Ch. 13] WATER CONTENT 235 



minerals proper, through their habit of cleaving into minute flakes, 

 present tremendous surface area. Moreover the arrangement of mole- 

 cules on the crystal surfaces of certain clay minerals is favorable to 

 the adsorption of water or to the adsorption of certain ions which 

 themselves may attract thick water films. Montmorillonitic clays in 

 which the exchangeable ion is Na+ adsorb thick oriented films; they 

 require somewhat more water to become plastic, and 5 or 6 times as 

 much water to become liquid as do clays carrying exchangeable Ca+ + 

 or H + . Thus they remain plastic over a considerable range of water 

 content. Some clays also adsorb organic molecules and form gels in 

 suitable organic liquids. Electron-microscope photographs show that 

 clay particles tend to cluster around bacteria and bits of organic mat- 

 ter (see Jackson, Mackie, and Pennington, 1946, and its bibliography). 



Water Content 



In fine-grain sedimentary deposits, the water films around clay par- 

 ticles profoundly influence the forces holding the particles together, 

 and also influence the arrangement of both the clay size and larger 

 particles into structures that affect the porosity, shear strength, con- 

 solidation properties, and permeability of the deposit. All these fac- 

 tors in turn depend ultimately on the amount, kind, and size distribu- 

 tion of the clay and non-clay size fractions, the kind and concentra- 

 tion of exchangeable ions and soluble .salts, the type and amount of 

 organic material present, the mechanics of deposition, the past loading 

 history of the deposit, and the past history of wetting and drying 

 cycles. 



Tchourinov (1945) has explained the slides in Cretaceous clays 

 along the banks of the Volga River somewhat as follows. During 

 the process of weathering, pyrite in the clay decomposed, calcium 

 carbonate in the clay was attacked, magnesium ions increased through 

 the decomposition of glauconite, and the ratio of sodium to calcium 

 and magnesium in the clay decreased markedly. Presumably through 

 base exchange involving the calcium or magnesium ions or both, the 

 thickness of bound-water films decreased, and the clay assumed a 

 granular structure leading to increased porosity, permeability, and 

 free water content. Cohesion and shearing strength decreased and 

 sliding occurred. Only the weathered clay was involved in movement. 



When unfissured natural earth materials are mechanically mixed 

 and remolded at constant water content or even disturbed by vibra- 

 tion, they commonly lose much of their shear strength. In clayey 

 materials the mechanics of the change is imperfectly understood, but 

 it is thought that loss of strength is due to a breaking down of more 



