Abstract: To study the unloading characteristic of marble with different initial damages, conventional triaxial loading-unloading tests were conducted under the same initial confining pressure but different axial pressures. Moreover, the rock samples subjected to loading-unloading conditions were performed to investigate the nuclear magnetic resonance (NMR) behaviour. The rock porosity, strain, nuclear magnetic resonance image (NMRI) and crosswise relaxation time T2 distribution were obtained. The results showed that: (1) the rock porosity increased during the unloading process. When the initial axial pressure was at 90% of the triaxial compressive strength (TCS), the unloading process was more steady than at either 70% or 80% TCS. (2) The initial damage advanced the increase of strain, in particular the hoop expansion, which in turn accelerated the growth of microcracks. (3) When the initial damage was low, the deformation of rock sample changed from elastic to plastic. Spectral peaks of the small microcracks moved from left to right, while spectral peaks of the large microcracks continuously extended to right, which indicated that small microcracks were generated at the initial stage of unloading, and both small and large microcracks propagated and coalesced at the last stage. When the initial damage was sufficiently high, T2 spectrum curves just moved towards right, which meant that the propagation of large microcracks played a vital role and the rock had undergone only the plastic deformation. (4) When the initial damage was increased, both the intensity and area of NMRI white spot and the rock porosity were increased at the same unloading confining pressure ratio.

Key words: nuclear magnetic resonance (NMR), initial damage, unloading, porosity, T2 spectrum distribution