Aging Effects on the Relation between Liquefaction Resistance and Shear Wave Velocity in Sand Deposits

Although shear wave velocity as an index property can be measured in field at low cost without necessarily drilling boreholes, there are some concerns over its use, mainly because of the liquefaction being associated with medium to large shear strain in contrast to the infinitesimal shear strain involved in the propagation of the shear wave. In spite of such potential shortcoming, there has been a lot of work towards the use of shear wave velocity, attempting to establish some charts correlating the cyclic strength and shear wave velocity. Thus, if there is a link between small-strain shear-wave velocity and medium-to-large strain liquefaction, the use of the shear wave velocity will be justified as an effective tool to assess the liquefaction resistance of in-situ deposits of sandy soils, not only for uncemented soils of Holocene age, but also for cemented soils of Holocene and Pleistocene age. In view of this shortcoming, a newly introduced parameter “cyclic yield strain” is proposed to indicate that shear wave velocity can be used to differentiate between the cyclic resistance of fresh and aged sand deposits. This parameter is defined as the ratio of the cyclic stress ratio required to cause liquefaction in a specified number of cycles to the shear modulus at small strains which are determined by the velocity of shear wave propagation. Large numbers of cyclic triaxial tests were performed on undisturbed and reconstituted samples of sand deposits obtained from areas of known liquefaction at the time of the 2011 Great East Japan Earthquake. In this test scheme, shear wave velocity was measured first in the laboratory, followed by the application of cyclic loads to determine the cyclic shear strength. The undisturbed samples were classified into two groups, namely; old alluvial (or diluvial) deep deposits and near-surface shallow deposits which had apparently been disturbed by the liquefaction in the 2011 event. The data were plotted in terms of the cyclic stress ratio versus the shear wave velocity and a curve of equal cyclic yield strain was drawn through average points in the plot for the two groups of soils, that is, one for undisturbed and the other for liquefaction-disturbed soils. It was found that the cyclic yield strain was larger for the latter type of soils as compared to the undisturbed soils from the old aged deposits. Similar sets of laboratory tests were performed as well on several sand samples reconstituted with different relative densities. Similar plots for these reconstituted samples indicated the same average value of the cyclic yield strain as that of the liquefaction-disturbed soils. In view of the fact that the larger the cyclic yield strain, the more ductile behavior the soils exhibit, and vice versa, it was concluded that the cyclic yield strain could be utilized as a yardstick parameter for taking into account the effect of aging or cementation of sand deposits. Accordingly, it is hypothesized that the cyclic yield strain will take larger values for ductile fresh sand deposits and smaller values for brittle aged sand deposits. It is suggested that two different curves pertaining to new and old sand deposits can be used to correlate the liquefaction resistance and the shear wave velocity in sites of natural deposits in order to consider the effect of aging or cementation. Keywords: Aging, Cyclic yield strain, Liquefaction, Shear wave velocity.