Effects of Stress and Suction on the Volume Change Behaviour of GMZ Bentonite During Heating

Unsaturated compacted bentonite is considered by several countries as sealing/backfill material in the deep geological repository for high-level radioactive waste (HLW). In the field conditions, where bentonite is subjected to coupled thermo-hydro-mechanical actions, its thermo-mechanical behavior may change with suction and stress variations. This work focuses on the compacted Gaomiaozi (GMZ) bentonite, which has been chosen as potential sealing/backfill material in the Chinese repository concept. The effects of vertical stress and suction on the volume change behaviour during thermal loading (heating) were experimentally investigated. A high pressure oedometer frame permitting simultaneous control of temperature, suction and pressure was used for testing. Compacted samples were heated from the original temperature of 20 to 80 °C at total suctions ranging from 4.2 to 110 MPa, and vertical stress of 0.1 or 5 MPa. Results show that heating at constant suction and vertical stress induces either swelling or contraction. At a constant vertical stress, samples with higher suctions swell during heating, and the lower the suction the lower the swelling strain; on the contrary heating resulted in a thermal contraction with lower suctions. At a constant higher suction (110 MPa), sample swells during heating; however, for a relative lower suction, a contraction was induced by heating for a higher vertical stress of 5 MPa. In short, at high pressure and low suction, heating tends to induce contraction, while at low pressure and high suction, heating induced expansion.