Direct and Indirect Tensile Tests for Measuring the Equivalent Effective Stress in a Kaolinite Clay

Laboratory tests performed on unsaturated kaolinite clay show that the shear strength is function of the water content. Water contents used for the tests were obtained by drying the clay. Three different intervals of water contents in the clay were identified for which the shear strength varies in different way. When the clay has high water content, close to saturation, the shear strength increases as the water content is reduced. If the clay has a water content in an intermediate interval the shear strength is almost constant. For the interval of low water contents the clay presents lower shear strength as the water content is reduced, which is the opposite tendency for the interval of water contents that is close to saturation. In this article, the three water content intervals are related to three water distribution states of the water in the pores namely: the saturated-funicular state, the complete pendular state and the partial pendular state. On the other hand, the shear strength in the soil is represented as a function of the equivalent effective stress and the friction angle in the clay. The equivalent effective stress is determined through direct and indirect tensile tests. The friction angle that results from the above relationship is very similar to the friction angle of the clay when it is saturated.