Volumetric Threshold Shear Strain for Cyclic Settlement

The volumetric cyclic threshold shear strain for cyclic settlement, ${\gamma }_{\mathrm{tv}}\text{,}$ is evaluated for seven different sands and clays at different degrees of saturation, $S\text{,}$ from the results of 11 Norwegian Geotechnical Institute-type direct simple shear, multistage cyclic settlement tests. Each test included several cyclic strain-controlled stages at cyclic shear strain amplitudes, ${\gamma }_c\text{,}$ slightly smaller and slightly larger than ${\gamma }_{\mathrm{tv}}\text{,}$ such that ${\gamma }_{\mathrm{tv}}$ could be evaluated from each single test. At ${\gamma }_c<{\gamma }_{\mathrm{tv}}\text{,}$ specimens did not experience a permanent change in volume or settle even after a large number of cycles, while at ${\gamma }_c>{\gamma }_{\mathrm{tv}}$ they did. The data show that ${\gamma }_{\mathrm{tv}}$ is larger for clays than for sands and that it generally increases with the soil’s plasticity index (PI). For example, ${\gamma }_{\mathrm{tv}}\approx 0.01-0.02\%$ was obtained for sands and ${\gamma }_{\mathrm{tv}}\approx 0.04-0.09\%$ was obtained for clays having $\mathrm{PI}\approx 30\text{.}$ A rather consistent ${\gamma }_{\mathrm{tv}}$ versus PI correlation for cyclic settlement is presented. No obvious trends were found between ${\gamma }_{\mathrm{tv}}$ and $S$ and vertical stress.