Ch. 5] SCOPE OF SOIL MECHANICS 97 



simple granular concept becomes somewhat complicated in clays, for 

 it is known that in these colloidal soils the boundary between the solid 

 grains and the fluid-filled voids is not always sharply defined. The 

 presence of relatively thick layers of adsorbed water molecules on the 

 surfaces of clay colloids introduces forces in the soil system which 

 cannot be readily explained in terms of an ideal granular soil. 



Most soils possess appreciable shearing strength. In the more 

 coarsely grained and non-cohesive soils, the source of this strength is 

 the friction developed at the points of contact of the soil grains. The 

 greater the pressure between soil grains, the greater is the force neces- 

 sary to displace one grain in relation to another. 



The importance of intergranular friction on soil strength can be 

 demonstrated by a simple laboratory experiment. A soft rubber blad- 

 der is filled with loose, dry sand. As can be readily visualized, the 

 sand inside the bladder is easily displaced by small stresses, such as 

 the prodding of a finger. However, when a suction pump is applied 

 to the bladder and the air from inside the bladder is excluded, the mass 

 of sand is seen to develop a rather surprising rigidity. It will be found 

 to resist not only the prodding of a finger but also the weight of a 

 heavy book placed on top of it. This sudden development of strength 

 is the result of increased intergranular pressure in the sand which is 

 brought about by the sand grains being made to bear the weight of 

 the atmosphere pressing against the outside of the bladder. The 

 intergranular pressures developed in this little experiment are roughly 

 of the same magnitude as those in dry sand buried at a depth of about 

 20 feet, for at that depth the weight of overburden is about equal to 

 the atmospheric-pressure load in the experiment. 



This experiment demonstrates another important property of soil, 

 namely, that the stresses in the pore fluids have a direct effect on the 

 strength of the soil. Indeed, the strength of a soil is affected by all 

 stresses within the soil system. This interdependence can be simply 

 demonstrated in the laboratory in another way. The apparatus con- 

 sists of a tank containing loose sand. Water is poured into the tank 

 to a level above the top of the sand. When we attempt to dig a hole 

 in the submerged sand, we discover that it is impossible to maintain a 

 steep face in the excavation. Sand flows in from all sides, and the 

 slopes immediately assume a low angle of repose. On the other hand, 

 when water is allowed to flow down through the sand and out an open- 

 ing in the bottom of the tank, we discover that we are able to dig a 

 steep face in the submerged sand, and, in fact, with care we can 

 make a vertical face. Finally, if the direction of flow in the experi- 

 ment is reversed and water is forced up through the sand, allowing the 



