A field study for understanding thermally driven coupled processes in partially saturated fractured welded tuff

As part of a multi-laboratory team, we are carrying out two in situ thermal tests - the single Heater Test and Drift Scale Test, in an underground fac ility at Yucca Mountain, Nevada, USA, the proposed site for a high-level nu clear waste repository. Our objective in these tests is to gain a more in-d epth understanding of the coupled thermal-hydrological-mechanical-chemical processes likely to exist in the fractured rock mass around a geological re pository. These coupled processes are monitored continuously by numerous se nsors emplaced in boreholes. while cross-hole radar tomography, neutron log ging, electrical resistivity tomography, and interference air-permeability tests all serve to measure moisture change in the rock mass. Thermal-hydrol ogical processes for both tests have been simulated (using a 3-D numerical model) and compared to the extensive data set. In this paper, we present examples to illustrate how an iterative approach requiring close integration of modeling and measurements enables us to trac k the complex coupled processes we seek to understand. The main manifestati on of coupled thermal-hydrological processes is in the time evolution of th e drying and condensation zones. Good agreement exists between model predic tions and measurements, specifically the decrease in air-permeability value s within zones of increased liquid saturation in the fractures and the incr ease of radar velocity in cross-hole radar survey in zones of decreased mat rix liquid saturation. A heal-pipe signature in the temperature data arisin g from liquid-vapor counter-flow occurs in both the measurements and simula ted results. The good agreement between predictions from the numerical simu lations and measurements in the thermal tests indicates that our basic unde rstanding of the thermal-hydrological processes in a potential repository a t Yucca Mountain is sound. However, detailed behavior is impacted by site-s pecific heterogeneity, in the form of discrete fractures that are not likel y to be predictable a priori. One emphasis of the on-going Drift Scale Test is to build on the present understanding and to assess the impact of heter ogeneity to the repository performance. (C) 2000 Elsevier Science Ltd. All rights reserved.