Volume change behavior of bentonite-sand mixture under constant suction condition

The unsaturated state of soil can raise many geotechnical problems upon wetting-drying cycles such as volume change resulting in collapse or swell, change in the shear strength, and change in the hydraulic conductivity. The classical principles of soil mechanics for saturated soil are often not suitable for explaining these phenomena. An experimental work was carried out to study the volume change behavior of Bentonite-Sand mixture under constant suction condition. A mixture of 30% Calcigel bentonite and 70 % Haider sand (30B) were tested in this study. The applied suction was kept constant through the tests. Thirteen 30B specimens were tested under constant suction condition. Two different initial states (slurry and loose) were chosen to examine the effect of initial state on the volume change at unsaturated state. The results of first group (initially slurry state) show that once the suction was applied the compressibility of the specimens in this group followed the isotropic NCL up to 255 kPa suction. Subsequently, the slope of compression curve decreased. The results of increasing suction for initially slurry state show that the slope of compression lines became near to zero for suction higher than 600 kPa. Moreover, when applied suction is lower than the air-entry value, s(aev), the soil behaviors as the saturated soil. Therefore, the effective stress equation can be applied. The results of second group (initially loose state) show that the unsaturated preconsolidation pressure increases as the constant suction increases. The slopes of the unsaturated over consolidated paths in all tests were closed to the slope of saturated over consolidated path (Cr), while the slopes of the unsaturated normal consolidated (yield) state are higher than the slope of saturated NCL. Finally the results show that the slope of the unsaturated NCL is higher (steeper) than the slope of saturated NCL.