UC Davis

Researchers have recently explored the effects of gradation on earthquake-induced liquefaction behavior, showing that broadly graded sands have a distinct response compared to poorly graded sands. However, current state of practice applies liquefaction procedures developed from poorly graded sands to liquefiable natural alluvial deposits that are much more broadly graded. The goal of this research is to better understand and quantify the systematic effects of sand gradation on embankment performance and the consequences of liquefaction. The soil’s coefficient of uniformity (Cu) is used as a proxy for gradation as it is a single parameter that affects multiple index and density metrics, which in turn generate distinct responses.

The testing series evaluated herein incorporates results from two centrifuge tests, including four different soil types with Cu values ranging from 2 to 12. The centrifuge testing was performed on the 9 m-radius centrifuge at the Center for Geotechnical Modeling (CGM) at UC Davis. The model container was designed with two identical parallel sloping embankments of different gradations, prepared to the same relative density (Dr) of 40%, and configured with arrays of accelerometers and pore pressure transducers. High speed cameras mounted outside the model container recorded embankment plane-strain deformations of dyed soil columns through the clear polycarbonate sidewalls. The models were subjected to four strong motion events of increasing intensity. The results reveal that at the same relative density, horizontal displacements and, strains decreased by over 50% with increasing Cu, whereas settlements reduced to zero and even resulted in net upward displacement due to the strong dilatancy of the broadly graded sands. Despite similar numbers of cycles to liquefaction triggering, the rate of dissipation of excess pore pressures occurred 3-4 times faster for the more broadly graded sands, reducing the duration of soil matrix instability. The accelerations in the broadly graded embankments were also approximately 3 times more amplified than the poorly graded embankments. Together, these results show that gradation plays a significant role in the consequences that proceed from liquefaction triggering, which should be explicitly considered in the evaluation of liquefaction susceptibility and consequences.