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Experimental characterizations of the dynamic property changes in aged sands

Authors Tsui, King Yuen
Issue Date 2007
Summary This study investigates dynamic property changes in aged sands and offers explanations for the measured results based on the concept of contact-force homogenization, as well as whether aging behavior is isotropic in nature. Resonant column tests of aged sands under various aging conditions were conducted. The results show that loose sands exhibit greater aging effects than dense sands do at a confining pressure of 35 kPa and the effects are completely opposite when the aging pressure is increased to 100 kPa. The aging effects can be partially erased by unloading-reloading; the remaining effects can be restored when the pressure is the same as the original pressure and the effects cannot be erased by additional unloading-reloading cycles. The stress history is also a factor that affects aging behavior: unloading-reloading and overconsolidation can reduce the aging rate in terms of the shear-modulus increase. The aging effects, however, can be wiped out by large strain shearing. An addition of fines (kaolinite) can increase the aging rate of the sand samples because of higher creep made by the softer kaolinite. The results of resonant column tests on Ottawa sand samples reveal that the medium-dense sample had the highest aging rate as determined by its shear-modulus increase followed by the dense and loose samples. This result is attributed to two competing processes, the tendency toward force-chain homogenization and the susceptibility to local structure collapse. This attribution does not apply to Toyoura sand samples, which suggests that the effect of packing density on the aging behavior of sand indeed varies with the type of sand. This deviation could be a result of the difference in grain size distribution and particle shape of the two types of sand tested. A hypothesis of the influence zone of particle creeping is proposed as well to explain the larger aging gain in shear modulus in dense samples. Bender element tests in the true triaxial box demonstrate that aging behavior is not isotropic by nature because the increase of shear wave velocities in different polarization planes varies under isotropic stress states. Increasing the stress in one direction leads to more significant increases in the velocities that are either polarized or propagating in that direction.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007
Language English
Format Thesis
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