178 NATURE AND PROPERTIES OF SOILS 



As valuable a figure may be obtained by calculation, pro- 

 viding the specific gravity and volume weight of the soil is 

 known together with its percentage of moisture based on dry 

 weight when it is capillarily satisfied. The following formu- 

 lae 1 may be used: 



1. Percentage pore space = 100 — I x — — I 



Lsp. gr. 1 J 



2. Percentage free water == % ^"w^ ~~ % water at 



(based on dry weight Vo1 - wt - maximum 



of soil) capillarity 



Suppose, for example, that a sand with a specific gravity of 

 2.6 and a volume weight of 1.56 contains 20 per cent, of water 

 when at its maximum retentive power. Its pore space would 

 be 40 per cent. If this pore space were filled with water, the 

 soil would contain 25.6 per cent, based on the dry weight of 

 the soil (per cent, pore space -^ vol. wt.). If the total capac- 

 ity of the soil for water is 25.6 per cent, and the hygroscopic 

 plus the capillary capacity is 20 per cent., the free water must 

 be 5.6 per cent. 2 



95. Importance of the study of the flow and composi- 

 tion of drainage water. — A clear understanding of the 

 factors governing the flow of gravitational water is of special 

 importance in tile drainage operations, particularly regarding 

 the depth of and interval between tile drains. Since percola- 

 tion is so slow in a heavy soil it is evident that the tile must 

 be near the surface in order to secure efficient drainage. In 

 a sand the depth may be increased, because of the slight re- 



1 Percentage of pore space represents the percentage of water by 

 volume that would occupy such a space. Percentage of water by volume 

 divided by volume weight gives percentage of water based on dry weight 

 of soil. Conversely, multiplying percentage of moisture calculated on 

 dry weight of soil by volume weight will give percentage of water by 

 volume. 



The air space in a soil at any particular moisture content may be cal- 

 culated as follows: 



Percentage of air space = % pore space — (%H 2 X Vol. Wt.) 



2 Below will be found some generalized moisture data on two distinct 



