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- Thörn, Johan, 1986, et al.
(författare)
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Hydraulic and Hydromechanical Laboratory Testing of Large Crystalline Rock Cores
- 2015
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Ingår i: Rock Mechanics and Rock Engineering. - : Springer Science and Business Media LLC. - 1434-453X .- 0723-2632. ; 48:1, s. 61-73
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, fracture stiffness in rock samples is determined by means of hydromechanical (HM) laboratory testing. The aim is threefold: to develop a procedure for sampling, to update testing equipment and to relate fracture stiffness to the geological history (e.g. stress history and fracture infillings). The hydraulic properties of twenty rock cores (diameter 190 mm, c. 100 mm high) from the Äspö Hard Rock Laboratory (HRL) were tested in a permeameter cell under different isotropic pressures up to 2.5 MPa. The flow rate through individual fracture samples was recorded. Four of the samples were re-tested in the permeameter cell using an updated hydromechanical procedure with deformation measurement across the fracture. Four load cycles of gradually increasing cell pressure were applied, resulting in a clearly observed hysteresis effect in the first and second cycles. Hydraulic aperture changes calculated using the cubic law were compared with their mechanical equivalents. The aperture changes followed similar trends, although these differed between the samples. Fracture stiffness was determined from the tests and the stiffness to hydraulic aperture relationship was found to follow previously published patterns linked to the storativity of fractures. Differences in stiffness are explained in the context of the geological history of individual samples, particularly their stress history. The paper presents a conceptualisation of the stiffness behaviour, which includes flow properties, geometric properties and the geological stress history of the tested samples.
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- Thörn, Johan, 1986
(författare)
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The Impact of Fracture Geometry on the Hydromechanical Behaviour of Crystalline Rock
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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