Static liquefaction of fibre reinforced sand under monotonic loading

This paper explores the possibility of improving the monotonic undrained response of a loose clean sand which normally appears susceptible to the phenomenon of static liquefaction by mixing the sand with discrete flexible fibres. It is shown that the reinforcement inclusions reduce the potential for the occurrence of liquefaction in both compression and extension triaxial loadings and convert a strain softening response (typical for a loose unreinforced sand) into a strain hardening response. Fibre orientation distribution and the apparent sand matrix densification due to the presence of fibres in the voids appear important for the fibre reinforced soil behaviour. Normalisation of the effective stress paths with the mean effective stress at the end of consolidation shows a common path once the characteristic state is reached irrespective of the fibre concentration. The mobilised angles of friction coming from the slopes of the stress paths at large strains are very different for compression and extension and this is a consequence of the anisotropic nature of the distribution of fibre orientations. When full liquefaction of reinforced specimens is induced by strain reversal, the lateral spreading of soil seems to be prevented. Analytical developments including the key aspect of fibre orientation distribution, have shown that once the tensile contribution of fibres has been taken out of the composite stresses, the experimental data in the stress plane for all the tests at large shear strains nicely collapse onto a unique line corresponding very closely to the mobilised steady state or critical state angle of friction of the tested sand.