234 varnes. LANDSLIDES [Cn. 13 



may cause sufficient swelling to damage structures (Joly and Ninck, 

 1935). Gypsum or, more properly, the calcium ion also contributed 

 to landsliding in clays along the banks of the Volga River (Tchouri- 

 nov, 1945). 



This reference to base exchange in clay leads to the subject of the 

 clay mineral group, which as a factor in composition ranks in im- 

 portance with water in the determination of stability or instability 

 of slopes. Clayey soils and sedimentary deposits are widely used as 

 foundation and construction materials; they are involved in land- 

 slides throughout the world, and they pose problems of maintenance 

 in countless railroad and highway cuts. The needs of engineering, 

 economic geology, ceramic technology, and many other fields have 

 resulted in intensive research on the physics and chemistry of clays, 

 but only the briefest mention of a few of the results of this research 

 can be made here. 



For further information on clays, the reader's attention is directed 

 to the many papers and their bibliographies in the First and Second 

 International Conferences on Soil Mechanics and Foundation Engi- 

 neering (at Harvard in 1936, and at Rotterdam in 1948) ; to papers in 

 Soil Science of America; to texts on soil mechanics such as those by 

 Terzaghi (1943) and Terzaghi and Peck (1948) ; and to articles by 

 Grim (1942), Casagrande (1932), Terzaghi (1941), Hendricks (1942), 

 Rutledge (1944), and Winterkorn (1942). 



Physical Properties 



The physical properties of earth materials that contain fine-size 

 fractions depend very largely on the character of the smallest-size 

 particles, especially those particles of less than 2 microns. The prop- 

 erties of these small particles are influenced by the intensity of the 

 negative electric charge on their surface. Because water molecules 

 are polar, the positive (hydrogen) ends of the water molecules are 

 attracted to the solid particles, where they orient into an adsorbed 

 layer. Near the mineral particle, the film is practically solid, farther 

 away the film is viscous, and near the outer surface the water is 

 liquid. If little water is present, the rigid, adsorbed layer on one 

 particle meets that on another, and the whole mass is rigid or highly 

 cohesive. With more water, the films thicken and lubricate the par- 

 ticles, making the mass plastic. With still more water the mass be- 

 comes fluid. The thickness of the water films, their degree of orienta- 

 tion, and their efficiency as binding or lubricating agents depend on 

 the amount of water available, the charge on the mineral surface, and 

 the surface area available among the mineral fragments. The clay 



