Gas permeability evolution of granite under confining pressure unloading tests

Abstract

Tunnel excavations in rock usually cause initial stress release, which is different from the stress path in classical triaxial compression tests. To analyze the permeability variations in excavation disturbed zone, two different stress paths, e.g. classical triaxial compression and confining pressure unloading, are applied on granite, and the permeability at different damage levels is measured using gas as transport medium. The differences in deformation and failure mechanism and their impacts on gas permeability variation are discussed. The test results show that lateral deformation of samples in confining pressure unloading tests is obviously larger than that in classical triaxial compression tests. Unlike the permeability decrease-increase transition in classical triaxial compression tests, the permeability in confining pressure unloading tests shows an overall progressively increase trend. The permeability in both tests increases significantly near failure and achieves the same order of magnitude. The differences in permeability evolutions under the two stress paths are directly attributed to their different deformation mechanisms. The test results could provide a new understanding of permeability properties during the underground excavation.