Experimental study on unloading rate effect of confining pressure on dilatation damage of surrounding rock in salt cavern gas storage

The rapid declination of the internal pressure in salt rock storage caverns leads to the dilatation and damage of the surround rocks. The effects of the initial confining pressure and unloading rate on the dilatation damage characteristics of salt rocks are studied in the present work，through the triaxial test with unloading confining pressure at constant axial stresses. The experimental results show that: at the same unloading rate，the higher the initial confining pressure，the larger the dilatation in the unloading stage. The mean volume strain at the confining stresses of 12 and 18 MPa increases by 7.1% and 21.4%，respectively，compared with that at the initial confining pressure of 6 MPa. Additionally，slow unloading rate of confining pressure is conductive to dilatation. As the unloading rate decreases from 0.2 MPa/s to 0.02，0.002 and 0.000 2 MPa/s，the mean volume strain increases by 60.2%，103.5% and 282.1%，respectively. In the unloading process，the larger the dilatation，the more serious the internal damage. The deformation modulus in the unloading stage decreases with the unloading of confining pressure. The larger the initial confining pressure and the unloading rate，the larger the deformation modulus reduces. The damage variable and the energy absorbed by salt rock in the unloading stage increase linearly with the increase of initial confining pressure，and on the other hand，decrease exponentially with the increase of unloading rate. The more energy absorbed，the more serious the internal damage. © 2022 Academia Sinica. All rights reserved.