مهندسی عمران شریف (Aug 2018)
EFFECT OF ANISOTROPY ON THE DYNAMIC PROPERTIES OF THE LOOSE BABOLSAR SAND WITH CYCLIC HOLLOW CYLINDER APPARATUS
Abstract
Direction of loading and magnitude of the intermediate principal stress have significant effect on the soil responses. In many of in-situ loading, the direction of major principal stress does not coincide with the deposition direction of the soil. In addition, the magnitude of the intermediate principal stress should be exerted in the three dimensional loading condition. A reliable assessment of the soil behavior and good estimation of the soil parameters need to do tests in similar condition with in-situ. Therefore, the testing apparatus should be able to control the loading direction in various stress paths. Typical equipment used in the geotechnical laboratory does not the ability to control the magnitude and direction of principal stresses. The cyclic hollow cylinder apparatus can control the magnitude and direction of the principal stresses and impose minimum non-uniformity on the specimens. The hollow cylinder apparatus used in this study is fully automated and can simultaneously control five loading axe (i.e., the vertical load, torque, inner cell pressure, outer cell pressure and back pressure). The main specifications of the specimens are as follows: 100mm outside diameter, 60mm inner diameter, and 200mm height. The consolidation control options allow various isotropic and anisotropic consolidation statuses. Moreover, the load could be exerted as cyclic or monotonic. The test material used in this study was Babolsar sand, obtained from the South coast of the Caspian Sea. A total of 30 undrained cyclic torsion shear tests were performed in a cyclic hollow cylinder apparatus in order to specifically investigate the effect of mean confining stress, $\sigma^{'}_{0m}$, intermediate principal stress ratios, as indicated by b=$(\sigma_2-\sigma_3)/(\sigma_1-\sigma_3)$ and principal stress directions $\alpha$.Results showed that the dynamic shear modulus is minimum and the material damping ratio is maximum as $\alpha$ is close to $45^\circ$. This observation is contrary to the current understanding in which progressively weaker responses are to be expected as the major principal stress aligns itself toward the weaker horizontal direction. The intermediate principal stress ratio had no significant effect on the dynamic shear modulus and material damping ratio. The effects of loading direction or intermediate principal stress ratio on the dynamic shear modulus and material damping ratio were magnified by increasing confining stress.
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