The mechanical behaviour of granular materials is known to be influenced by the addition of fibres. However, most previous research has been carried out on materials with relatively small grains (sands), and its application to larger grains is not well documented. This paper reports an investigation into the mechanical behaviour of a fibre reinforced granular material having a relatively large grain size corresponding to 1/3 rd and 1/5 th scaled railway ballast. The investigation was carried out by means of triaxial tests incorporating a fullfield, image-based deformation measurement technique; this enabled detailed observations to be made of each triaxial test specimen during shearing. The test data demonstrate the benefits of random fibre reinforcement for aggregates having a relatively large grain size. Analysis taking into account the effect of fibre tension on the effective stresses experienced by the granular skeleton provides new insights into the mechanisms of reinforcement in larger sized granular materials.
Previous research on random fibre reinforced granular materials has shown that the relative dimensions of the grains and fibres significantly affect the macro-mechanical behaviour of the mixture. However, quantitative data are scarce and most previous work has focused on fine to medium sands, leaving uncertainties regarding the applicability of current knowledge to larger size aggregates such as railway ballast. In this paper, triaxial test data on 1 /3 and 1 /5 scale railway ballast are used to develop scaling relationships for the size and quantity of fibres needed to achieve the same reinforcing effect in granular materials of differing grain size. It is shown that, to maintain consistency across scales, fibre content should be quantified as a numerical (i.e. number of fibres per grain) rather than a volumetric ratio. It is further shown that increasing the fibre length increases the resistance of the mixture to deviator stress if the fibres are wide enough; and that provided an allowance is made for the effect of fibre tension, the changes in the stress-strain-strength behaviour of the granular matrix resulting from the changes in void ratio associated with the addition of the fibres are consistent with conventional soil mechanics theory across scales.
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