This paper refers on the shear strength of soils reinforced with short randomly distributed fibres, which is a new and effective ground improvement technique. The shear strength of these soils is usually anisotropic because of compaction. In the paper, some of the main characteristics of these reinforced soils, and some considerations on anisotropy in strength are reported. Than, the expression recently introduced by Lirer et al. (2011) to simply model the failure envelope of the reinforced soil is discussed. This expression is based on simple micromechanical considerations and on a number of experimental results, and takes into account the main characteristics of the soil and of the fibres as well as the effect of fibre to grains relative dimensions. However, it only allows to calculate the maximum value of the shear strength of the composite material, being related to the results obtained in triaxial compression tests with fibres mostly oriented in the horizontal direction. In the paper, a possible evolution of the formulation of this shear strength envelope is proposed, to take into account anisotropy. A simple trigonometric reduction function is introduced, which complies with the expected behaviour of reinforced soils under different loading conditions. The proposed approach has the advantage of being a rather general expression that can be used by knowing just some simple and basic information on the host soil and on the fibres.
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