Geomechanical analysis of the large block test

Abstract

The Yucca Mountain Site Characterization Project is investigating the Topopah Spring tuff at Yucca Mountain, Nevada, to determine whether it is suitable as a host rock for the disposal of high-level nuclear wastes. The Large Block Test (LBT) at Fran Ridge was planned as part of the project to investigate coupled thermal-mechanical-hydrological and geochemical processes that may occur in the repository near-field environment. This test would be performed on an excavated block of Topopah Spring tuff and would provide information at an intermediate scale (1-10 m) that would help evaluate existing models for repository performance. As part of the LBT, we are analyzing the coupled thermal-mechanical- hydrological behavior of the block in response to heating. Our objectives are to aid in the experimental design of the test, to evaluate different thermal and constitutive models, and to evaluate several different numerical methods. In this report, we present results of thermal-mechanical simulations of the heat-up phase of the LBT conducted using two different numerical codes that are commercially available: a two-dimensional (2D), finite- difference model called FLAC and a three-dimensional (3D), finite- element model, called ABAQUS. The purpose of this initial numerical modeling is to calculate temperatures, stresses, and displacements in two and three dimensions for a simplified representation of the large block. In reality, numerous joints and fractures complicate the behavior of the large block significantly. Nonetheless, these 1218 simulations provide a general understanding of the thermal-mechanical behavior to be expected in the LBT.