Determination of the soil–water characteristic curve of unsaturated bentonite–sand mixtures

To investigate the effect of the changes of the soil suction on volume changes, laboratory experiments were conducted on pure bentonite and bentonite mixed with sand of proportion: 30, 40 and 50% at different initial water contents and dry unit weights that were chosen from the compaction curves. In addition, large-scale model with dimensions (700 × 700 × 650) mm was used for testing soils prepared at the same initial water content and dry unit weight to show the effect of water content changes on different relations (swelling potential, swelling pressure and soil suction). The soil water characteristic curve (SWCC) was measured by the tensiometer at depths 200 and 500 mm from the surface footing. The SWCC was obtained experimentally from the tensiometer method. Different curves were obtained due to different initial water contents and different soils. It was concluded that the suction tends to decrease steeply with increase of the initial water content. The increase in bentonite content shifts the SWCC slightly. The residual water content increases as the bentonite content increases when the residual water content decreases from 44 to 32% when adding 30% sand to pure bentonite due to that the samples with higher bentonite content had a higher volumetric change potential on wetting than samples with low bentonite content. The air-entry value (AEV) increases with increase of initial water content and clay content, where AEV for B1 (pure bentonite) soil sample is 10 kPa, while AEV is 25 kPa for B2 (pure bentonite) soil sample due to increase of initial water content from 25 to 34%; however, AEV for BS1 (70:30, bentonite/sand) soil sample is 8 kPa, on the other hand, AEV reaches 6 kPa for BS3 (60:40, bentonite/sand) soil sample due to decrease of the clay content in this sample.