Crack propagation behavior in sandstone during unloading confining pressure under different seepage pressures

The unloading confining pressure tests were conducted on sandstone under the seepage pressure to reveal the relationships between the deformation characteristics and permeability and the stress paths and seepage pressure of the rock. The weights of influences of the confining pressure and seepage pressure on the damage of sandstone are quantified by symbolic statistics. Experimental results show that with the increasing confining pressure, the characteristic stress increases, and the growth rates of crack strain corresponding to the damage stress as well as the peak stress both decrease. However, with the increase of the seepage pressure, the characteristic stress decreases, the initial crack volumetric strain corresponding to the initiation stress reduces, the crack-growth volumetric strain corresponding to the damage stress increases and the growth rates of crack strain corresponding to the damage stress as well as the peak stress all increase. The permeability decreases slowly at first and then increases dramatically in the process of unloading confining pressure. Under the same confining pressure, the brittleness index and crack strain growth rates are the highest in the unloading test with seepage pressure, while they are moderate in the conventional unloading test, and the lowest in the loading test. The confining pressure is extremely correlated with the peak strength, axial strain, circumferential strain, crack circumferential strain corresponding to the peak strength. The seepage pressure is extremely correlated with growth rates of both crack axial strain and crack circumferential strain.

Automated summary

Unloading confining pressure tests were conducted on sandstone under seepage pressure to investigate the relationship between deformation characteristics, permeability, stress paths, and seepage pressure of the rock. The results showed that increasing confining pressure leads to an increase in characteristic stress and a decrease in crack strain growth rates, while increasing seepage pressure leads to a decrease in characteristic stress and an increase in crack strain growth rates.