Viscous behaviour of unbound granular materials in direct shear

The viscous properties of a variety of poorly graded unbound granular materials were investigated by direct shear tests on 12 cm-cubic specimens. A number of natural sands having different particle shapes and sizes as well as uniform glass beads having different particle sizes were used. The viscous properties were evaluated by changing the shear displacement rate many times during otherwise monotonic loading (ML) at constant shear displacement rate and normal pressure. Creep loadings were performed in two tests. Different types of viscous properties, which are affected by the particle shape but essentially independent of the particle size, are reported. The viscosity type varies as the shear displacement increases from the pre-peak regime towards the residual state. A new viscosity type, called "Positive & Negative", was found with relatively round granular materials in the pre-peak regime and with relatively angular granular materials in the post-peak softening regime and at the residual state. Peculiar "rate-independent unstable behaviour" is observed with round natural sands and glass beads in the post-peak regime, which is more significant and frequent with glass beads. Controlled by the particle size, this behaviour is caused by the so-called stick/slip phenomenon. The viscous properties observed in the DS tests are quantified by the rate-sensitivity coefficient defined in terms of the shear and normal stresses, which are then converted to those defined in terms of the major and minor principal stresses, beta(13). These beta(13) values are consistent with those directly obtained by the triaxial and plane strain compression tests. The effects of particle size on the beta(13) value are negligible and the beta(13) value tends to decrease as the particle shape becomes more round.