WATER DIVERSION CAPACITY OF INCLINED CAPILLARY BARRIERS

Transcription

1 WATER DIVERSION CAPACITY OF INCLINED CAPILLARY BARRIERS Bruno Bussière, Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management, UQAT, Rouyn-Noranda, Québec Senami Aurore Apithy, Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management, École Polytechnique, Montréal, Québec Michel Aubertin, Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management, École Polytechnique, Montréal, Québec Robert P. Chapuis, CGM Department, École Polytechnique, Montréal, Québec ABSTRACT Covers with capillary barrier effects (CCBE) are an interesting alternative to low hydraulic conductivity covers to limit water infiltration into waste disposal sites, especially for arid and semi-arid climates. This type of cover system typically includes a fine-grained material layer overlying a coarser one. In sloping systems, water can be diverted at the interface between the two materials, hence reducing water infiltration into the underlying wastes. However, at a certain down dip location, the moisture-retaining layer can become wet enough to reduce the efficiency of the capillary barrier effect at the interface. Infiltration of moisture into the coarse material then becomes possible as suction reaches its water entry value (WEV). This paper presents the main results of a laboratory investigation and a numerical study designed to better understand the diversion capacity of inclined CCBE. The results show that cover inclination, precipitation rate, and soil properties and thickness are the main parameters influencing the diversion capacity of an inclined CCBE. RÉSUMÉ Les couvertures avec effets de barrière capillaire (CEBC) constituent une alternative intéressante aux couvertures à faible conductivité hydraulique saturée pour limiter l infiltration d eau dans des sites de rejets, surtout pour des climats arides et semi-arides. Ce type de recouvrement est typiquement constitué d une couche de matériau à granulométrie fine placé sur un matériau plus grossier. Lorsque la CEBC est inclinée, l eau peut être déviée à l interface entre les deux matériaux, ce qui permet de limiter la percolation vers les rejets sous-jacents. Cependant, à un certain endroit le long de la pente, le matériau qui retient l eau peut devenir suffisamment humide pour entraîner une baisse d efficacité des effets de barrière capillaire. Il peut alors y avoir infiltration à travers le matériau grossier lorsque la succion atteint sa pression d entrée d eau. Cet article présente les principaux résultats d une étude en laboratoire et d une étude numérique visant à mieux comprendre la capacité de diversion de CEBC. Les résultats montrent que l inclinaison de la couverture, le taux de précipitation, les propriétés des matériaux et l épaisseur de la couche de rétention d eau sont les principaux paramètres qui influencent la capacité d une CEBC à dévier l eau.. INTRODUCTION In a number of field applications, covers with capillary barrier effects can advantageously be used instead of low saturated hydraulic conductivity covers to prevent water percolation into waste disposal sites, and related problems such as the production of Acid Mine Drainage (AMD). This type of cover relies on the capillary barrier effect phenomenon appearing under unsaturated conditions when a relatively fine-grained material overlies a coarser one (e.g. Rasmuson and Erikson 86; Morel- Seytoux 2). The contrast in hydraulic properties between the two materials restricts vertical water flow at their interface. The lower unsaturated hydraulic conductivity, at similar suctions, of the coarser material may limit the downward flow of water from the fine-grained material to the coarse-grained one underneath. A detailed description of unsaturated flow and capillary barrier effects in layered systems can be found in many recent publications, including Morel-Seytoux (2), Aubertin et al. (,6) and Bussière et al. (200). In arid and semi-arid climates, inclined covers with capillary barrier effects (CCBE) constitute a means of storing, diverting and releasing water to protect wastes (industrial, domestic, or mining) from percolation (e.g. Frind et al. 6; Ross 0; Stormont, 6; Zhan et al. 200). The main advantages of such covers lie in design relative simplicity, long-term stability and lower construction costs compared to low saturated hydraulic conductivity covers (Morris and Stormont ). Recent studies indicate that, when properly designed for a relatively dry climate, the performance of a CCBE can exceed that of a low saturated hydraulic conductivity cover (Wing and Gee 4; Morris and Stormont ), particularly when the layered system is inclined (e.g. Ross 0; Oldenburg and Pruess ; Stormont, 6). This paper recalls the main features of inclined capillary barriers used to divert water. Results of a laboratory study that illustrates the influence of inclination and precipitation rate on diversion capacity are presented first. These results are complementary to the ones presented previously by the authors (Bussière et al., 8; 2002, 200), where the emphasis was placed on pressure and water distribution in inclined layered systems after a prolonged drainage period. Numerical simulations representative of field situations for arid and semi-arid

