On the Long-Term Consolidation of Undisturbed Fully-Saturated Clays

Abstract

Some experimental studies have been performed to investigate the deformation characters of undisturbed fully-saturated clays. It has been well-known that the secondary compression proceeds even after the dissipation of water from clay specimens has ceased, which cannot be explained by the famous Terzaghi's theory. From the author's point of view, it seems there exists a common tendency in the conventional approaches to investigate the secondary compression of clay that the phenomenon has been treated through somewhat formal model analyses.In the one-dimensional consolidation one should consider the anisotropy of stresses applied to soil specimens, which causes the shear creep by movement of the relative position of clay particles.Considering the stress and strain relation during the oedometer test, one can recognize that the coefficient of lateral earth pressure (K=σ3/σ1) is equal to unity at the beginning of consolidation. With the time elapses a lateral displacement (-δ) due to the volume change occurs which compensates the lateral shear displacement (+δ), the lateral pressure σ3 decreasing steadily. Thus K becomes smaller than unity with time. This mechanism has been confirmed by the triaxial consolidation test controlled lest any lateral strain should occur (see Fig. 5 (c)). The change of the coefficient K during consolidation can be represented on the Rendulic's stress plane (Fig. 1 (a), (b)) and the stress locus in terms of the effective stress should be determined by the behaviour of the pore pressure in the specimen.It has been found from the test result indicating the correlation between the volume change ΔV/V and σm', the mean effective stress that ΔV/V increases rapidly at the end of 100%-primary consolidation for any value of K (Fig. 2).A series of long-term oedometer test (for 13 weeks) using an undisturbed fully-saturated silty-clay (Fig. 3) shows:(1) the rate of the secondary compression does not depend on the sample thickness, the change of void ratio being taken as the ordinate, (2) the time of the end of primary consolidation t100 is exactly porportional to the square of the sample thickness, (3) the time of the end of secondary compression is about 2 weeks for the standard sample of 2cm height, and(4) the secondary and primary compression ratio increases with decreasing the sample thickness.On the other hand, a series of triaxial consolidation test under anisotropic stress condition shows the fact that the rate of the secondary compression increases with decreasing the effective principal stress ratio (K'=σ3'/σ1') (see Fig. 4).As a result it can be concluded that the rheological parameters of clay have to be determined by the triaxial consolidation test in which no lateral strain is permitted, under the same stress condition at the site where the settlement of clay layer is to be estimated.