INTRODUCTION
We estimate how thermomechanical processes affect the spatial variability of fracture perme- ability for a 3-D model representing Topopah Spring tuff at the nuclear-waste repository horizon in Yucca Mountain, Nevada. Using a finite-difference code, we compute thermal stress changes. We evaluate possible permeability enhancement resulting from shear slip along various mapped fracture sets after 50 years of heating, for rock in the near-field environment of the proposed repository. Our results indicate permeability enhancement of a factor of 2 for regions about 10 to 30 m above drifts, for north-south striking vertical frac- tures. Shear slip and permeability increases of a factor of 4 can occur in regions just above drifts, for east- west striking vertical fractures. Information on how permeability may change over the lifetime of a geologic repository is important to the prediction and evaluation of repository performance.
This paper describes current results for modeling geomechanical behavior of the Topopah Springs tuff in the near-field environment (NFE) of the potential repository at Yucca Mountain, Nevada. The model- ing focuses on estimating how thermomechanical (TM) processes affect the spatial variability of frac- ture permeability. A more detailed description of the method and our modeling is given in Berge et al. (1998). Our results were obtained by a thermohy- dromechanical (THM) modeling procedure (i.e. modeling coupled thermohydrologic [TH] and TM processes). These results are needed for modeling changes in repository-level moisture movement and thermally induced seepage and for assessment of rock fall and tunnel collapse.
Specifically, we used the TM three-dimensional (3-D) finite-difference code Fast Lagrangian Analy- sis of Continua in 3 Dimensions (FLAC 3D) version 2.0 (Itasca Consulting Group Inc. 1997) to compute changes in stress and displacement in an elastic model subjected to temperature changes over time. Output from TH modeling (Hardin et al. 1998, Chapter 3) using the code Nonisothermal Unsatu- rated-Saturated Flow and Transport (NUFT) (Nitao 1998a, 1998b)provided the temperature changes for input to FLAC 3v. We then estimated how the stress changes could affect permeability.
By using a model based on the work of Barton et al. (1997, 1995) and the Mohr-Coulomb criterion (Jaeger & Cook 1976, p. 399), we determined which regions of the model would experience increases in permeability because of shear slip along fractures having particular orientations. As done in our previ- ous work (Blair et al. 1997) incorporating a model developed by Brown (1994, 1995, Brown & Bruhn 1997), we estimated that the permeability would in- crease by approximately a factor of two for regions where shear slip could occur along parallel fractures. If slip occurred along two different fracture sets, the permeability could increase by a factor of at least four.
