The aim of this research was to show the effect of initial creep damage on unloading failure of rock from macro-mesoscopic perspective. A series of triaxial creep tests were carried out on sandstone to simulate initial creep damage and then unloading confining pressure tests were performed, respectively. A creep damage variable was proposed to analyze the degree of initial creep damage and the relationship of it with the macroscopic strength parameters was established. The results showed that the unloading amount of confining pressure and residual strength all tend to decrease when the degree of initial creep damage increases. The critical challenge was how to describe the effect of initial creep damage from mesoscopic perspective. This aim was achieved through two steps. In the first step, the mesoscopic properties were analyzed using experimentally obtained SEM images of the rock samples with different levels of initial creep damage. By comparative analysis of porosities in different magnifications, it can be concluded that porosity can’t reflect the effect of initial creep damage very well, thus, other pore parameters are further proposed. In the second step, three pore parameters were calculated by using the Matlab and IPP software, then, the average value of mean pore diameter is determined as the proper evaluation parameter and, finally, the agreement was verified between the mesoscopic pore parameter and creep damage variable.

UF cracks in rock masses commonly occur due to the unloading effect, which constantly happens after the variation of in-situ stress field or rock excavation. When undergoing periodic water fluctuation, rock mass with UF cracks is vulnerable to deterioration or even disintegration, especially for clay-bearing sandstone. To study the effect of changes in moisture on rock samples with UF cracks, clay-bearing sandstone from the Triassic Badong group in the Three Gorges Reservoir Area were chosen and investigated. The rock samples with UF cracks are obtained by conducting triaxial unloading confining pressure experiment. The effect of wet-dry cycles on the morphology properties and microstructure of the UF surface was investigated. The characteristics of particle-size uniformity from the sieve test were obtained by the calculation of RMS of particle contents. The test results show that UF cracks widen significantly and the disintegrated mass increases rapidly in the first three wet-dry cycles, while the fractal dimension of UF surface decreases sharply, but afterwards the disintegrated mass changes gently and the UF surface tends to be flat and smooth. Then, the RMS calculation of particle contents quantitatively evaluate the clay-bearing sandstone’s disintegration properties, which indicate the particle uniformity plays a key role on its disintegration mechanism. During wet-dry cycles, the tested samples tend to disintegrate more rapidly and entirely with the decrease of particle uniformity.