2 climates follow. This numerical study focuses on the influence of different parameters (precipitation rate, material properties, and cover thickness) on the capacity of an inclined CCBE to divert water. Some of the main results are presented and discussed. 2. DIVERSION CAPACITY OF INCLINED CAPILLARY BARRIERS When a fine-grained material overlies a coarser one under unsaturated conditions, the infiltrating water is retained by capillary forces in the fine medium and doesn t tend to move into the coarse material layer. The water accumulates above the contact interface until the pressure reaches the water entry pressure (or water entry value, WEV) of the coarse material. This pressure corresponds to the suction at which water starts to penetrate the coarse layer, hence increasing its degree of saturation and hydraulic conductivity. If the hysteresis of the water retention curve is neglected, the WEV approximately corresponds to suction ψ r at the residual water content θ r (Fredlund and Xing,4). Infiltration through the interface is usually reduced when the contact between the two materials is tilted. In this case, the moisture that builds up above the contact can flow along the sloping interface. However, when there is a significant inflow of water, at a certain point downdip, the fine soil may become wet enough so the pressure at the interface reaches the WEV of the coarse-grained material. At this location, water can start to infiltrate the coarse material and the capillary barrier effects progressively disappear. The location of this point along the slope is called Down Dip Limit or DDL point (Ross, 0). The amount of water flowing laterally up to the DDL point (Breakthrough point in Figure ) is taken as the maximum that a CCBE can divert and is called the diversion capacity. The horizontal projection of the distance between the top of the slope and the DDL point is another important characteristic, usually referred to as the effective length of the capillary barrier (L cb ). COARSE SOIL FINE SOIL analytical solutions have been reviewed previously by the authors (Bussière et al., 8; Bussière, ), who showed they have a limited applicability due to their inherent simplified assumptions. Hence, further investigation was deemed necessary to better define the responses of inclined covers with capillary barrier effects used to store, divert and release (SDR) water.. LABORATORY STUDY The first part of this investigation consists in evaluating, under controlled laboratory conditions, the diversion capacity of a bi-layered system. Four different tests have been performed using a new laboratory setting to look at the diversion capacity, with two inclinations and two precipitation rates. Other aspects, such as water flow, pressure distribution, and local desaturation in the capillary barrier during drainage, were also investigated; they have been presented elsewhere (Bussière, ; Bussière et al., 2002, 200).. Laboratory Setting The apparatus used in this study is a rectangular box with dimensions of 2.m x m, built with a steel frame and Plexiglas walls 0.2 m thick (see Figure 2). Holes were drilled in the bottom plate every 20 cm to recover water that percolated through the system and to evaluate the location of the DDL point. Also, 2 valves were placed at the bottom and three other valves were installed in the wall to monitor water flow at the bottom, at the interface between the gravel (coarse-grained material) and sand (fine-grained material) layers, and at the top of system (run-off). The slope angle α was adjusted to the desired inclination by using a jack (0 α 0 ). A precipitation simulator, located above the box, was made of sprinklers linked to a water reservoir. The flux of the sprinklers (or precipitation rate) was adjustable. The hydraulic behavior of the bi-layer capillary barrier has been evaluated using TDR probes and tensiometers, which measured volumetric water content θ and matric suction ψ respectively. More details on the inclined box tests can be found in Bussière et al. (8, 2002, 200) and Bussière (). BREAKTHROUGH COARSE SOIL.2 Materials description L cb Figure. Schematic representation of water movement in inclined CCBE A few analytical solutions have been proposed to evaluate the diversion capacity and the length