Undrained Stress-Strain-Time Behavior of Clays

A single phenomenological equation is shown to provide an adequate and complete description of the undrained stress-strain-time behavior of clays under axisymmetric loading conditions for cases not involving any rotation of the principal stress axis directions. The basic form of the relationship is thereafter idealized so that mathematical solutions may be achieved in closed form. The rheological properties of clays are shown to greatly influence the results of undrained pressuremeter tests. It is demonstrated that a simple analysis of pressuremeter test results which disregards rheological effects leads to a large discrepancy between the predicted and the actual stres-strain behavior of the material. Detailed comparisons are made. Even if the material is actually strain-hardening for constant shear strain-rate loading conditions, the derived stress-strain curve from constant strain-rate pressuremeter tests will exhibit strain-softening properties at conventional testing rates.