Permeability evolution of grout infilled fractures subjected to triaxial compression with low confining pressure

Fractures in rock masses not only are main flow conduit for groundwater, but also provide planes of weakness on which further displacement can more readily occur. Grouting has been widely used to reduce the permeability of fractured rocks, as well as to improve the stability of fractured rocks. Therefore, fractures infilled with grout appear extensively in practice, and may be subjected to different in-situ stress conditions during the construction and operation life of tunnels and underground spaces in fractured rock masses. To examine the stress induced changes in permeability of grout infilled fractures, we performed triaxial compression tests on fractured granite samples infilled with grout, during which continuous permeability measurements were also conducted. Both planar and rough fractures with different apertures were tested. The permeability of infilled fractures slightly decreased with increasing deviatoric stress from 0 to the crack damage stress, followed by drastic enhancement in permeability with further increasing deviatoric stress. The fracture permeability increased by about 5-10 times at the peak deviatoric stress compared with that at the deviatoric stress of 5 MPa. Both CT techniques and particle discrete element models were used to investigate the changes in the microstructures of grouts and granites, as well as the interface between them. The results show that compression induced cracks in grouts and grout-granite interfaces are the main sources for the enhancements in permeability of grout infilled fractures under triaxial compression with low confining pressures.