of homogenous inclined capillary barriers (e.g. Ross, 0; Steenhuis et al., ; Morel-Seytoux, 4; Warrick et al., ). These solutions have been typically developed for steadystate conditions, with a pressure (or suction) that does not depend on the location along the slope (except for the Morel-Seytoux, 4 solution s); also, they usually consider the coarse-grained layer to be infinitely thick (phreatic surface far from the interface). Some of these In order to observe capillary barrier effects in the inclined box, it was necessary to use materials sensitive enough to the anticipated suction variations. A sand and a gravel were selected. The grain size curve (see Figure ) of the sand shows that about % of particles are smaller than mm and % smaller than 0. mm. As shown in Figure, the gravel has a uniform distribution with about % of the particles between and 0 mm. The saturated hydraulic conductivity k sat of the two materials, measured in a rigid wall permeameter, is.x0 - m/s and 4.x0 - m/s for the sand and the gravel respectively. The air entry value (ψ a ), evaluated by simultaneous measurements of suction ψ and volumetric water content θ in the box, are about 0.0 m of water (or less) for the gravel and

3 approximately 0. m of water for the sand. The WEV are approximately 0. and 0. m of water for the gravel and the sand respectively. The materials were placed in the box and densified to the desired unit weight. The average porosity was 0.0 for the sand and 0.42 for the gravel. Table. Description of the tests performed Test Slope angle P (m/s) Duration of precipitation S..x0-2h00 min S2. 2.x0 - h00 min S x0 - h00 min S4 8.4.x0-2h00 min.4 Main Results During the tests, water was recovered through the valves at the bottom of the box. The results presented in Figures 4 to 8 show the cumulative percentage for water recovery at each valve. From these, one can infer the approximate position of the DDL point. % Passing Figure 2. Description of the inclined box testing set up Sand Gravel Particles size (mm) Figure. Particle size distribution of the two soils used for the inclined box tests. Tests Performed The results of four tests are presented in the following paper (see Table ). Details on other tests are given in Bussière (). Two precipitation rates P have been investigated: P ½ k sat and k sat of the sand (top layer). The duration of the precipitation was set to 2 hours for the highest precipitation rate and to hours for the lowest P value. These time periods were long enough to reach a quasi steady-state regime in the box. Two inclination angles α were studied:. (6H:V) and 8.4 (H:V). The thickness of the bottom layer (made of gravel) was 0. m, and the top sand layer was 0. m thick. For Test S, water slowly started to percolate through the cover at valve #8. The cumulative percentage of water recovered reached more than 0% at valve #. Runoff (approximately 4% of the total precipitation) was recovered at the extremity of the box. At the bottom, most of the water was collected at valves #2 and #; this is due to the presence of the wall at the box extremity. Approximately 20% of the total precipitation was nevertheless recovered through valves #6, #8, #0, and #. Therefore, the location of the DDL point can be estimated to be at a distance of between. and 2. m along the slope (between valve #8 and #). It is also interesting to notice in Figure 4 that some water was recovered at valve #6, confirming that the ingress of water through the coarse-grained material is progressive and not instantaneous as postulated by most analytical solutions. For Test S2, the precipitation rate P was reduced to 2.x0 - m/s. In this case, a small amount of water started to percolate into the gravel at a position corresponding to valve #. As mentioned previously, water recovered in the last three valves (#2, #, and #4) is mainly due to wall effects, as was shown by numerical calculations (Bussière, ). In this case, despite the small quantity of water recovered at valve # (less than 0% of the total precipitation), it was estimated that the location of the DDL point would be at a distance greater than the length of the box (2. m). When compared to S, this test also showed that the precipitation rate influences the position of the DDL point in a sloping capillary barrier (this is confirmed by numerical results shown below). As the precipitation rate decreases, the diversion capacity increases and so does the effective length of the capillary barrier (L cb ). Test S is similar to Test S2, except that the box is further inclined to 8.4. The results are similar to Test S2 with most of the water recovered in valve #2, #, and #4 (see Figure 6). This means that the location of the DDL point would be at a distance greater than the length of the box (2. m).

