PLASTICITY OF SOIL 97 



This table shows that the finer the soil the lighter 

 its absolute weight. Clay soils may range from 60 to 90 

 pounds in weight, according to their fineness and state 

 of granulation. Sand soils weigh from 90 to 110 pounds. 

 In practice, soils are spoken of as "light" and "heavy," 

 but this use of these terms does not apply to the weight 

 of the soil. The term light is applied to sandy soil 

 because the particles move freely. On the other hand, 

 a clay is termed heavy because of its cohesiveness. 



38. Plasticity. The property of stickiness of soils, 

 when mixed with water, is termed plasticity. Soils 

 exhibit it in very different degrees. In general, it may 

 be said that the finer the soil the greater the plasticity, 

 and therefore the finest-textured clays generally. exhibit 

 the greatest degree of plasticity. On the other hand, 

 plasticity is not absolutely lacking in sandy soil, for, 

 when moist, this material adheres together and may 

 support a considerable weight. But, when the water is 

 removed by drying, the sand will fall apart readily, 

 and therefore the cohesiveness exhibited was largely 

 due to the surface tension of the water between the 

 particles. However, when the clay is dried out, it be- 

 comes a hard mass, and it has a superior adhesive and 

 cohesive property when dried from the wet puddled 

 state. 



But while plasticity and great tensile strength appear 

 to be very closely associated with fine texture, the 

 fineness does not appear to be entirely responsible for 

 the property, as is shown by the results of numerous 

 studies reviewed by Ries. Writers in the past have 

 dwelt much on the effect of colloidal clay in this connec- 



