AGU 1999 Fall Meeting
San Francisco, Calif., December 13-17, 1999

Special Session H31 - Unsaturated or Multiphase Flow/Transport in Fractured Rock: Where Do We Stand?

Drift Seepage in Unsaturated Fractured Rock

Stefan Finsterle and Robert C. Trautz

Lawrence Berkeley National Laboratory, Earth Sciences Division
University of California, Berkeley, CA 94720

Abstract. We performed numerical modeling studies to investigate water seepage into underground openings excavated from unsaturated fractured formations. Water seepage is a key factor affecting the performance of a nuclear waste repository at Yucca Mountain. The amount of water dripping into an underground opening is determined by the capability of individual fractures to hold water by capillary forces and by the permeability and connectivity of the fracture network, which allows water to be diverted around the drift. We developed a high-resolution numerical model of an unsaturated fracture network and examined the appropriateness of seepage predictions using a fracture continuum model. Effective capillary strength and effective permeability are highly influenced by the geometry and hydraulic properties of the fracture network in the immediate vicinity of the drift wall. Model calibrations were performed using liquid-release tests conducted at Yucca Mountain to determine unsaturated flow parameters relevant for seepage calculations. The calibrated model was then used to determine the seepage threshold in a fractured formation. This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.