Laboratory investigation of the stiffness and damping properties of binary and gap-graded mixtures of granular soils




Shin, Boonam

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Sandy and gravelly soils are encountered in many natural soil strata such as alluvial, fluvial, residual, and glacial deposits. These granular materials are used in the construction of many types of man-made geotechnical structures, fills, and ground improvements. A need exists to understand the dynamic behavior of these sandy and gravelly mixtures. A “binary packing model” has been proposed by some researchers to represent a simplified model for some sandy and gravelly soils that are composed of only a few, quite different, particle sizes (typically more than a factor of 20, and a factor of 34 used in this study) created during construction of geotechnical systems or in the natural environment. In other cases, gap-graded materials have been created. The packing condition and void distribution in the two types of gradations are complicated due to large variations in particle size and construction conditions. Up to now, no systematic studies have been conducted to evaluate key factors in the soil matrix (i.e., the parent materials in these mixtures) affecting the dynamic properties (i.e., shear modulus, G, and material damping ratio, D) of the binary and gap-graded mixtures considering the packing state of the matrix particles.

In binary granular soils, estimating the critical packing condition, which represents the case when all void space between the large particles are completely filled with each group of small particles, is important. Additionally, the associated critical small-particle content (SPC*), which is the small-particle content (SPC) under the critical packing condition, can be calculated using the void ratios of the small-particle and large-particle materials.

In this research, systematic variations of the parent materials in binary and gap-graded mixtures (i.e., the small and large particles in binary mixtures) were created, and the dynamic torsional resonant column (RC) testing was performed on a wide range of particle packing conditions. Some findings are: 1) the binary and gap-graded specimens for SPC ≥ 76% behaved very similarly to their parent poorly graded sand in both the small-strain and nonlinear dynamic properties; 2) the binary specimens for 15% ≤ SPC ≤ 100% showed a clear trend in the relationship of small-strain shear modulus at one atmosphere (A [subscript G]) and void ratio (e); 3) the binary specimens for 23% ≤ SPC ≤ 36%, which were in the range of SPC*, showed the highest stiffnesses and most nonlinearity; and 4) overall, the void ratio was a better parameter than the uniformity coefficient to estimate the nonlinear behavior of the specimens for 0% ≤ SPC ≤ 39%.


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