Fibre-reinforced sand: interaction at the fibre and grain scale

For fibre-reinforced granular soils, the efficiency of the fibres is governed by the local fibre-grain interaction mechanism. This local interaction mechanism is evaluated, in this paper, by using a modified version of the shear-lag stress theory. While this theory provides a description of the stresstransfer mechanism at fibre-matrix interface level, it also generates the stress distribution along the fibre. The proposed model explicitly accounts for the effects of the geometrical fibre and granular size characteristics, fibre stiffness, global stress level, soil density and the non-linearity of soil behaviour. An analytical expression for the ratio of strains in the fibre and in the composite, which is fundamental for any prediction of fibre contribution, is further derived. A discussion on the effects of the controlling parameters is presented, while the scale-up of the problem at the composite level is then conducted by using a continuum constitutive model appropriately modified to account for the strain ratio between the fibre and the composite. The model is validated against a series of triaxial compression tests on two different sands mixed with polypropylene fibres of different aspect ratios.