Residual and Postliquefaction Strength of a Liquefiable Sand

Seismic design of geotechnical structures often requires estimates of how shearing resistance of liquefiable soil is reduced to its minimum value (residual strength, s r ) as pore pressures build up and is subsequently regained as pore pressures dissipate. It was envisioned that the shear strength of liquefying sands could be measured in-flight in a seismic geotechnical centrifuge model by pulling thin coupons (plates) horizontally through the sand models before, during, and after shaking to simulate the large strains and strain rates associated with liquefaction flow failures. This paper presents the results of seismic centrifuge tests that were used to make such measurements. Companion ring shear (RS) tests also are described. Although centrifuge and RS residual strengths were generally similar and increased with relative density, the centrifuge s r and s r /σ ′ vo (s r divided by preshaking effective vertical stress) increased significantly with small changes in relative density, while the RS test s r and s r /σ ′ vo increased only slightly with changes in relative density. Furthermore, many measured s r and s r /σ ′ vo values fell below penetration test-based design curves used in practice. Dilative tendency during shearing is believed to have resulted in a dramatic increase in s r and s r /σ ′ vo in the centrifuge tests that consisted of soil dense of the critical state. In contrast, RS tests likely resulted in shear band development, where intense shearing and particle damage occurs, leading to suppressed soil dilative tendency and divergence from the centrifuge test results at higher relative densities. The centrifuge tests provided evidence that, as expected, postliquefaction strength recovery is linearly proportional to effective stress as excess pore pressures dissipate.