The Impact of Fracture Geometry on the Hydromechanical Behaviour of Crystalline Rock

Abstract: Effective construction of tunnels in fractured crystalline rock requires a unified approach for handling rock mechanics and hydrogeological issues. Traditionally, rock mechanics and hydrogeology not only use different nomenclature, they also measure parameters such as e.g. aperture differently. A description of fractures that includes both fracture surface- and void geometry could be used as a basis for a conceptual model that allows complexity to be added to the descriptions of hydraulic and mechanical behaviour without contradictions. In this work, hydromechanically coupled experimental setups and methods were developed and used to improve a conceptual model of hydromechanical (HM) fracture behaviour at low compressive stress. Key aspects of the model are hydraulic aperture, fracture normal stiffness, the number of contacts between the surfaces, and the aspect ratio, i.e. the relationship between contact point distance and aperture, thus describing the voids between the surfaces. The experimental setups that were developed comprised equipment for in situ measurements of mechanical deformation due to stepwise hydraulic injection of fractures close to a tunnel, and a laboratory HM permeameter used in conjunction with fracture topography and aperture scanning. The latter produced high-resolution aperture maps of samples at 1.0 MPa, which were related to the flow rates, estimated hydraulic aperture and stiffness from the HM permeameter tests of the samples. Aiming at a common aperture-stiffness relationship for laboratory and in situ tests at different scales, the results were compared to a previously suggested relationship linking hydraulic aperture and normal stiffness. A relationship that has been devised from in situ hydraulic interference tests and is assumed to be valid for low comp-ressional stress across fractures with limited prior deformation. The few laboratory samples tested and the in situ tests performed show agreement with the aperture-stiffness relationship. A relationship and a conceptual model that have potential to provide support to future studies on hydromechanical behaviour of crystalline rock.