Effect of frequency of minute-amplitude oscillatory shear loadings on ultimate yield stress of cohesive sediments

Abstract

The ultimate yield stress of cohesive sediments changes with oscillatory loading or vibration. Given a strong enough vibration, the sediment may be fluidized. In this study, the effect of the frequency of minute-amplitude oscillatory shear loadings on the ultimate yield stress of cohesive sediments was experimentally and analytically investigated in a laboratory. The test samples were prepared with cohesive sediments with a median particle size of 31 μm and at four water content (31.8% to 37.03%). The sediments were subjected to continuous oscillatory shear loadings with a constant frequency (0–131.6 Hz). The findings of the study indicated that the ultimate yield stress of cohesive sediments under minute-amplitude oscillatory shear loadings was dependent on the oscillatory shear frequency, water content, and the ultimate yield stress of the sediments prior to the application of the oscillatory shear loading. This study revealed that there exists a critical shear loading frequency fcr. When the oscillatory shear frequency (f) was less than the critical shear frequency (fcr), the ultimate yield stress decreased as f increased. However, when f was greater than fcr, the ultimate yield stress was constant, irrespective of further increase of the oscillatory shear frequency. The former condition is identified as the non-equilibrium fluidization stage and the latter, the equilibrium fluidization stage. The fcr value decreased as the water content increased. In the equilibrium fluidization stage, the dimensionless yield stress Su/Su0 did not vary with the water content or oscillatory shear frequency. Two discrete formulae were proposed to estimate the ultimate yield stress of cohesive sediments subjected to minute-amplitude oscillatory shear loadings for the non-equilibrium and equilibrium fluidization stages. To explore the engineering applications of the formula for the equilibrium fluidization stage, a series of vertical pull out tests were performed using a dynamic torpedo anchor. This study provides an important method for distinguishing the degree of fluidization and sheds lights on potential applications in coastal and marine engineering, such as pile or anchor installation and extraction and scour protection in cohesive bed.