178 SOILS : PEOPEUTIES AND MANAGEMENT 



have been determined either on samples that have been 

 worked to a maximum plasticity and then brought to 

 the required moisture content, or on uniformly compacted 

 samples that have been allowed to take up water by capil- 

 larity. In general the curve that would occur with normal 

 granulation, w^hile lower, will follow the direction of a 

 maximum plasticity curve. 



117. Factors affecting cohesion. — It is obvious, from 

 what has already been said regarding the general char- 

 acteristics of soils, that texture must play an important 

 role in the determination of the cohesion factor. In 

 general, the finer the texture, the greater is the cohesion, 

 since, w^hether the soil is wet or dry, the forces that tend 

 to hold the particles together are stronger than in a coarser 

 soil. The drier the soil, however, the greater is the tex- 

 tural influence in this regard, due to the very great in- 

 crease in the binding capacity of the colloidal matter 

 on drying. In a coarse soil this binding effect is small or 

 entirely absent. 



Another factor is the granular condition of the samples. 

 In general granulation may be said to be due to an exer- 

 tion of cohesion between a limited number of particles, 

 resulting in a crumb, or granular, structure. This granu- 

 lation, by loosening the soil mass, lowers not only plastic- 

 ity but cohesion also. The addition of organic matter 

 to a soil, by hastening and increasing granulation, will 

 tend to lower cohesion at ®every moisture content ranging 

 from a dry to a saturated condition. The following 

 data, taken from Puchner,^ bring out the points just 

 discussed: — 



1 Puchner, H. Untersuehungen iiber die Kohareszenz der 

 Bodenarten. Forseh. a. d. Gebiete d. Agri.-Physik., Band 12, 

 Seite 195-241. 1889. 



