A Multiaxial Constitutive Model for Fibre-reinforced Sand

Fibre orientation in fibre-reinforced sand (FRS) is highly anisotropic due to compaction during sample preparation or field construction. This makes the mechanical behaviour of FRS, such as strength and dilatancy, highly dependent on the strain increment direction. While constitutive models able to capture such anisotropic behaviour of FRS have been proposed for conventional triaxial compression and extension conditions only, this paper proposes for the first time a full anisotropic model for FRS formulated in the general multiaxial stress space. The new model is developed based on the assumption that the strain of FRS is dependent on the deformation of the sand skeleton. In turn, the fibre presence affects the void ratio and effective stress of the soil skeleton, which governs the elastic properties, dilatancy and plastic hardening of the FRS. The effect of anisotropic fibre orientation on the FRS behaviour is considered through an anisotropic variable which characterises the relative orientation between the loading direction tensor and fibre orientation tensor. The model does not require direct measurement of the stress-strain relationship of individual fibres. Though the model is for FRS under multiaxial loading conditions, the parameters associated with the fibre inclusion can be determined based on triaxial test results, provided that the orientation of fibres is known. The model has been used to predict the stress-strain relationship of fibre-reinforced Hostun RF (S28) sand under multiaxial loading conditions. Satisfactory agreement between the experimental data and model predictions is observed.