Velocity features of shear wave in sand in liquefying progress under cyclic tri-axial test with bender elements

During the liquefying progress of soils in cyclic tri-axial tests, excess pore water pressure increases gradually until it could be balance to the initial interactive force between soil particles. In macroscopic, soil effective stress and shear stiffness keep decreasing by this process until being reduced to zero while soil is completely liquefied. Utilizing of the propagation characteristics of shear wave in different soil states in tri-axial tests, this study is expected to find out the variation features of velocity of shear wave in the whole liquefying progress, and to discuss about the relations between shear modulus G, which is directly related to the velocity of shear wave, and effective stress σ'. Bender elements were used for the measurements of velocities of shear wave in the tests. Saturated fine sand with different relative densities was conducted with the same test conditions in the study. The results mainly indicated that initial velocities of specimens in all cases achieved approximate same value of 200m/s under a same consolidation conditions. Contrarily, the velocities, measured immediately when specimens liquefied, were quite different in each case. Meanwhile, less than 100m/s of velocities of share wave were measured in all cases while sand was liquefied, and the velocities further reduced to less than 50m/s in two cases with greater relative density. The test progresses showed that the velocities decreased rapidly in the initial stages and approaching liquefaction stages. Besides, the results revealed the relation between shear modulus G and effective stress σ' of that the σ' decreased faster than the G in a very short period after test start, on the contrary, decrease of G became faster than σ' in a short period before the liquefaction.