Influence of coefficient of uniformity and base sand gradation on static liquefaction of loose sands with silt

When occurred on site static or flow type liquefaction could result in catastrophic consequences due to its sudden occurrence with large strains. Presented here is an experimental study based on monotonic undrained triaxial compression tests conducted on three clean sands and their mixtures with three different non-plastic silts at three different fines contents (≤25%). The results demonstrate that base sand gradation has significant influence on the static liquefaction potential of clean and silty sands. It was observed that clean sands become more liquefiable as their mean grain size got smaller and/or they became more uniform. However, it was found that the order of liquefaction resistance of the same base sands were reversed when they were mixed with silt (i.e. resulting silty sands become more liquefiable as the mean grain size of base sand got larger and/or base sand became relatively well graded). Possible reason for such a reversed behavior was hypothesized and then experimentally justified with extra tests. It was also found that the influence of base sand gradation on static liquefaction of loose specimens was most significant at low fines content (e.g. 5%) and almost erased at relatively high fines contents (e.g. 25%). In the last part of the study, the relationship between the normalized peak deviator stress (qpeak/σ'3c) and coefficient of uniformity (CU) is discussed. It was shown that unlike clean sands, for which liquefaction potential decreases with increasing CU, the liquefaction potential of sand-silt mixtures reconstituted in the laboratory increases with increasing coefficient of uniformity (i.e. technically as they became more well graded). Two equations were proposed to represent the discussed relationship between qpeak/σ'3c and CU; one for stable and temporarily liquefied specimens, the other for liquefied specimens. Finally, the applicability of these equations to other types of silty sands in literature was shown.