Effective Stress Change and Cyclic Resistance of Saturated Sands Under Uniform and Irregular Cyclic Shears

In this study, two marine sands collected from Vietnam (Nam O sand) and Japan (Toyoura sand) with similar grain size were tested under undrained uniform and irregular cyclic shears and the effective stress change and liquefaction resistance of the soils were then observed for different cyclic shear conditions. For the uniform cyclic shear, the tests were carried out with various cyclic shear directions, different number of cycles ( n ), and a wide range of shear strain amplitudes ( γ ). Meanwhile for the irregular one, the two-directional cyclic shear tests were performed with different amplitudes of maximum shear strain ( γ max ). It is shown from the experimental results that firstly, the threshold number of cycles ( n tp ) and cumulative shear strain ( G* tp ) for effective stress reduction in saturated specimen at D r = 50 ± 5% of Nam O sand are about n tp = 0.1–0.7 cycle and G* tp = 0.1%–0.5%. For specimen at D r = 70 ± 3% of Toyoura sand, values of n tp are about 0.03–0.3 cycle while those of G* tp is constant at 0.1%. Secondly, based on relationships between the cumulative shear strain G* and γ at the occurrence of liquefaction, the threshold cumulative shear strain for liquefaction was obtained as G* tfU = 15% and G* tfM = 11% for Nam O sand and as G* tfU = 24% and G* tfM = 19% for Toyoura sand under uni-directional and two-directional cyclic shears, respectively. Thirdly, the results of G* tp and G* tfM obtained from uniform cyclic shear were applied to the earthquake-induced irregular cyclic shears, and practical applicability was confirmed for different amplitudes of the maximum shear strain.