Experimental study on three-dimensional swelling pressure of compacted GMZ bentonite–sand mixtures

The three-dimensional swelling pressure tests on the compacted Gaomiaozi (GMZ) bentonite–sand mixtures are carried out under different dry densities, sand contents and temperatures. The results show that anisotropy of swelling pressure is observed in the mixtures and the vertical swelling pressure is greater than the horizontal swelling pressure in the range of test conditions (dry densities of 1.6, 1.7, 1.8 and 1.9 g/cm3, sand contents of 0, 10%, 20% and 30%, temperatures of 30, 45, 60 and 75 ℃). The development of swelling pressure with time shows significant stage characteristics. The three-dimensional swelling pressure increases rapidly before 400 min, basically reaching 85% of the ultimate swelling pressure, and reaches equilibrium within 1400 min. The ratio of horizontal swelling pressure to vertical swelling pressure decreases with increasing dry density, which is about 0.87 when the dry density is greater than 1.8 g/cm3. When the sand content is above 20%, the increase of sand content has little effect on anisotropy. When the sand contents are the same, the vertical swelling pressure of the bentonite–sand mixtures increases with the enlargement of the dry density at temperature of 30 ℃. Under the same temperatures and dry densities, the three-dimensional swelling pressure decreases linearly with the increase of the sand content. Moreover, the slope of the relation curve increases linearly with the dry density. In the range of dry densities, when the temperature is higher than 45 ℃, the vertical swelling pressure decreases with the increase of temperature. The experimental results are of reference value for design of buffer/backfill materials in high-level radioactive waste (HLW) repository.