Behavior of rounded granular materials in direct shear: Mechanisms and quantification of fluctuations

Fluctuations of vertical displacement and shear stress are routinely observed during shear tests on granular materials. This study was designed to examine these fluctuations to gain insight into their shear behavior. Three distinct particle size mixtures of glass beads were tested by an intermediate direct shear apparatus at four normal stress levels and three shearing rates. Based on examination of the onset and nature of growth/decay patterns of the fluctuations, four distinct stages were identified during shearing of these model materials. The fluctuations were demonstrated to reflect roughness of shear surfaces and variations in the driving force to accommodate this roughness. The Fast Fourier Transform (FFT) method was used to determine the dominant frequency bands and the amplitudes and wavelengths of the corresponding sine waves. These characteristic wave parameters were then linked to the shear strength parameters through the volumetric dilation rate, enabling determination of the friction coefficient of the system. It was demonstrated that the fluctuations are directly related to the intrinsic properties of soils and test conditions, and the current practice of averaging these fluctuations leads to the loss of valuable information. It is therefore suggested that residual strength of granular materials be presented in a way to recover and process this information.

Research Highlights
► Fluctuations of vertical displacement and shear stress were quantified; the fluctuations’ characteristic amplitude and wavelength were linked to the dilation component of shear stress; the friction coefficients were calculated directly from the raw data, making curve smoothing unnecessary.
► Shearing of rounded granular materials takes place in four stages: contraction, dilation, shear-induced packing and residual state.
► Three main factors influences the fluctuations: uniformity coefficient and normal stress increases whereas shearing rate decreases the amplitude and wavelength.