Quasi-steady state: a real behaviour?

Most loose sand samples exhibit the so-called quasi-steady state (QSS), which is a limited liquefaction behaviour under undrained loading conditions. However, the QSS behaviour is not in agreement with the concepts of the critical void ratio and the steady state of deformation. Undrained and drained stress-controlled triaxial tests with a rapid data collecting system were conducted on Unimin sand to investigate the QSS behaviour. The test results indicate that all loose Unimin sand samples exhibited the QSS behaviour under undrained triaxial loading conditions. A quasi-steady state usually commenced prior to a strain of 10% and continued until a strain of about 15%; subsequently, the deviator stress increased with further strain. In drained triaxial tests on the loose sand, after peak, the deviator stress could remain constant, while the void ratio remained constant temporarily, then continuously increased with strain. Detailed test data indicate that the QSS behaviour may be a test-induced behaviour. The post-phase transformation increase in deviator stress in the QSS may result mainly from end restraint. It may also result from the undrained volume change due to the variation of membrane penetration and the compression of pore fluid, and from the conventional area correction. Therefore, the minimum shear resistance at phase transformation should be used as the steady-state strength.