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.