Undrained Monotonic and Cyclic Triaxial Strength of Sand

The undrained behavior of saturated sand in monotonically loaded triaxial tests is discussed using steady state concepts. The relations among various drained and undrained strength envelopes are first examined. These monotonic loading concepts are then used to explain the effect of static shear stress, ${\tau }_S\text{,}$ on the undrained cyclic triaxial strength of anisotropically consolidated specimens. The effect of ${\tau }_S\text{,}$ is found to depend on whether a specimen is contractive, dilative or partially‐contractive. For contractive specimens, the cyclic strength always increases with increasing ${\tau }_S\text{,}$ if there is stress reversal and if the peak shear stress, ${\tau }_p\text{,}$ is smaller than the steady state shear strength, $S_{\mathrm{us}}\text{.}$ This effect of stress reversal on cyclic strength is explained by the larger cyclic strains generated in the extension part of the cycle. However, if ${\tau }_p>S_{\mathrm{us}}\text{,}$ cyclic strength can increase or decrease with increasing ${\tau }_s;$ if there is no shear stress reversal, the cyclic strength always decreases with increasing ${\tau }_S\text{.}$ For dilative specimens, cyclic strength always increases with increasing ${\tau }_S\text{.}$ Partially‐contractive sand under cyclic loading behaves like contractive sand for small accumulated failure strains, and similar to dilative sand for large failure strains.