Undrained Shear Strength and Monotonic Behavior of Different Nonplastic Silts: Sand-Like or Clay-Like?

The common assumption in geotechnical engineering research and practice has been that nonplastic silts behave sand-like, whereas, as silts become more plastic, they start to behave clay-like. To date, there are practically no studies focusing solely on various nonplastic silts and their monotonic response, perhaps because of the solid belief that nonplastic silts behave sand-like in all aspects. A series of constant volume direct simple shear tests were conducted on three different nonplastic silts. Because the specimens were reconstituted to have loose grain structures, volumetrically contractive tendency was observed for all the silts with relatively small effective stress friction angles. It was observed that Ko compression lines and critical state lines are parallel to each other for all three silts, which is considered to be a clay-like behavior in literature. It was shown that undrained shear strengths can be normalized with the effective vertical consolidation stress, which is also considered to be a clay-like behavior in literature. Normalized values are compared with the ones obtained for plastic silts compiled from previous studies. Normalized undrained shear strength values for plastic silts in literature were seen to be typically greater. Possible reasons such as density effect and strength anisotropy were discussed.