The diversion capacity of capillary barriers

Abstract

An arrangement of unsaturated fine‐grained soil overlying unsaturated coarse‐grained soil along a sloping contact can, under appropriate circumstances, divert infiltrating water away from the coarser material. Such an arrangement is called a capillary barrier. The water diverted by a capillary barrier flows downdip above the contact. The volume of water moving laterally increases in the downdip direction as additional infiltration is diverted by the barrier. Sufficiently far downdip, the laterally moving water wets the contact to the point that an amount of water equal to the infiltration flows downward through the coarse soil. The lateral flow at such a point represents the diversion capacity of the capillary barrier because this flow will not increase farther downdip. If the width (measured in the direction of dip) of the system is large enough that total infiltration exceeds the diversion capacity, the downdip portion of the barrier will not be effective. The diversion capacity can be calculated exactly in the quasi‐linear approximation where the relationship between relative permeability krel and pressure potential ψ takes the form krel = eαψ. This calculation shows that an upper bound on the width of capillary barriers is Ks tan ø/qα, where Ks is the saturated hydraulic conductivity of the fine soil, ø is the dip angle of the contact, and q is the infiltration rate.

Development of a unit cell model for interim performance assessment of vitrified low level waste disposal

“…Therefore, the e f f e c t i v e length o f the c a p i l l a r y b a r r i e r under the proposed Concept 1 w i t h the f i n e s o i l assumed t o be represented by the 241-AP-1 s o i l sample i s not more than about 1800 meters on one side from equation (1). This example estimate I s comparable t o one given by Ross (1990), taking i n t o account the difference i n assumed recharge rate, even though the two f i n e s o i l s considered here and there are d i s s i m i l a r i n terms o f t h e i r respective hydraulic parameters. For c l a r i t y , i t should also be noted t h a t L-1800 m i n t h i s example i s an estimate o f maximum e f f e c t i v e length f o r one side o f a b a r r i e r t h a t i s peaked i n the center.…”

“…a l . (1991) and Ross (1991) agree t h a t nonequilibrium conditions produced by periodic wetting and drying can imorovg the c a p i l l a r y b a r r i e r performance i n terms o f the e f f e c t i v e length L.…”

“…One of the fundamental differences between the two competing facility concepts is in capillary barrier length (Eiholzer 1995). effective length of one side of a capillary barrier has been shown by Ross (1990) The exponential conductivity c h a r a c t e r i s t i c parameter u can be determined by using the f a c t t h a t the r e l a t i v e hydraulic conductivity i s…”

“…The capillary barrier is composed of fine soil overlying coarser soil. At equilibrium, capillary pressure in the finer soil will adjust to a value at which the infiltration rate equals the hydraulic conductivi$y (Ross 1990). The underlying coarser material will equilibrate to a similar potential.…”

Inverse and predictive modeling of seepage into underground openings

“…For example, analytical solutions to the so-called seepage-exclusion problem were developed in a series of papers by Philip and coworkers for a variety of idealized cavity shapes, assuming steady, uniform downward flow in homogeneous, isotropic porous media with an exponential relationship between hydraulic conductivity and water potential (Philip et al, 1989a,b;Philip, 1989a Philip, ,b, 1990. Several analytical and numerical studies were performed in which capillary-barrier performance in engineered or naturally layered systems was investigated (Frind et al, 1977;Ross, 1990;Oldenburg and Pruess, 1993;Warrick et al, 1997;Webb, 1997). Ho and Webb (1998a) and Birkholzer et al (1999) examined the impact of heterogeneity on the distribution and rate of water flow across a capillary barrier.…”