Dynamic properties of polypropylene fibre-reinforced silica quarry sand

The study reports on the dynamic behavior of fibre-reinforced sand with a focus on the shear modulus at very small strains. A set of twenty four specimens composed of a silica quarry sand reinforced with different percentages of polypropylene fibres was constructed in the laboratory using two different resonant columns embedded with bender element inserts. One resonant column is of the fixed-free type and the second apparatus is of the fixed-partly fixed type. The percentages of fibre ranged from 0% to 2% and the specimens were tested in a fully saturated state following an isotropic stress path. The shear modulus of the specimens obtained from the bender element tests was computed based on the measured shear wave velocities considering an equivalent density instead of the bulk density. This is because the bender element test method corresponds to a high-frequency technique and adjustment of soil density should take place to account for the relative movement between the solid skeleton and the water that fills the pores. This adjustment provided a better comparison between the obtained moduli from resonant column and bender element tests. Based on the set of experiments, it was found that the small strain shear modulus of the specimens reduced with an increase of the fibre content, which trend was more pronounced for fibre contents greater than 0.5%. The use of a granular void ratio in typical power-law type formulae for the expression of soil modulus could not work effectively, as for example it happens in granular binary mixtures. Based on the experiments, a formula was developed that correlates stiffness to pressure. In this regard, an additional set of dynamic tests was conducted including twenty newly created specimens in order to further validate the new expression, which is more applicable for a well-graded quarry sand as physical portion and for contents of polypropylene fibre from 0% to 2%.