Experimental characterization of the influence of fines on the stiffness of sand with inherent fabric anisotropy

This paper reports the experimental findings regarding the influence of fines on the stiffness reduction, the stiffness anisotropy, and the stiffness changes (during aging) of Toyoura sand and kaolinite (fines) mixtures. The fines content of the mixture samples ranged from 0% to 30% by weight. A tailor-made true triaxial apparatus was used and equipped with a bender element system to measure the shear modulus and with the I-Scan system to monitor the contact forces within the soil sample. By adding fines, the shear moduli of the samples, i.e., Ghh and Ghv (or Gvh), were reduced, such that the higher the fines content, the higher the reduction and the higher the confining pressure, the smaller the reduction. For a given confining pressure σ′, the percentage stiffness anisotropy of the samples increased as the fines content was increased from 0% to 2% and then to 8%. As the fines content was increased from 8% to 15%, the percentage stiffness anisotropy gradually decreased. It ultimately reached a small value at which the fines content was 20% where the fines-controlled soil matrix minimized the stiffness anisotropy induced by the inherent fabric anisotropy of sand particles. The aging rates for Ghh and Ghv were both enhanced after adding fines, and the enhancement was larger as the fines content was increased because the amount of sample creep also increased. A larger amount of sample creep can lead to a greater degree of contact force homogenization which enhances the soil stiffness. This was evidenced by the change in the associated coefficient of variance (CV) of the contact forces measured by the tactile pressure sensors. The aging rate was always greater in Ghh than in Ghv regardless of the different fines contents. Yet, such aging rate anisotropy became minor as the fines content reached 15%.