Ch. 5] THE ROLE OF GEOLOGY IN SOIL MECHANICS 109 



some extent permeability, internal friction, and even the elastic prop- 

 erties of soils. The intergranular structural arrangements of clay 

 crystals are only imperfectly understood, although it is known that 

 they have an important effect on strength. What are the formational 

 environments, and what clay minerals produce the different cellular 

 and flocculated arrangements of clay crystals? The mortar structure 

 of many clays, that is, a structure characterized by an open skeleton 

 of large non-clay mineral grains embedded in a matrix of clay minerals, 

 may produce hybrid mechanical properties. Frictional strength may 

 be imparted to the soil by the non-clay framework. A shifting of the 

 contact points of these grains may suddenly destroy the strength of 

 the framework, and a weakening of the whole clay mass may result. 



The chemical activity of the clay minerals is known to affect perme- 

 ability and plasticity. The permeability of montmorillonite clays has 

 been reduced by introducing sodium ions into a calcium-rich clay. A 

 famous example of this was the treatment of the clay lining of the 

 lagoon on Treasure Island for the 1939 Golden Gate Fair in San 

 Francisco (Lee, 1940). 



The surface activity of clay minerals affects strength and perme- 

 ability. Plasticity is an index property of primary importance. 

 Highly plastic clays almost always bring with them difficult engineer- 

 ing problems. The nature of all the factors affecting the plasticity 

 and the cohesion of clays is yet to be determined. Is the theory of the 

 adsorbed water layer on the surface of the colloidal clay crystals the 

 only and entire explanation of cohesion, or will further research suggest 

 other forces? 



The correlation of plasticity data, as expressed by the Atterberg 

 limits, with geological observations may be of far-reaching consequence 

 in furthering knowledge of the mechanical properties of clays. Geolo- 

 gists will do well to recognize that plasticity is a property as im- 

 portant to study in cohesive soils as are grain size, rounding, and min- 

 eral content in the study of sands. 



The loss of the cohesive bond of many clays on disturbance and 

 manipulation and the slow redevelopment of cohesion on standing 

 (thixotropy) is a soil property concerning which are many ques- 

 tions. There are a number of examples of the practical importance of 

 this property. For example, the movement of a large soil mass on the 

 shores of Lake Gerzen, Switzerland, was brought about by the weak- 

 ening of the cohesive bond of a lake marl due to the vibrations set 

 up by the blasting of the stumps of trees (von Moos and Rutsch, 1945) . 



The phenomenon of the movement of pore water in soil under elec- 



