Zhongze Xu

The combined dynamic torsional resonant column and cyclic torsional shear (RCTS) device is employed to study the generation of excess pore-water pressure, especially the threshold shear strain of excess pore-water pressure, γtPP, in an uncemented poorly graded granular soil. Reconstituted specimens of uncemented sand were constructed under water and then tested in the RCTS device using a newly designed base pedestal, with which pore-water pressure is measured at four points systematically located at 0.75 times the radius of the base pedestal. Specimens were tested under different mean effective confining pressures, and all specimens had B-values above 0.95 at the start of testing. During testing, the excess pore-water pressure was measured continuously during 30 cycles of loading at multiple increasing loading levels. The three major factors influencing γtPP that were investigated are as follows: (1) mean effective confining pressure, (2) relative density, and (3) loading frequency. The results are presented in terms of the following: (1) excess pore-water pressure ratio versus shear strain, and (2) threshold shear strain versus number of loading cycles. Other related behaviors such as softening during cycling and post-cyclic moduli changes are also discussed.

This paper illustrates two novel methods for computing the small-strain hysteretic material damping ratio, λ_min, of soils from the cyclic torsional shear (TS) and computing the small-strain viscous material damping ratio, D_min, from the free-vibration decay (FVD) testing. Both λ_min and D_min are challenging to measure, due to the significant level of ambient noise at small strains (−4%). A two-step method is proposed combining the Fourier Transform and a phase-based data fitting method for torsional shear testing, and this method can effectively eliminate the ambient noise at small strains. A Hilbert Transform-based method is proposed for the free-vibration decay testing in order to achieve a more accurate measurement of the viscous material damping ratio, D, at different strain levels, especially at small strains. The improved λ_min and D_min at small strains are compared to data available in the literature. The two novel methods are shown to be accurate in computing the small-strain damping ratios.