2.65, and this figure is 

 used in all calculations here given. 



Since the pore space enters into the calculation of 

 the weight of any volume of field soil, this figure is 

 much more variable for different soils than the one just 

 given. It is directly related to pore space, and the 

 larger the volume of pore space, the smaller the unit 

 weight. 



Combining the figures for pore space given above 

 with that for average specific gravity, the figures in 

 the following table are obtained. 



The weight of a given volume of soil may be deter- 

 mined from the pore space and specific gravity of the 

 material, by use of the following formula. 



