Based on a series of calibration tests on the Stokoe combined torsional shear test resonant column apparatus, two novel simple and reliable procedures were proposed for determination of the mass polar moment of inertia of drive system in resonant column test and torque factor for torsional shear test, respectively. These proposed procedures were used to investigate dynamic behavior of Berlin sand and vibration history effects on shear modulus and damping properties of sand by resonant column tests. The proposed method for calibration of drive system can reduce the deviation of shear modulus for stiffer sample based on the conventional method suggested by GDS instruments Ltd from 20% for to around 2%. And the method for calibration of torque factor is able to reduce the deviation of testing results from 19% to below 2% for torsional shear test.
The influences of confining pressure, void ratio, water content, sampling method, stress history, confinement duration, and shearing strain amplitude on dynamic properties of Berlin sand were examined by resonant column tests. An empirical equation was proposed to predict the small-strain shear modulus, Gmax, based on void ratio and confining pressure; and two models were proposed to evaluate the normalized shear modulus and damping ratio, a brief comparison of Gmax by resonant column (RC) and bender element (BE) tests was addressed as well, and the relationship between the Gmax by RC and BE methods was initially developed.
For the investigation of vibration history effects on dynamic sand properties, the author initially introduced preloading concept, which is quite different from the prestraining concept conventionally employed in previous similar investigations. For the preloading concept, the previous vibration applied to specimen is supplied by a non-resonant vibration of the stress-controlled shear by resonant column apparatus. This concept can apply any desired number of loading cycles to the sample under the constant shear stress. By contrast the prestraining concept cannot investigate the effects of low number of cycles on dynamic properties of the tested sample due to several hundreds of cycles are necessary to determine the resonant frequency. In addition, the preloading concept can also ensure the applied preloading stress constant during previbration at given vibrating frequency and input drive voltage. However, prestraining concept may fail to guarantee the applied shearing strain amplitude constant because the set frequency is not the resonant frequency due to the stiffness of tested sample varies with number of cycles. Therefore, take the set frequency as the resonant frequency will result in greater deviation in the evaluation of the applied prestraining amplitude.
With the preloading concept, effects of various factors on the vibration history effects on dynamic sand behavior were systemically explored herein. On the basis of a large umber of tests, some valuable findings were given, three findings are: (1) there exists a threshold number of cycles below which the stiffness of sand decreases, this reduction may be up to over 20% at this threshold number of cycles for medium dense samples, and beyond which increases with number of cycles, which is inconsistent with the previous conclusion that shear modulus always increased with number of cycles; (2) even though significant densification of dry sample might occur after preloading under higher confining pressure, the Gmax at the unloading path of confining pressure are significant smaller than those without preloading under the same confining pressures, and the relationship between Gmax and confining pressure at the unloading path dose not follow the exponential relationship as before preloading, but linear relationship; (3) water content has significant influence on the development of shear modulus with number of cycles and effects of unloading confining pressure on the Gmax after preloading, which is seldom concerned in previous studies. Finally, a microstructure evolvement theory of sand was proposed considering wear process of interparticles and particles reorientation in vertical direction, which can well interpret relative testing results.