ABSTRACT
A method to predict the stress-strain response of a fractured rock mass is presented. The method involves using a finite element model as a numerical testing apparatus such that variations in rock-mass deformability properties can be calculated as a function of load level and direction. Results for the particular tuff unit studied suggest that the stress-strain behavior of the unit is anisotropic and stress- dependent.
1 INTRODUCTION
The Nevada Nuclear Waste Storage Investigations (NNWSI) Project, administered by the Nevada Operations Office of the U.S. Department of Energy, is in the process of evaluating a proposed site for geologic disposal of high-level nuclear wastes in the volcanic tuffs at Yucca Mountain, Nevada. Sensitivity and tradeoff studies are an integral part of these activities. In many calculations performed as part of these studies, the rock mass is assumed to be a linear elastic, isotropic medium. For these calculations, estimates of the rock-mass mechanical properties (tangent modulus of deformation and Poisson's ratio) are required. Until now, values for these properties often have been recommended based on engineering judgment, supplemented by limited field measurements. Studies of the structural geology at Yucca Mountain (Carr et al., 1986; Scott and Castellanos, 1984; Spengler et al., 1984; Spengler et al., 1981) indicate that the mountain is composed of a layered sequence of fractured volcanic tuffs. The fractures tend to have near-vertical to steep westward dips and strike north to northwest. Fracture densities are greater in welded tuffs than in nonwelded tuffs. It has been argued that the fractures impart a mechanical anisotropy to the rocks (Swolfs and Savage, 1985). Scott and Castellanos (1984) conclude that the fracture anisotropy caused the observed deviation in direction of deep drill holes. Further complications in understanding the behavior of fractured rock are implied by the observed nonlinear and stress-dependent mechanical response of individual fractures (cf. Goodman, 1976). Also, recent analyses of the thermomechanical response of the tuff at Yucca Mountain (St. John, in preparation) indicate that as a result of excavation and thermal loading, the directions of principal stress rotate 90° . From the above discussion, the potential for anisotropic, stress- dependent, nonlinear mechanical response of the fractured rock at Yucca Mountain exists. In this paper, we propose a method to be used as a tool to help understand the mechanical response of fractured rock. The method is proposed to help assess the impact of the mechanical response of fractured rock upon calculations that address site evaluation, repository design, and performance assessment. We use a finite element model (Thomas, 1982; Chen, 1986) as a numerical testing apparatus to calculate the stress-strain response of fractured rock as a function of load direction. The goal of our work is to provide a quantitative understanding of the potentially anisotropic and stress-dependent material response of fractured rock. Our application is specific to a tuff at Yucca Mountain, Nevada, yet our approach is general; we hope the method can be applied to other fractured rock masses.
2 MODELING APPROACH AND METHODS
Our approach involved generating a two-dimensional finite element mesh of 50 by 100 m to represent a section of the rock mass. We used the finite element model in a computer code to simulate unconfined compression tests upon this section of the rock mass.
