Influence of Surface Layer Permeability on Ground Liquefaction Strength During Long-Duration Motions

Conventional assessments of soil liquefaction risk are based on simplified methods using penetration test results. In this approach, soil density is estimated from penetration resistance, and liquefaction strength is calculated based on the estimated soil density. However, this method primarily evaluates risks associated with excess pore water pressure build-up during an earthquake and does not account for changes in water pressure caused by the movement of pore water during and after the earthquake. It is anticipated these effects will be significant when an impermeable layer exists at or below the ground surface, or when long duration seismic motion occurs. Considering the above, two cases of geotechnical centrifugal model tests with and without impermeable surface layer were performed to evaluate its influence on the liquefaction behaviors of soil strata. Long duration sinusoidal motions of various intensities were applied to the specimens. Based on the test results, the surface layer permeability had a significant influence on pore water pressure buildup in the liquefiable strata beneath the surface layer, especially in the shallower portion of the soil. In addition, by plotting the relationships between response accelerations and excess pore water pressure, the liquefaction strength of the ground was assessed. These relationships showed a high potential for evaluating the liquefaction strength of “geo-structures,” and consider not only the properties of “geo-material” determined from cyclic triaxial tests but also the influence of surface layer permeability. Defining the geo-structural liquefaction strength under various conditions is expected to lead to the establishment of a more reasonable method for assessing soil liquefaction risk.