CAPILLARY WATER 201 



Moisture in Soil Columns. A number of particles are ar- 

 ranged vertically as in figure 92. The film around (1) is held 

 by the attraction of that particle alone. The film around (2) is 

 held by the attraction of the particle, plus the outward pull of the 

 waist at a. The water film of (3) is held by its own force, plus 

 those of a and b. No. (4) is held by its force, plus a, b, and c. The 

 film of the lower soil particle must be held by the greatest force, and 

 as a result this particle would have the thickest film. If the lower 

 end of the soil column contains free water the films may be so thick- 

 ened by the combined force of the film above that nearly all pore 

 spaces may be filled with water. The pore spaces of the soil column 

 in contact with the free water may act as tubes. 



Effect of Size of Soil Particles. It will he seen from this, 

 then, that the movement of capillary water is due to the difference 

 in curvature of the film, and the greater the curv- 

 ature the greater will be the capillary pull or 

 pressure. The smaller the amount of water pres- 

 ent in the soil, the greater will be the curvature 

 of the film between the soil particles. As a gen- 

 eral rule, the larger the number of soil particles 

 the greater the number of films present, and 

 consequently the greater pull will be exerted per 

 unit of volume of the soil. Hence the water 

 in capillary tubes represented by a single film 

 at the top of the water column will not rise as 

 high as in the soil, where the number of films 

 is many times greater than in the tube. 



FIO. 02 show-in* r ^ ne nt> 'rjht< to which the water will rise de- 

 reticiiiiy th<- thick- i H . M( l s upon the difference between the combined 



of films in u vcr- J _ 



tical soil column. force of the films and the force of gravity rep- 



resented by the weight of the water. In coarse-grained soils, 

 where the total film surface is small, this force representing the 

 upward pull will soon be overcome by the force of gravity or 

 weight of water, and hence water will not. rise very high. In liner- 

 grained soils where the total surface of the films is very large it 

 will require the weight of a very high column of water to balance 

 this force, hence in fine-grained soils water will rise much higher 

 than in coarse soils. 



In two soils, one fine-grained and the other coarse, having the 

 films of the same thickness and the same curvature, there will be no 

 tendency for water to move from one to the other when brought in 



ness 



