مهندسی عمران شریف (Nov 2020)
DYNAMIC BEHAVIOR OF GRANULAR SOILS FROM THE POINT OF VIEW OF EQUIVALENT SKELETON VOID RATIO
Abstract
The shear modulus $(G_o)$, the shear modulus versus shear strain $(G/G_o-\gamma)$ and damping ratio versus shear strain $(D-\gamma)$, are important parameters for the design of structures subjected to dynamic loading and can be obtained by in situ and laboratory measurements. Advanced laboratory testing techniques such as bender element (BE), resonant column (RC) and cyclic triaxial (CT) testing have been developed to study the dynamic properties of soil. Despite extensive studies on the dynamic properties of granular soil, previous research has lacked a quantitative study of the effects of mean effective stress, relative density and gravel content on the dynamic properties from the point of view of equivalent skeleton void ratio. The results from a laboratory experimental study on granular soil are presented. In the current study, $G/G_o-\gamma$ and $D-\gamma$ curves of sand-gravel mixtures have been evaluated using a triaxial cyclic apparatus along with local axial strain measurement and also a resonant column apparatus. The effects of mean effective stress, relative density and gravel content on $G/G_o-\gamma$ and $D-\gamma$ curves have been investigated. Also, using the soil skeleton void ratio, the microscopic changes of particles are discussed. Skeleton void ratios are used to describe the idealized packing conditions of the dominant particle fraction. For soils with distinct coarse and fine particles, the skeleton void ratio is defined as the volumetric ratio between the voids formed by the soil skeleton and the volume of particles that make up the skeleton. Using the $G/G_o-\gamma$ curve, variations of threshold shear strain, $\gamma_{th}$ are also investigated. The results show that decreasing relative density and mean effective stress increase the stiffness degradation and damping ratio in the granular soil. The amount of $\gamma_{th}$ has also increased with increasing mean effective stress and relative density in all subgroups of the soil. In addition, the results presented on the basis of the skeleton void ratio indicate that the dynamic properties of the mixed granular soil can be evaluated using the skeleton void ratio. The dynamic properties of gravelly soils depend on the gravel skeleton void ratio $(e_{gk})$, which is a more representative parameter for the soil packing condition than the global void ratio and the dynamic behaviour of sand-like gravelly soils can be estimated from the sand skeleton void ratio $(e_{sk})$.
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