Seepage into an underground opening constructed in unsaturated fractured rock under evaporative conditions

Abstract

[1] Liquid-release tests, performed in boreholes above an underground opening constructed in unsaturated fractured rock, are used in this study to evaluate seepage into a waste emplacement drift. Evidence for the existence of a capillary barrier at the ceiling of the drift is presented based on field observations (including spreading of the wetting front across the ceiling and water movement up fractures exposed in the ceiling before seepage begins). The capillary barrier mechanism has the potential to divert water around the opening, resulting in no seepage when the percolation flux is at or below the seepage threshold flux. Liquid-release tests are used to demonstrate that a seepage threshold exists and to measure the magnitude of the seepage threshold flux for three test zones that seeped. The seepage data are interpreted using analytical techniques to estimate the testspecific strength of the rock capillary forces (a À1 ) that prevent water from seeping into the drift. Evaporation increases the seepage threshold flux, making it more difficult for water to seep into the drift and producing artificially inflated a À1 values. With adjustments for evaporation, the minimum test-specific threshold is 1600 mm/yr, with a corresponding a À1 value of 0.027 m.

Unsaturated flow and transport through a fault embedded in fractured welded tuff

“…While capillary effects have been well documented for porous media, the existence of such impedance to flow in unsaturated fractured rock has not been extensively demonstrated. Most recently, the capillary barrier effect corresponding to underground openings in unsaturated fractured rock was demonstrated by Trautz and Wang [2002], who observed the spreading of a wetting front across the ceiling of a drift and up into fractures during liquid release tests.…”

“…[35] While both laboratory and field observations [e.g., Trautz and Wang, 2002;Dahan et al, 1998;Su et al, 1999], including those in this study, consistently show the existence of dynamic flow behavior in unsaturated fractured rock at different scales, the temporal and spatial scales of the problem under consideration largely determine the practical importance of this dynamic behavior. For example, if our concern is the flow process for a given flow path, the dynamic behavior can be a dominant factor, one that needs to be considered (e.g., tray 6 in Figure 6).…”

“…A typical example is the flow path(s) associated with tray 6, within which there was a large degree of temporal variability in seepage rates. Trautz and Wang [2002] and Dahan et al [1998] have also reported similar dynamic flow behavior in unsaturated fractures, which may result from a combination of mechanisms such as gravity-driven instabilities [Nicholl et al, 1992[Nicholl et al, , 1993a[Nicholl et al, , 1993b[Nicholl et al, , 1994, the development of capillary islands [Su et al, 1999], or switches along fracture intersections [Glass et al, 2002]. Additional mechanisms include alterations to the fault surface brought about by the shrinking/swelling of infill material within the fault [Salve and Oldenburg, 2001] and other alterations to fracture surfaces [e.g., Weisbrod et al, 1998Weisbrod et al, , 1999Weisbrod et al, , 2000.…”

Boundary layer analysis of unsaturated seepage into cylindrical cavities

“…[12] The planned drifts at Yucca Mountain will have a radius ' of 2.75 m [Bechtel SAIC LLC, 2004b], and in the surrounding fractured tuff a À1 is a few centimeters [Trautz and Wang, 2002]. Thus a' is on the order of 100.…”

Seepage Calibration Model and Seepage Testing Data

“…Trautz and Wang (2002 [160335], Section 5) estimated the seepage threshold directly from the liquid-release test data, based on a number of simplifying assumptions (with regard to the crosssectional area of the flow path between the borehole and the ceiling, evaporation, and the steadystate flow field). Once the seepage threshold was determined, a capillary-strength parameter was derived assuming seepage into a cylindrical cavity excavated from a homogeneous porous medium (Trautz and Wang 2002 [160335], Section 6).…”