Swelling characteristics and permeability of bentonite

Swelling characteristics and permeability of bentonite are the essential factors for designing the deep geological repository of nuclear waste. A series of one-dimensional swelling tests due to wetting were first carried out on unsaturated GMZ Ca-bentonite by using oedometer, for analyzing the swelling characteristic of unsaturated bentonite. The relationship between montmorillonite void ratio and vertical pressure after full swelling can be represented as a unique linear equation in their log-log plot. From this line, the volumetric strain from the initial state to saturated state under a given vertical stress and the swelling pressure when reaching saturation can be predicted for bentonite with different initial states. After the swelling tests, GMZ Ca-bentonite reaches saturated state. A series of one-dimensional compression tests on saturated GMZ Ca-bentonite were conducted. The permeability coefficient was calculated by Terzaghi’s one-dimensional consolidation theory after the consolidation coefficient was obtained by the square root of time method. The result shows that in elastoplastic stage the calculated permeability coefficient decreases linearly with increasing vertical stress in double-logarithmic scale. Moreover, calculated permeability coefficient decreases linearly with decreasing void ratio in semi-logarithmic scale. The test results indicate that the indirect method derived from consolidation theory is feasible to calculate the permeability coefficient of saturated bentonite, and thus the indirect method is suitable for the permeability coefficient measurement of low permeable saturated clays.