A laboratory experiment was conducted on a 0.5-m scale block of Topopah Spring tuff, to measure fluid flow and mechanical deformation properties under conditions that approximate the near-field enviroment of a potential nuclear waste repository, and to provide an intermediate-scale test case for numeri- cal model validation. The test specimen is a 0.25 x 0.25 x 0.50 m rectangular prism bisected by an artificial (saw-cut) fracture orthogonal to the tuff fabric. Water was supplied by a point source at the center of the frac- ture under various pressures of up to 0.04 MPa. Both fluid flow and mechanical properties were found to be anisotropic and strongly correlated with the ash flow fabric. Fluid mass-balance measurements revealed that only minor imbibition of water occurred through the fracture surfaces and that flow rates were independent of normal stress to 14.0 MPa and temperature to 140øC. Flow through the fracture occurred largely through un- correlated porosity that intersected the fracture plane.
The hydrologic response of fluids in the proposed nuclear waste repository horizon at Yucca Mountain (YM) is a critical factor in the repository design. Re- cent analysis ofhydrologic data implies that the rate of water infiltration into YM rock may have been as much as 30 mm/yr, although estimates for the pres- ent flux range between 5 and 15 mm/yr (Taylor 1997). Because the repository horizon contains a significant number of fractures, water transport in the YM hydrologic model is dominated by episodic fast flow in fractures (Nitao et al. 1993, Taylor 1997). The amount of water that reaches the reposi- tory horizon also depends on imbibition into the host rock through the fracture surfaces, and recent discussion (Hardin et al. 1998) indicates that imbibi- tion values currently used in many hydrologic mod- els must be re-assessed. Moreover, results from the Large Block Test (LBT) (Lin et al. 1995) at Fran Ridge near YM indicate that thermal regimes in par- tially saturated fractured rock can be dramatically perturbed by small fluctuations in water infiltration rates or by small changes in thermal and mechanical boundary conditions, including temperature excur- sions and rock-mass displacements not currently ac- counted for in thermal-hydrologic-mechanical (THM) models of the potential repository. The thermal field and the permeability of the fracture network in the near-field environment (NFE) clearly dominate seepage into the repository. The thermal field itself is perturbed significantly by "heat pipes" formed by fluid in fractures and connected porosity. Further, the sum of the thermally induced stresses and the relaxation stresses owing to the drift excava- tion determines the rock-mass stability.
Experiments on intermediate-scale (0.5-m) sam- ples contribute to the understanding of these phe- nomena, especially flow in discrete fractures under well-characterized thermal and mean stress fields. Results of these experiments provide phenomenol- ogical data on the fluid flow in fractured rocks at the characteristic temperature and stress conditions of the NFE. In addition, they contribute to the quanti- tative physical parameter database used in simula- tions of the potential repository. Finally and most importantly, they provide laboratory-controlled cases with known boundary and initial conditions for computer-model validation tests. In contrast to cen- timeter-scale samples, heterogeneities such as po- rosity, vugs, and healed fractures can be treated sto- chastically. Introduction of a single, well- characterized fracture provides an ideal test case for the physics required in discrete fracture and dual continuum models.
The work presented here is a conti
