A comparative study of the influence of fines on the liquefaction susceptibility of sands: Field versus laboratory

Widely used field-based liquefaction evaluation procedures account for the influence of non/low-plastic fines by adjusting or "correcting" SPT N-values and CPT tip resistances with "fines content correction factors." In general, the correction factors increase with increasing non/low-plastic fines content (FC) until reaching a "cap" at FC - 35%. This implies that for a given in-situ index value (e.g., N1,60 value), liquefaction resistance increases as the non/low-plastic fines content increases up to FC - 35% and remains constant thereafter. However, laboratory studies show a significant decrease in the cyclic resistance of silt-sand mixtures at non/low-plastic fines contents greater than the limiting fines content, which ranges from FC ~ 30 to 40% for many silt-sand mixtures. Although fines content correction factors reflect both the influence of fines on the in-situ indices (e.g., N1,60 values) and on the cyclic resistance of the soil, the laboratory results suggest that there should be a reduction in the fines content correction factors at higher fines contents. To evaluate whether field evidence corroborates the limiting fines content phenomenon, the authors re-analyzed SPT liquefaction case histories. The results of the re-analysis indeed show trends consistent with those observed in the laboratory (i.e., a significant drop in the cyclic resistance of soils is observed in soils having FC > 35%), thus implying that the currently used fines content correction factors may overestimate the liquefaction resistance of sandy soils having non/low-plastic fines contents greater than ~35%.