Role of boundary condition, stress-suction coupling, and effective stress in swelling of densely compacted Gaomiaozi bentonite

The swelling behavior of compacted bentonite needs to be analyzed in detail to ensure the buffer functions in deep geological repositories. In this study, various swelling tests were conducted on densely compacted Gaomiaozi bentonite and role of the boundary condition, stress-suction coupling, and effective stress was analyzed. Results show that the porewater density, total suction, and swelling pressure after water saturation increased with the final dry density. The void ratio of the saturated specimen depends solely on the vertical pressure, and the relationship between the water ratio (void ratio times the degree of saturation) and total suction is also unique. In addition, total suction of the specimen after stress release exceeded the imposed target suction and the difference increased with the vertical pressure. Moreover, total suction of the saturated specimen increased linearly with the vertical pressure, and the ratio of the swelling pressure to the total suction increased exponentially with the degree of saturation. In view of this stress-suction coupling, total suction of the specimen after stress release was adopted as the microscopic effective stress. At last, an aggregate dry density (mass of the solid phase divided by volume of the clay aggregates) was employed to characterize the extent of swelling. The relationship between the effective microscopic stress and aggregate dry density is unique, indicating that the effective stress principle is valid during swelling of the bentonite microstructure.