Assessment of suction measurements in saturated clays

Three laboratory methods of suction measurement were investigated: the pressure plate technique; measurement of positive pore water pressure after undrained isotropic loading in a triaxial cell; and the filter paper technique. Tests were performed on reconstituted and undisturbed saturated samples of four different clays. For these clays, the pressure plate method appeared to give the most reliable and consistent results. In the case of reconstituted samples, where the stress history was known, pressure plate measurements gave values of suction that were similar to, but slightly higher than, theoretically predicted values, suggesting slight compression of the water within the apparatus during application of the axis-translation technique. Values of suction measured by undrained loading in a triaxial cell were generally higher than measured in the pressure plate apparatus, suggesting a B value less than 1 (B is the pore water pressure parameter), and that use of a pressure plate is preferable even with ostensibly saturated samples. Filter paper measurements of suction were highly dependent on the calibration curve employed, emphasising the importance of deriving an appropriate calibration curve for the particular batch of filter papers being used. Using an appropriate calibration curve, filter paper measurements of suction on reconstituted samples showed good agreement with theoretically predicted values and were generally consistent with pressure plate measurements (although the variability was greater). Filter paper measurements of suction on undisturbed samples, however, were dramatically different from the other methods and often indicated impossibly high values of suction.