AbstractEngineers have worked greatly on measuring the strength of soils but relatively little on the fundamental geologic causes of strength. Strength depends principally upon the content of (1) water, (2) clastic materials and (3) plastic materials. Soils are primarily of two types (1) cohesionless soils in which the strength is produced mainly by the friction of clastic particles against one another, and (2) cohesive soils in which the strength, among other things, is influenced by forces between clay particles. The present investigation is a study of the effect of clay content upon the strength of cohesive soils. The strength was measured by a shear vane device working upon synthetic mixtures of clays of known composition. In each mixture strength varies inversely with water content in a straight line relationship when strength is plotted logarithmically and water arithmetically. Mixtures of glycerine with vol-clay (a montmorillonite) give a curvilinear relationship. For given water content the strength increases with respect to type of clay from kaolin through illite, ball clay to montmorillonite. Strength also increases progressively with increasing clay-sand ratio for all types of clay. In clay-sand mixtures of given clay composition strength increases with increasing fineness of grain of the sand mixed with clay. The liquid limit likewise increases regularly with increasing clay concentration and varies with clay type fci the same way as does strength. Strength varies inversely with temperature to a slight extent, changingjess than one percent per degree Centrigrade. Hydrogen kaolin clay, for given water content is several times stronger than sodium clay.
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