Pore-pressure cycling experiments on Mx80 Bentonite

The Swedish concept for geological disposal of radioactive waste involves the use\udof bentonite as part of an engineered barrier system. A primary function of the bentonite is its\udability to swell when hydrated by its surroundings. One particular uncertainty is the impact on\udthis function, resulting from deviations in pore-water pressure, pw, from expected in situ hydrostatic\udconditions. We present results from a series of laboratory experiments designed to investigate\udthe form of the relationship between swelling pressure and pw, for compacted Mx80 bentonite, from\udlow to elevated applied water pressure conditions. The experiments were conducted using constant\udvolume cells, designed to allow the total stresses acting on the surrounding vessel to be monitored\ud(at five locations) during clay swelling. The results demonstrate that swelling pressure reduces nonlinearly\udwith increasing pw, becoming less sensitive to changes at elevated pressures. After cyclic\udloading a marked hysteresis was also observed, with swelling pressure remaining elevated after a\udsubsequent reduction in applied water pressure. Such behaviour may impact the mechanical and\udtransport properties of the bentonite and its resulting performance. However, such hysteric behaviour\udwas not always observed. Further testing is required to better understand the causes of this\udphenomenon and the controls on such behaviour