SOIL STRUCTURE 171 



not only when the pressure is removed, but also when dry, 

 is said to be plastic. Putty with a proper admixture of 

 oil is a very good example of a plastic body. As is well 

 known, the various plastic materials differ in their plastic- 

 ity. Not only this, but such substances as clay or some 

 other soils vary in plasticity with their moisture content, 

 their granulation, and their texture. The great diffi- 

 culty in the study of plasticity has been in finding a 

 means of estimation allowing an exact numerical expres- 

 sion. The amount of hygroscopic water that a soil will 

 hold has been used as an expression of plastic qualities, 

 as well as shrinkage on drying, the ability to adsorb dyes, 

 tensity, and other characteristics. None of these has 

 proved satisfactory, since one quality of a clay or other 

 soil is used as a measure of another quality. 



Atterberg 1 has suggested that the difference in mois- 

 ture content of a clay at the point at which it ceases to 

 be plastic, as compared with the moisture content at 

 which it becomes viscous, might be used as an expres- 

 sion of plasticity. He has called this figure the plasticity 

 coefficient. Thus, a soil may cease to be plastic at 20 

 per cent of moisture and may flow at 40 per cent. The 

 plasticity coefficient would then be 20. While this is one 

 of the latest methods, it is open to the objection already 

 stated that one quality of a soil is used as a measure 

 of another. Two soils showing the same plasticity coeffi- 

 cient by this method may exhibit undoubted differences 

 in actual plasticity. Kinnison, 2 in testing several methods 

 of expression, found Atterberg's no better than others 



1 Atterberg, A. Die Plastizitat der Ton. Internal. 

 Mitt. f. Bodenkunde, Band I, Heft 1, Seite 10-43. 1911. 



2 Kinnison, C. S. A Study of the Atterberg Plasticity 

 Method. Trans. Amer. Cer. Soc., Vol. 1G, pp. 472-484. 1914. 



