Permeability changes induced by excavation in fractured tuff

For a niche seepage test site in the Exploratory Studies Facility at Yucca Mountain, air-injection tests using multiple packer strings were performed to measure air-permeability values along boreholes drilled before niche excavation. These tests were repeated after niche excavation. Increases of nearly two orders of magnitude were measured between the pre-excavation air-permeability values and the post-excavation air-permeability values. These increases could be interpreted as the opening of pre-existing natural fractures induced by stress releases associated with niche excavation. For the horizontal fractures above the niche, the effective fracture apertures increased with the unloading and removing of rock mass in the niche opening. The pre-excavation permeability distribution was used to characterize the initial fracture aperture distributions. Stress redistribution was evaluated with simple continuum models. The fracture opening and the corresponding fracture permeability increases associated with the stress releases were calculated. Calculated post-to pre-excavation permeability ratios compared well with the measured values for most of the borehole intervals.