Mechanical behaviour of coarse grains/fines mixture under monotonic and cyclic loadings

Micro-ballast/fines mixtures were tested at various contents of coarse grains in triaxial cells under monotonic and cyclic loadings. X-ray µCT scans were undertaken to visualise the coarse grains distributions in the compacted samples. Results show that inclusion of coarse grains has significant effect on the mechanical behaviour of micro-ballast/fines mixture. While the volumetric content of coarse grains fv increases, the cohesion decreases, the friction angle increases, the dilatancy becomes more pronounced, the maximum deviator stress increases, the permanent deformation decreases, the resilient modulus increases and the damping ratio decreases. There is a characteristic volumetric content of coarse grains fv-cha, that defines two zones: when fv < fv-cha, the mechanical behaviour is governed by the fines proportion. On the contrary, when fv > fv-cha, the mechanical behaviour is controlled by the coarse grains proposition. This is totally supported by the X-ray µCT scans which revealed that when fv varies from 0% to 10% (below fv-cha), the fine particles constitute the skeleton of the sample, with coarse grains drowned in the matrix of fines. When fv increases to 20% (still below fv-cha), the connections between coarse grains start to develop. As  fv > 35% (beyond fv-cha), almost all coarse grains are connected to form a skeleton of grain-grain contacts.