Modulus Reduction and Damping Increase of Two Sands Reinforced with Polypropylene Fibers

Mixing of soils with synthetic fibers has been a well-established approach in geotechnical engineering practice for ground improvement purposes. Of particular interest for most design problems in soil mechanics and earthquake engineering are the properties of geomaterials at medium strains, between approximately $5\times {10}^{-3}\%$ and $5\times {10}^{-1}\%$, where soil modulus and damping are highly sensitive to the strain magnitude (or strain amplitude involving cyclic-dynamic problems). The present paper reports on the small- ($<5\times {10}^{-3}\%$) to medium-strain behavior of two different sands mixed with polypropylene fibers. The study implemented high-amplitude resonant column tests to capture the behavior of a uniform natural sand and a well-graded crushed rock mixed with contents of reinforcing fiber between 0 and 2%. For this purpose, a fixed–partly fixed resonant column was used and the experiments were conducted on fully saturated specimens subjected to isotropic pressures from 50 to 800 kPa. The analysis of the results, in the range of medium strains, was conducted adopting a commonly used hyperbolic-type model and the model parameters were best fitted to the experimental data, allowing the investigation of the fiber content effect on the reference strain and curve shape parameter.