Characterizing Fractured Rock for Fluid-Flow, Geomechanical, and Paleostress Modeling: Methods and Results from Yucca Mountain, Nevada

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Fractures have been characterized for fluid- flow, geomechanical, and paleostress modeling at three localities in the vicinity of drill hole USW G-4 at Yucca Mountain in southwestern Nevada. A method for fracture characterization is introduced that integrates mapping fracture-trace networks and quantifying eight fracture parameters: trace length, orientation, connectivity, aperture, roughness, shear offset, trace-length density, and mineralization.

A complex network of fractures was exposed on three 214- to 260-m2 pavements cleared of debris in the upper lithophysal unit of the Tiva Canyon Member of the Miocene Paintbrush Tuff. The pavements are two-dimensional sections through the three-dimensional network of strata-bound fractures. All fractures with trace lengths greater than 0.2 m were mapped and studied.

The networks consist of two fracture types. The first type is distinguished by low surface- roughness coefficients and by open, anastomosing, matched half-tubes on opposing fracture faces. These fractures show only face separation without shear and are termed joints. Spherulites adjacent to joint faces suggest that the joints formed, opened, and their surfaces were quenched before or during devitrification of the tuff. The tubular structure is interpreted to be analogous to bread- crust structure on volcanic bombs. The cooling joints make up two well-defined sets striking 25 to 85° and 270 to 355°, both dipping perpendicular (plus or minus 6°) to foliation. Abutting of the two sets against each other suggests that they developed coevally. Both sets exhibit 3- to 5-m-wide swarms spaced 150-200 m apart. The second fracture type is distinguished by higher surface-roughness coefficients and by the absence of tubular structures on fracture faces. A few of these fractures have demonstrable shear offset and are thus termed faults. For most of these fractures, it was not possible to determine whether there was any shear displacement, and they are referred to as fractures. The fractures abut against and the faults offset the cooling joints and thus both postdate the joints. Unlike the cooling joints, the fractures do not define sets based on orientation or surface roughness.

The frequency distribution of surface-roughness coefficient (RC) for fractures and faults combined is fitted with a normal distribution and peaks at RC = 10. The RC frequency distribution for the cooling joints is also fitted with a normal distribution and peaks at RC = 2. The aperture frequency and trace-length frequency are best fit- ted by power laws. Anisotropy in aperture for the fracture networks is interpreted to result from a combination of tectonic and topographic stresses.

The spatial patchiness of fractures, joints, and faults in each of the networks is shown to be fractal, and the fractal dimensions D are 1.5, 1.4, 1.5.


U.S. Geological Survey Open-File Report 93-269.

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