Author(s): Sridharan A, VenkatappaRao
The soft clay of Ariake Bay, in western Kyushu, Japan covers several hundred square kilometers. Ariake clay consists of the principal clay minerals namelysmectite, illite, kaolinite and vermiculite, and other minerals in lesser quantity. The percentage of the principal clay mineral can vary signi®cantly. The percent clay size fraction and the salt concentration can also vary signi®cantly.In view of the importance of undrained shear strength in geotechnical engi-neering practice, its behavior has been studied with respect to variation in saltconcentration. Basically two mechanisms control the undrained strength in clays,namely (a) cohesion or undrained strength is due to the net interparticle attractive forces, or (b) cohesion is due to the viscous nature of the double layer water.Concept (a) operates primarily for kaolinitic soil, and concept (b) dominates pri-marily for montmorillonitic soils.In Ariake clay, different clay minerals with different exchangeable cations and varying ion concentration in the pore water and varying nonclay size fraction are present. In view of this while both concepts (a) and (b) can coexist and operatesimultaneously, one of the mechanisms dominates.For Isahaya clay, concept (a), factors responsible for an increase in level of ¯occulation and attractive forces result in higher undrained strength. Increase in saltconcentration increases the remolded undrained strength at any moisture content.For Kubota and Kawazoe clays, concept (b) factors responsible for an expansion of diffuse double layer thickness, resulting in higher viscous resistance, increase theundrained shear strength, that is, as concentration decreases, the undrained strengthincreases at any moisture content.The liquid limit of Isahaya clay increases with increase in ion concentration and a marginal decrease is seen for both Kubota and Kawazoe clays, and their behaviorhas been explained satisfactorily.