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.