Volumetric Threshold Shear Strain for Cyclic Settlement

The volumetric cyclic threshold shear strain for cyclic settlement, γtv, is evaluated for seven different sands and clays at different degrees of saturation, S, 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, γc, slightly smaller and slightly larger than γtv, such that γtv could be evaluated from each single test. At γc<γtv, specimens did not experience a permanent change in volume or settle even after a large number of cycles, while at γc>γtv they did. The data show that γtv is larger for clays than for sands and that it generally increases with the soil’s plasticity index (PI). For example, γtv≈0.01-0.02% was obtained for sands and γtv≈0.04-0.09% was obtained for clays having PI≈30. A rather consistent γtv versus PI correlation for cyclic settlement is presented. No obvious trends were found between γtv and S and vertical stress.