The influence of the fluid dielectric constant on the shear strength of a unsaturated soil

Results of triaxial tests performed in saturated and unsaturated compacted soil specimens with different interstitial fluids are presented. Tests were carried out in order to study the influence of the fluid relative dielectric constant, epsilon(r), on the soil shear strength of a granite-gneiss clayey residual soil from Salvador, Bahia, Brazil. It is shown that the soil shear strength is affected not only by the interstitial fluid saturation degree (or suction) but it is also a nonlinear function of the interstitial fluid value of epsilon(r). The shear strength of the saturated samples decreased with epsilon(r), following the order (air epsilon(r) = 1, diesel epsilon(r) = 2.13, ethanol epsilon(r) = 24.3 and water epsilon(r) = 80), whereas fluids with higher dielectric constants presented a more pronounced increase in shear strength under drying (replacement of the interstitial liquid with air). An empirical model is proposed to predict soil shear strength as a function of (epsilon(rw) - epsilon(f)), the difference between the relative dielectric constant of the water and the interstitial fluid. Good adherence between experimental and fitted results was obtained.