Specimen geometry-dependent small-to-large strain stiffness

초록

This paper examines the behavioral characteristics of soil across various diameter-to-height ratios and relative densities using an oedometer system, a method of laboratory testing. We design a floating-ring oedometer cell combined together with bender elements for shear wave transducers. The ratio lambda between specimen diameter D and height H is lambda = 1.0, 1.3, 1.7, and 2.0, and remolded sand specimens with relative densities of Dr = 50%, 63%, 76%, and 90% are prepared to simulate the initial fabric conditions for medium-dense to dense states. All specimens experience static step loading to 500kPa followed by unloading step. Load cell data recorded at the top and bottom of the specimens confirm that the soils experience the same vertical load at the both ends. In the void ratio versus vertical effective space, the compression index Cc tends to increase as the diameter-to-height ratio lambda increases. Furthermore, alpha-value and beta-exponent in the power equation of vertical stress vary with the size ratio. The force-equilibrium setup for conventional fixed-ring system under zero-lateral strain conditions reveals that the specimen geometry determines the friction at the soil-wall interface and leads to the exponential decay of vertical effective stress imposed on the top of the specimens. More significant volume contraction occurs in the higher size ratio specimen; then, this vertical deformation leads to the shorter tip-to-tip distance for bender elements. Clearly, the greater size ratio will provide a more reliable maximum shear modulus at small-strain regime. These findings indicate that careful consideration is necessary for selecting equipment specifications when conducting oedometer tests in laboratory experiments, especially using the bender elements.

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