Abstract
Mechanical response, deformation behaviour and permeability evolution of surrounding rock under unloading conditions are of significant importance in rock engineering activities. In this research, triaxial experiments of sandstone subjected to different initial confining pressures and unloading rates under fixed axial stress were conducted. The results showed that sandstones experienced shear dilatancy before failure. However, the dilatancy factor did not decrease with increasing confining pressure, i.e. the dilatancy behaviour was not suppressed, which contradicted the phenomenon under increasing axial stress. The crack density also increased with increasing initial confining pressure. Furthermore, the normalized permeability was positively correlated with unloading rates. The sandstone permeability was closely related to the shear dilatancy behaviour. In the accelerated dilatancy stage, the relationship between normalized permeability and volumetric strain was linear at low unloading rates and nonlinear at high unloading rates. The linear/nonlinear relationship between them can directly affect the temporality of respective mutation, so as to guide the prevention of geological disasters at different excavations rates.
Highlights
Unloading behaviour of rock mass is inevitable in rock engineering activities, such as tunnel/underground roadway/royalsocietypublishing.org/journal/rsos R
The crack density parameter increased with the increase of initial confining pressure or peak deviator stress, which was consistent with the change of volumetric strain
Triaxial experiments of sandstones subjected to different initial confining pressures and unloading rates were performed to investigate the deformation behaviour and permeability evolution of sandstone under the engineering conditions related to the unloading path
Summary
Unloading behaviour of rock mass is inevitable in rock engineering activities, such as tunnel/underground roadway/. The dilatancy behaviour appears from the beginning of the yield stage to the strain softening stage It is mainly dependent on the plastic strain and confining pressure [5,6], which is consistent with the mechanical response of the surrounding rock near the excavation boundary of the underground engineering [7]. Roche & van der Baan concluded that the anisotropic Young’s moduli tended to reverse the effect of plasticity strain, decreasing the likelihood of failure in the shales and coals [12] These studies show that sandstone has obvious dilatancy behaviour. The loading and unloading rates are closely related to the deformation behaviour, strength and permeability evolution of rock mass. Electric motor axial hydraulic cylinder triaxial pressure cell data monitoring and acquisition system confining pressure rate (UCT) under fixed axial stress are performed in order to investigate the dilatancy behaviour, permeability evolution and their relationship
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