98 kaye. PRINCIPLES OF SOIL MECHANICS [Ch. 5 



overflow to drain out the top of the tank, the sand suddenly becomes 

 a "quicksand." This is shown by the fact that a small weight, which 

 had previously rested on the submerged surface of the sand, now 

 sinks down into the sand. 



The explanation of these three related states of strength lies in the 

 effect of the pore fluids on the intergranular pressure. The friction of 

 downward-moving water on the sand grains acted as an additional 

 downward stress and thus increased intergranular pressures. The re- 

 sulting increase in strength was expressed by the steep slopes that 

 were maintained in the sand. On the other hand, the upward-moving 

 pore water produced the same frictional effect, but in the opposite 

 direction. The upward stresses it induced cancelled the effect of the 

 gravitational stresses. If the water moved upward with a great enough 

 velocity, the sand grains would go into suspension as sediment and be 

 carried out of the top of the tank. 



In addition to intergranular friction, cohesion is another property 

 which contributes directly to the shearing strength of soils. As is well 

 known, it is futile to attempt to mold a sand castle in dry beach sand; 

 whereas the same sand, when moistened, can be fashioned into near- 

 vertical walls and turrets. Clay, on the other hand, can be molded 

 into vertical walls, and even into intricate overhangs. Moreover, dry 

 clay maintains its shape and, unlike sand, develops a high strength. 

 This ability of soil granules to hold together when unsupported is 

 called cohesion. 



Cohesion can result from a number of conditions in a soil. For ex- 

 ample, the cohesion of the moist beach sand is the result of capillary 

 tensions, whereas that of the clay is ascribed to the cohesive bonds 

 between adsorbed layers of oriented water molecules on the surfaces 

 of the clay particles. Cementation of different kinds may also be con- 

 sidered a form of cohesion. 



It is apparent, therefore, that the shearing strength of soils is the 

 sum of the two factors, cohesion and intergranular friction. This was 

 expressed by Coulomb (1776) in an equation that has become classical 

 in mechanical theory: 



s — c + n tan <j> 



where s is shearing strength along any plane, c is cohesion, n is the 

 pressure normal to the plane, and tan <£, or the tangent of the angle of 

 internal friction, is an expression of the frictional properties of the 

 material. However, today it is known that the validity of this equa- 

 tion is somewhat limited. The relationship between s and n for co- 

 hesive soils is more complicated than the equation indicates. 



