Discussion of “Viscous behaviour of clays in one-dimensional compression”

Abstract

Wong and Varatharajan hypothesize that the time-dependent or creep deformation under one-dimensional constant effective stress involves creep deformations of individual particles and creep shear deformation at interaggregate contacts. They have conducted different experiments on reconstituted kaolinite clay samples to investigate certain creep mechanisms. In the rheological model they tend to advocate, an implicit assumption made is that the creep strain starts at end of primary (EOP) consolidation and the void ratio at EOP, eEOP, is unique. Consequently, the authors interpreted their test data with this assumption and ended up asserting that “... the creep compression behaviour of claymay be uniquely related to its eEOP ...”. This is in contradiction to the opening statement of their Introduction where they state that creep deformation occurs in the entire compression process involving primary consolidation and secondary compression. The authors’ attempt to investigate the effect of drainage path on resulting void ratio, by conducting tests with top and radial drainages, is too limited to appreciate and be conclusive about this important effect. However, Degago et al. (2009) interpreted experimental data by Feng (1991) and showed that EOP tests on 127 and 508 mm thick samples, under similar loading and drainage boundary conditions, yielded a nonunique eEOP. Differences in eEOP can further be demonstrated by differences in vertical deformation of a soil element located closest to and farthest from a drainage boundary. Tests by Feng (1991) configured in such a way revealed that soil elements closer to the drainage boundary have lower eEOP values when compared with elements farthest from the drainage boundary (Degago et al. 2011) The authors tried to demonstrate unique EOP values by comparing multiand singlestage tests. The discussers do not see how the authors “proof” for unique eEOP values holds true. In Figs. 4 and 7, the authors plot log (that can be reformulated into inverse of strain rate over stress) versus strain after EOP. The inclination of this line is the reciprocal value of C (for a remolded sample in practice a constant value is expected) and the shift in vertical direction as plotted by the authors is close to log( ), which comes from multiplying the inverse of strain rate with stress. If the authors had instead presented a plot of the inverse of the rate of void ratio change versus void ratio for the different stress levels, the plot would have shown a unique relation between void ratio, rate of change in void ratio, and effective stress level as stated in the isotache concept (Suklje 1957). This demonstrates that the isotache concept also holds for the tests conducted and presented by the authors. This, thus, implies that that eEOP cannot be unique, but depends on the experienced strain rate, i.e., time it takes to reach “EOP”, and the position of the material with respect to the drainage boundary. In conclusion, the discussers believe the primary aim of such laboratory tests is to enable proper characterization of the field behavior of soils and in this aspect the extension from laboratory to field condition is observed to follow isotache formulation (Leroueil 2006) — this implies that creep occurs during the entire soil compression process and eEOP cannot be unique and therefore cannot be considered as a soil parameter.