Hydromechanical Behaviour of Fractures Close to Tunnels in Crystalline Rock

Licentiate thesis, 2013

The deformation and stiffness properties of rock fractures are important measurable parameters when describing their hydromechanical behaviour. Deformation refers to aperture change. Stiffness refers to the amount of deformation per stress change to which a fracture is subjected. This thesis aims to investigate the stiffness and deformation behaviour of fractures in crystalline rock through in situ and laboratory experiments. The focus in this work is on fracture geometry due to geological stress history. This will result in increased conceptual understanding and accordance between hydromechanical and geomechanical fracture description and behaviour. The in situ measurements consisted of deformation measurements in boreholes and were conducted at the Äspö Hard Rock Laboratory (HRL) and in the Hallandsås Tunnel. The total deformation across the instrumented borehole sections was measured as an effect of hydraulic pressurisation of the fractures in the nearby rock volume. The results were assessed in terms of deformation and fracture stiffness. The laboratory experiments were conducted as cyclically loaded permeameter measurements of fractured rock core samples from Äspö HRL with simultaneous deformation measurements across the fracture. The tested samples had various geological properties and revealed differences in hydraulic aperture and mechanical deformation behaviour across the experimental cycles. The stiffness to hydraulic aperture relationship followed a trend identified in the literature and deviations were given plausible explanations related to the geology and geometry of the samples. The results were discussed in the light of the sampled geology and the measurement methods. The measured deformations and corresponding stiffness were found to be reasonable in the light of available knowledge of the local geology and stress situation at the sites.