Sensitivity of total stress to changes in externally applied water pressure in KBS-3 buffer bentonite

This report describes two laboratory tests undertaken to examine the effect of increasing porewater
pressure and its relationship to total stress and swelling pressure in KBS-3 specification
bentonite. Laboratory data clearly demonstrate that at porewater pressures of 46 MPa the
bentonite retains a significant component of its original swelling pressure. Analysis of the total
stress data demonstrates significant hysteresis between ascending and descending porewater
pressure histories. The amount of hysteresis appears to be linked to the magnitude of the
porewater pressure applied to the specimen, suggesting some form of “stress memory” within
the clay. By the end of the second test the average swelling pressure had increased from 7.2 MPa
to 14.4 MPa, which if correct, has important implications for repository performance assessment.
No evidence for classic liquefaction of the clay at high water pressures was observed in this
experimental study.