HR: 1340h
AN: H23A-1020    [Abstracts]
TI: Stereological analysis of fractures in the Roselend tunnel and permeability determination
AU: Patriarche, D
EM: delphine.patriarche@gdf.com
AF: CEA BP12 Bruyères le Châtel now at GDF, Saint Denis, La Plaine, 93211, France
AU: Patriarche, D
EM: delphine.patriarche@gdf.com
AF: CEA, BP12, Bruyeres le Chatel, 91680, France
AU: Pili, E
EM: eric.pili@cea.fr
AF: CEA, BP12, Bruyeres le Chatel, 91680, France
AU: * Adler, P M
EM: padler@ccr.jussieu.fr
AF: UPMC-Sisyphe, place Jussieu, Paris, 75252, France
AU: Thovert, J F
EM: thovert@lcd.ensma.fr
AF: LCD, SP2MI, Futuroscope, 86960,
AB: Fractures are often present in geological formations over a large range of scales. They determine the macroscopic mechanical, hydraulic, and transport properties of many natural rocks. In hard rock environments such as granitic media, the role of fractures in flow and transport is enhanced since the matrix can be considered as impervious. This work shows that field measurements of fractures in conjunction with records of water fluxes in an underground tunnel, can be used for estimating the permeability of a fractured medium. Orientations and length of fracture traces, and fluxes of percolating water were manually recorded along the 128 meter-long Roselend dead-end tunnel drilled in the granite of the Méraillet massif (French Alps). The thickness of rock overburden increases from 7 m at the tunnel entrance to 55 m at the tunnel dead end. The tunnel has a roughly cylindrical shape with a 2.4 m diameter. The fractures can be classified in two families: large fractures which intersect the tunnel, and small fractures which partially intersect it. Three different zones in the tunnel are distinguished with mild, low and high water fluxes, starting from the entrance. A stereological analysis of the trace length probability densities of small fractures provides the fracture diameter probability density distribution which is best approximated by a power law. Large fractures are assumed monodisperse, with a 5 m estimated radius. The generated fracture networks obtained by combining large and small fractures do percolate while networks consisting of small fractures only do not percolate. Various assumptions can be made on the permeability of each fracture. It is usual to assume that the fracture permeability is a power law of its lateral extent. The macroscopic permeabilities of the generated fracture networks can be systematically computed with our numerical technique based on the meshing of each fracture and a finite-volume formulation for the Darcy equation. Observed water fluxes are best modelled when the fracture permeability is a power law of its lateral extent with an exponent equal to 3. Ref: D. Patriarche, E. Pili, P.M. Adler, J.F. Thovert, Water Resour. Res., in press.
DE: 1835 Hydrogeophysics
DE: 1847 Modeling
DE: 1859 Rocks: physical properties
SC: Hydrology [H]
MN: 2007 Fall Meeting