Dynamic properties of soils at very small strain, i.e. small strain shear modulus (G0) and shear wave velocity (Vs), are used frequently in geotechnical applications. Many researchers have studied the effective factors on the small strain shear modulus in clean sands or sand-fine mixtures. However, less attention has been given to the dynamic properties of gravelly soils at small strain and they are poorly understood. Reviewing the technical literature, one may find it very interesting to study the impacts of non-plastic fine content on the small strain shear modulus of gravelly soils. A fundamental experimental study was designed to explore the influence of non-plastic fine content on the small strain shear modulus of gravel-sand-silt mixtures (common geomaterials in nature). Since gravel and silt mixtures with no sand particles are less common in nature, three types of sandy gravels with different sand-to-gravel ratios of 0.25, 0.43 and 0.67 were selected as base gravelly soils. In order to isolate the impacts of the fine content, sand-to-gravel ratio was kept constant in each soil type. Various percentages of silt (0~45%) were carefully added to the base soil. Eighteen mixtures of sandy gravels with different silt contents were prepared. Bender Element tests were carried out under saturated conditions to determine the small strain shear velocity. Samples were prepared by the moist tamping method due to its advantages for making loose and homogeneous samples without creating any segregation of grains. Following the saturation process, specimens were subjected to three isotropic confining pressure levels of 50, 100, 150 kPa. The relative densities of the samples were carefully kept constant to avoid the density effect on the responses. To keep the densities of samples constant while varying the fine content percentages, the initial relative density (Dr before consolidation) was selected in a way to ensure that the target relative density (Dr after consolidation) was approximately 35% (i.e., Dr=32% ~ 38%). The appropriate initial relative densities for each mixture and for various effective confining pressures were obtained using iterative efforts. Laboratory results show that the maximum shear modulus increases in all mixtures when the effective confining pressure increases. The small strain shear modulus is significantly impacted by the percentage of non-plastic fine content and sand-to-gravel ratio of base gravelly soils. Increasing the percentage of silt and also the sand-to-gravel ratio causes the shear modulus to decrease constantly. The reduction (rate of which is different for mixtures and effective confining pressures) can be explained from the micromechanical perspective and the formation of strong and weak force chains through interparticle contacts. Finally, Hardin's general equation is fitted to experimental data and fitting parameters are found using regression analysis. Empirical relationships are presented to estimate the small strain shear modulus in each of those three types of soil. These correlations are a function of non-plastic fine content, void ratio and effective confining pressure of mixtures. The comparison between the estimated and the measured small strain shear modulus clearly indicates that the purposed equations yield good agreement with experimental data. Therefore, these equations can be used to predict the small strain shear modulus for different mixtures of gravel-sand-silt in many geotechnical applications such as soil improvement, evaluation of liquefaction potential and the design of dynamic foundations.
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