Influence of stress anisotropy on small-strain stiffness of reinforced sand with polypropylene fibres

This paper reports on the small-strain stiffness (G_max) of a well graded crushed rock reinforced with polypropylene fibres. The experimental study was conducted on twenty-five fully saturated samples subjected to bender element tests. The contents of fibre used were equal to 0%, 0.5%, 1.0%, 1.5% and 2.0%. The bender elements were inserted in a resonant column and a stress path triaxial apparatus accommodating samples of 70 and 50 mm in diameter, respectively, with a ratio of length to diameter equal to 2:1. The samples were initially consolidated isotropically to pressures of 50, 100, 400 and 700 kPa. At a given isotropic pressure, G_max measurements were conducted increasing the deviatoric compressive stress (q) and keeping a constant mean effective confining pressure (p’). This allowed the study of the effect of stress anisotropy, expressed with the ratio (q/p’), and the role of fibre inclusion and content on G_max of the reinforced samples under variable mean effective confining pressures. The data were analysed by means of the normalized shear modulus, G_{max,normalized}, against the stress ratio, where G_{max,normalized} expressed the increase of the small-strain shear modulus under an anisotropic stress state with respect to the corresponding modulus under an isotropic stress state. For a given confining pressure, the results showed a clear increase of G_{max,normalized} with increased content of fibres which implied that the sensitivity of modulus to the stress anisotropy became more pronounced for greater contents of fibres. However, this influence of fibre inclusion and content on the small-strain stiffness of the samples was found weaker at greater magnitudes of p’.