Abstract
The 2016 Kumamoto earthquakes struck Kumamoto and Ōita regions and caused several devastating liquefaction induced damages. The primary damage was due to the extreme ground shaking of the foreshock and main shock sequence. Therefore, it is essential to develop a quick reliable approach with a high accuracy to assess the ground situation after foreshock or several shocks. Velocity of small shear wave (Vs) was widely used for evaluating the potential liquefaction. This study investigates the possibility of using Vs as a new indicator of the stress state in the soil after earthquake and evaluation of post liquefaction resistance of soil. Cyclic tri-axial apparatus equipped with bender elements were used to conduct two consecutives liquefaction tests on sandy soil. The Vs measured by bender elements was discussed on the relationship with effective stress during the liquefying processes. The results showed that for the sandy soils, a) Vs could not clearly reflect the significant reduction in resistance at re-liquefaction stages by directly comparing the Vs at the end of consolidation between first and stages, b) The shear wave velocity is significantly affected by effective confining pressure c) the shear wave velocity of 190 m/s was at confining pressure of 100 kPa. The shear wave velocity reaches to after 100 m/s after consolidations and re-consolidations. The shear wave velocity was found to reduce to 100 m/s when the effective confining stress reaches to 0 and to around 25 m/s when the double amplitude reaches to 5%; d) the velocity and effective stress decreases with a similar rate in liquefying process.
Highlights
In recent years, many earthquakes induced geo-disasters are reported after extreme events such as the 2011 Tohoku Great earthquakes and the 2016 Kumamoto Earthquakes in Japan
The results indicate that a) In first liquefaction test, Nc shows a linear relation with cyclic stress ratio σd/2σ'0 (CSR) in a logarithmic coordinate in each group with the similar relative densities
Comparing the initial Vs between first and second stage it could not reveal the correlation between Vs and the variation of anisotropy, as Besides, the excess pore water pressure increased gradually with the liquefaction potential increasing during cyclic test process in first and second liquefaction stages
Summary
Many earthquakes induced geo-disasters are reported after extreme events such as the 2011 Tohoku Great earthquakes and the 2016 Kumamoto Earthquakes in Japan. According to the field investigation conducted right after the earthquakes, the two major shocks were the main reason for the extreme liquefaction induced damage. K. Ishihara et al [36] found that the variation was mainly affected by the stress history in previous stage of loading by the cyclic triaxial tests. Ishihara et al [36] found that the variation was mainly affected by the stress history in previous stage of loading by the cyclic triaxial tests They defined the line of phase transition which makes the boundary with failure line. Some other researchers believe that the anisotropy was affected by the final load directions in cyclic tests They figured out that the liquefaction resistance depends on the loading directions between the last direction in previous stage and the direction in current stage [7]. This study assessed the possibility of using the shear wave velocity (Vs) to determine the liquefaction potential of post-liquefied soils by discussing the stress states and the probable changed anisotropy
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