The techniqueo f reinforcing soils with discrete flexible fibres is slowly gaining acceptancea nd support between practising engineers. However, little is known about the main factors governing the behaviour of this material and the main characteristics of its behaviour. This means that the applicability and potential of this soil improvement method have not been resolved. The present research aims to provide a considerable contribution in understanding the mechanisms governing the behaviour of this reinforced material and to develop a modelling tool which allows the-prediction of its mechanical behaviour. A procedure for the determination of the distribution of the orientation of fibre in reinforced specimens has been developed herein. It was found that the most common procedure for preparing reinforced specimens, moist tamping, leads to preferred sub-horizontal orientation of fibres. In view of these preliminary results an extensive campaign of compression and extension conventional triaxial tests for both drained and undrained condition has been perfon-ned.F ibres were found to be effective in increasingt he strengtho f the material but their effectiveness was found to be dependent on the orientation with respect to tensile strains. Fibres were also found to be effective in preventing the liquefaction of loose sand specimens: addition of fibres results in a densification of the sand matrix not only for the volume that they occupy but also for preventing the sand matrix from using some of the voids during the deformation process. A new constitutive modelling framework has been developed on the basis of the rule of mixtures of composite materials. This approach allows the adoption of separate constitutive laws for each constituent and to combine their effects in characterising the behaviour of the composite material. In this manner it is possible to select the complexity of the constitutive model of each constituent and to simulate peculiar aspect of its behaviour. Application of the model demonstrated how accurately the mechanical behaviour of fibre reinforced sand can be simulated with particular regard to the anisotropy of strength and the prevention of liquefaction