In Situ X-Ray Computed Tomography Experiment on Mesodamage Evolution of Subgrade Bimsoil during Cycle Loading

This work revealed the mesodamage evolution in blocks-in-matrix soils (bimsoils) under triaixal cyclic loading. Although the macroscopic cyclic stress-strain responses of bimsoil have been well investigated, the mesoscopic damage cracking mechanisms are still not well understood. This work used an artificially prepared bimsoil sample with a rock block percentage (RBP) of 40% (mass ratio) to perform cyclic triaxial testing with a 450-kV industrial X-ray computed tomography (CT) system. For the convenience of damage evolution investigation, the sample was tested with constant stress amplitude and at a low strain level. The mean CT value method and crack extraction method were both used to study the damage evolution during cyclic loading. Results showed that the hysteresis loop on the stress-strain curve had a sparse-dense-sparse pattern caused by the plastic strain accumulation. The damage field distribution in bimsoil is very inhomogeneous and affected by rock blocks. Clear CT images and the CT value of various stages-sample compaction, emergence of a low-density region, crack initiation at rock-soil interface, bifurcation, propagation, and coalescence-were observed. The CT value in the CT images, which is equivalent to sample density, was the most important parameter reflecting the mesoscopic damage evolution. Variation of the CT value in the region of interest (ROI) was influenced by the contact, occlusion, interlock, and separation between the soil matrix and rock blocks. Stress dilatancy caused by the damage cracking behavior had a close link with the evolution of the hysteresis loop, which is controlled by the mesostructural changes during cyclic loading. The paper also reveals the internal relationship between the hysteresis loop area and total crack area during deformation inside the bimsoil sample. The mesodamage evolution of bimsoil under cyclic loading was first documented in this study.