(1. Research Center of Coastal and Urban Geotechnical Engineering，Zhejiang University，Hangzhou，Zhejiang 310058，China；2. College of Architecture and Civil Engineering，Wenzhou University，Wenzhou，Zhejiang 325035，China)
Abstract：Undrained and constant shear drained(CSD) tests were conducted on loose Toyoura sand using high-precision variable confining pressure triaxial apparatus，which can maintain the deviator stress constant，more reliable results can be obtained from this study. Based on the test results，it can be concluded that the specimens of the same void ratio under different effective stresses have the same instability line，but the instability line slope of sand with the same effective stresses decreases gradually along with the increase of void ratio and tends to be stable. The instability can occur under completely drained conditions，instability as defined in this paper refers to a behavior in which large plastic strains are generated rapidly and continuously，and the axial strain and volumetric strain develop rapidly. In the previous CSD studies axial force were maintained constant；the decrease of the deviatoric stress during the process of test may affect the judgment of the instability；this may be the important reason for the researchers hold different conclusions. The instability occurring under drained conditions is different from that observed under undrained conditions. The differences and similarities between the two types of instability were elaborated. It can be included that instability of undrained tests and CSD tests occur on the similar stress ratios for a given void ratio of specimens. The test results in this study provide a more powerful evidence for the research of instability under drained condition；and they also can be used to analyze the failure mechanisms of granular soil slope.
2014, Vol. 33 
