There are obviously differences on deformation properties and strength parameters of rock under loading and unloading paths, the same as that under pre-peaking strength and post-peaking strength unloading paths. The practice also confirms the exist of timeeffect of deformation of rock under unloading action. With respect to these characteristics, in order to simulate the practical stress condition changes under excavation of underground rock mass, four different stress paths that are these of regular triaxial test under high confining pressure, unloading high confining pressure at pre-peaking strength with axial pressure fixed, unloading high confining pressure at post-peaking strength with axial pressure fixed, and keeping axial pressure stationary synchronously unloading lateral stress gradually are put forward. Following the paths, series of tests are carried out on the marble specimens sampled from the 3rd water tunnel of Jinping hydropower project. The full process curves of stress versus strain, deformation properties, failure characteristic and changing rule of major mechanical parameters and unloading rheological behavior are obtained in respective stress path. The experimental research reveals the differences in mechanical properties of Jinping marble.
A great deal of practical engineering examples have validated that the failure of almost all the underground surrounding rock mass isn't due to loading, but unloading effect for excavation of rock mass which can cause earth stress release in some directions and the change of stress boundary. For the completely different stress path between loading and unloading, the corresponding rock strength, deformation and failure mechanism are therefore different. And like loading stress behaving creep characteristic, unloading of rock have the same time-effect with loading stress path. As an illustrating example here, In order to describe the deep rock strength and deformation properties and failure mechanism of rock under high stress level more properly, the marble sampled from Jinping Grade II hydropower station was taken as research objective to obtain the law knowledge of unloading mechanics and rheological properities of Jinping marble. The water tunnel group of Jinping Grad II hydropower consists of 4 parallel stressed tunnels with diameter of 13 m. Its average depth is of 1610 m, and the maximum depth is of 2525 m. The length of tunnel whose depth is greater than 1500 m accounts for 75 percent of total length. According to the materials discovered from investigating cave and auxiliary tunnel earth stress test and hydraulic jacking test, the practical measured pressure between domain of 42MPa and 46MPa, the predicted maximum and minimum earth stress reach up to 70MPa and 26MPa respectively. The strata through which tunnels run across mainly consist of Triassic shallow sea to shore sea facies or alternating terrene and sea facies stratum. The lithology of tunnel surrounding rock is middle Triassic Zagunao Group marble (T2z), Baishan Group marble (T2b), Yantang Group limestone and crystal limestone (T2y), and upper Triassic sandstone and. slate, among which the majority is carbonate rock.