This paper investigates the meso-damage mechanism and the cracking characteristics of rock and soil aggregate (RSA) by X-ray computed tomography (CT) under uniaxial compressive loading. Cylindrical RSA specimens (50 mm diameter and 100 mm height) with rock percentage 40 % were produced to perform a series of uniaxial compressive strength test. The damage cracking characteristic is identified by the mean CT value, and also the meso-damage propagation law is analyzed based on the damage factor defined by the CT value. From the test results, the variation of CT value for region of interest is different at the same stress level, the mean CT value for the RSA specimen, and rock block inclusions, and their adjacent soil regions decrease with the increasing stress level. It is more sensitive for block inclusions than soil regions. Also, the meso-damage evolution equation and constitutive equation for RSA are established. The estimated result is in agreement with the measured result. Moreover, based on the image threshold segmentation method, length, area, and mean width of cracks obey to power function distribution. Crack statistical characteristics are closely related to the rock block's distribution and morphology. These results confirm that the elastic mismatch of RSA is the primary reason resulting in cracking under loading, damage characteristics of RSA is progressive, cracking begins from rock-soil interfaces, and then propagate into soil matrix.

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Non-linear mechanical behaviours of soil and rock mixture (SRM) are known to be the result of elasticity mismatch between soil matrix and rock blocks. Yet the rock–soil interactions, rock block interlocking phenomenon and their dependence under triaxial deformation are poorly understood. Here the authors investigate on the mechanical and ultrasonic properties of SRM by analysing the variation of ultrasonic pulse velocity (UPV) during triaxial compression. The results show that rock–soil interaction can be reflected by the variation of UPV during deformation. In this work, the change of UPV mainly depends on the axial stress level, and they present a consistent trend during deformation. With the increasing rock block percentage, both axial stress and UPV take on the fluctuation growth trend; the fluctuation is most obvious at the strain hardening stage. Internal friction angle increases with the increasing of rock block percentage, while the cohesion presents the opposite trend. The ultimate failure modes and crack coalescence behaviour were also evaluated, with the increasing rock blockpercentage, failure mode of SRM experiences such as shear failure and bulging failure.