4 % cumulative recovery 4.0% 40.0%.0% 0.0% 2.0% 20.0%.0% 0.0%.0% 0.0% # valve Run off Figure 4. Cumulative water recovery in the different holes for a precipitation rate of.x0 - m/s, with an inclination of. (Test S) Finally, Test S4 simulates an inclined system (with an angle of inclination of 8.4 ) with a precipitation rate of.x0 - m/s. The results presented in Figure show that most of the water is recovered at the valves located in the wall (valves #, #4, and runoff). Again, this indicates that the DDL point would be at a distance greater than the length of the box (2. m). Also, the fact that more than 0% of the water is recovered as runoff indicates that the diversion capacity of the cover is increased when the inclination of the system increases. The above results, including those shown in Figure, equally show that capillary barrier effects disappear progressively and not at a pin-point location. Indeed, in this latter test, small amounts (approximately 2%) of the total precipitation were recovered at valve #, #8, #, and #. Pressure measurements during the tests were also used to confirm the tendencies observed with water flux measurements. The pressure (or suction) measured by the tensiometers located in the gravel (near the sand/gravel interface) were usually, at the end of the precipitation events, close to zero near the bottom of the slope (which is less than the WEV of the gravel = 0. m of water), and greater than 0. m of water near the top of the sloping system. As water starts to infiltrate the coarse material when suction becomes less than its WEV, these results indicate that the DDL point was located close to (or beyond) the edge of the box. However, the exact location of the DDL point could not be determined experimentally by the pressure measurements due to the limited number of tensiometers ( to depending on the test) installed in the coarse layer. It should finally be noted that numerical modelling results have been successfully correlated to the behaviour of these layered systems. Also, results showed that numerical calculations can predict more precisely the L eff than analytical solutions for these particular inclined capillary barriers (Bussière, ; Bussière et al., 2002; 200). % cumulative recovery 0.0% 80.0% 0.0% 60.0% 0.0% 40.0% 0.0% 20.0% 0.0% 0.0% # valve Run off Figure. Cumulative water recovery in the different holes for a precipitation rate of 2.x0 - m/s and an inclination of. (Test S2) % cumulative recovery 0.0% 60.0% 0.0% 40.0% 0.0% 20.0% 0.0% 0.0% # valve Run off Figure 6. Cumulative water recovery in the different holes for a precipitation rate of 2.x0 - m/s and an inclination of 8.4 (Test S) % cumulative recovery 60.0% 0.0% 40.0% 0.0% 20.0% 0.0% 0.0% # valve Run off Figure. Cumulative water recovery in the different holes for a precipitation rate of.x0 - m/s and an inclination of 8.4 (Test S4)

5 4. NUMERICAL ANALYSIS OF A LARGE SCALE COVER The laboratory study showed that a DDL point exists, where capillary barrier effects are reduced and where the barrier becomes less effective to limit water percolation. The study also showed that precipitation rate and inclination of the layered system influence the location of the DDL point. These laboratory tests were useful to better understand the phenomena involved, but they do not offer the practical solutions necessary for field applications. A series of numerical analyses, based on a typical field case, were performed to quantify the effect of different parameters on the performance of a CCBE used to limit water infiltration in a semi-arid climate. Previous publications by the authors have shown that it was possible to predict water movement in horizontal or inclined CCBE with unsaturated-saturated numerical codes (e.g. Aubertin et al.,, 6, ; Bussière, ; Bussière et al., 2002, 200). The main objective of this part of the study was to investigate the effect of key influence factors such as precipitation rate, material property, and cover configuration. 4. Model Description A toe drain was simulated by applying a boundary pressure equal to atmospheric pressure at the bottom of the slope (in the cover at x = m and z = 0, m). The phreatic table was placed at the base of the grid. The initial pressure head at each node, required for transient calculations, was obtained from a steady-state analysis using the same model. For the steady-state simulation, a precipitation rate P of mm/day (a typical condition for an arid or semi-arid climate) was applied on top of the cover. During the transient analyses, the boundary conditions P applied on top of the model ranged from 0. to 0 cm/day. In each simulation, the precipitation was applied for days, followed by 2 days of free drainage (see Figure ). Élévation z (m) Distance x (m) y z x The program used is the commercial software SEEP/W developed by GEOSLOPE International (6). This Finite Element Method (FEM) code uses Richards () equation to simulate various situations in two dimensions, including variably saturated flow for both steady-state and transient conditions. SEEP/W was chosen because it has been successfully used for several unsaturated flow studies on layered systems in the past (e.g. Chapuis et al., 200; Yanful and Aubé ; Barbour and Yanful 4; Woyshner and Yanful ; Bussière et al. ; Aubertin et al. 6, ). The grid, which contains more than 200 elements, represents a covered circular waste rock dump (under axisymetric conditions) having a height of 24. m, a base radius of 0 m, and a slope inclination of 40. A typical representation of the mesh used for the numerical calculations is shown in Figure 8. The density of the mesh was increased in the single layer cover (placed on the coarse waste