Interpretation of the Upper Plastic Limit of Clays

Abstract

THE upper plastic limit, or liquid limit, of clays is regarded as that water content at which sufficient free water is present to allow clay particles to slip past one another under a certain applied force. It can also be interpreted as the distance between particles or between structural units of particles at which forces of interaction between the clay particles become sufficiently weak to allow easy movement of particles or units relative to each other. This interpretation allows an easier explanation of the influence on the liquid limit of such factors as exchangeable ion, salt concentration and pH. of the clay paste. For this interpretation it is convenient to define an ‘interaction volume’ for a unit mass of clay particles as that volume within which individual particles or units of particles interfere with the movement of adjacent particles or units. The interaction volume will decrease as the applied force increases.