Assigning Fracture Stiffness from In-Situ Deformation Measurements
Paper in proceedings, 2014
Fracture stiffness is varying between fractures and is influenced by its proximity to a tunnel opening; if the behavior close to the opening is of interest for modelling efforts, then it may be better to use such data as input rather than high-stress laboratory measurements. A handy method for in situ testing of deformation (stiffness) and transmissivity would be beneficial to obtain data for numerical modeling of the near field of an excavation. We describe a measurement method under development that uses an anchor in a borehole and measures deformations between the anchor and the rock surface. Measurement of deformations is done during a stepwise constant head injection test providing information about both hydraulic and mechanical properties. Deformations and applied pressure is used through the effective stress concept to calculate fracture stiffness. Deformation measurements have been conducted in the TASO and TAS04 tunnels at 410 – 420 m depth at Äspö Hard Rock Laboratory (HRL), and in the Hallandsås tunnel, Sweden. Results show deformations in tested fractures 0.2 – 3 m below tunnel floor in the order of a few to tens of micrometers for injection overpressures in the order of 0.5 – 0.6 MPa. Stiffness is traditionally described as either normal stiffness or shear stiffness, the design of the experimental setup here does not allow for this distinction directly from the results; however knowledge of the orientation of tested fractures and coupling of the results to injection rates may help in discerning type of deformation.