rock) in order to improve calculation precision in this critical part. The model contains three different materials: waste rock in green (Material ), the silty cover material (thickness of 0. or 0.2 m, depending on the simulation) in grey (Materials 2a, b, c, and d), and a 6 cm drainage/runoff material layer in yellow (Material ). The latter (yellow) material is used for water accumulation and movement at the surface (as runoff) of the cover, but it does not actually exist in a real world situation. In this particular case, the cover is made of only one material since the waste rock can create the necessary capillary barrier effect at the interface with the silty soil (e.g. Zhan et a. 200). Figure 8. Typical mesh used in the unsaturated flow calculations, with a close up view of the top part of the slope (Apithy, 200) 4.2 Material properties The main hydraulic properties of the different materials used in this numerical investigation are given in Table 2. The waste rock (Material ) has a low air entry value ψ a of about 0. m of water and a saturated hydraulic conductivity k sat of 0 - m/s. The residual and saturated volumetric water contents, θ r and θ s of this material are respectively, 0.06 (for a WEV of 8 kpa, or 8. m of water) and 0.. Four different silty material properties were used for this study. In each case, ψ a =. m of water, θ r = 0.06, and θ s = 0.8. The difference between the four materials is their saturated hydraulic conductivity k sat, which varies from 0 - to 0-8 m/s. This allowed (in a simplified manner)

6 the evaluation of this parameter s impact on the ability of the cover to divert water. More details on material properties can be found in Apithy (200). Precipitation rate P (m/s).00e E-0.00E E-0.00E E-0.00E-0 Boundary condition 0.00E Time (s) Figure. Example of top boundary conditions applied during transient analyses Table 2. Main hydraulic properties of the materials used in the numerical study at hand. More details on these and other results can be found in Apithy (200). 4.4 Influence of Precipitation Rate The effect of precipitation rate on the location of the DDL point, expressed here as elevation z 8, where suction near the interface between the waste rock and cover reaches 8 kpa, is presented in Figure 0. For these simulations, the cover material was Material 2a, which has a saturated hydraulic conductivity k sat of 0-6 m/s. For the boundary conditions applied to the model, the z 8 value varied between to 2. m. The results show that there is a critical value of P above which z 8 starts to move from its initial position (z=m) towards the top of the slope. In this series of calculations, the critical P value is between 2 to cm/day. Under this critical P value, the system is able to divert water along the slope (up to the maximum length imposed by the position of the water table) while at higher values water infiltrates below the cover at a DDL point with a L eff that diminishes as P increases. It is also interesting to see that, for P values greater than cm/day, the system does not reach a steady-state condition even after days of precipitation (z 8 is still increasing thus the DDL point continues to move upward). Material ψ a (m of water) θ s (or n) k sat (m/s) θ r WEV (kpa) x a. 0.8 x b. 0.8 x c. 0.8 x d. 0.8 x x < z8 position (m) q = 0, cm/day q = cm/day q = 2 cm/day q = cm/day q = 0 cm/day q = 20 cm/day q = 0 cm/day 4. Numerical Results Overall, more than 0 different simulations were performed. Different precipitations rates P were used: 0.,, 2,, 0, 20, and 0 cm/day. As mentioned previously, these were applied for days and the system was allowed to drain for 2 days. Also, for each P value, the response of four different cover materials with different k sat values was investigated. Finally, for precipitation rates of 0 and 20 cm/day and for a cover material with a k sat of 0-6 m/s, two cover thicknesses were studied: 2 and 0 cm. The main results of the modelling are presented in the next sections. The emphasis in this paper is placed on the location (called z 8 ) of the DDL point in the slope, where the capillary barrier effects begin to disappear. The DDL location corresponds to the point where the pressure near the interface is equal to the water entry value of the coarse material (i.e. WEV of 8. m of water or 8 kpa; see Table 2). It should be stated here that this large WEV was identified conservatively on the water retention curve of the waste rock; a larger value means that there is more possibilities for suction to become lower than the WEV, hence providing a pessimistic view of the DDL location and of the cover performance. The diversion length given here must thus be taken as lower bounds for the problems Time (days) Figure 0. Evolution of the DDL point z location (z 8 ) during the 28 days of modelling for different precipitation rates 4. Influence of k sat Another series of simulations attempted to evaluate the influence of the cover material saturated hydraulic conductivity (k sat ) on its ability to limit water infiltration. Four different k sat values were used: 0 -, 0-6, 0 -, and 0-8 m/s. Figure shows the location of the DDL point (z 8 ) for different cumulative precipitations on the cover after days (expressed in cm of water). For example, when a precipitation rate P of 0 cm/day is applied, the cumulative amount of water after days is 0 cm. The results show that for materials having a k sat less (or equal to 0 - m/s, the z 8 does not appear to vary with P (the DDL stays at its initial position). However, the DDL point is affected when the cover material has a hydraulic conductivity larger than 0 - m/s, especially when the cumulative precipitation is very large (greater than 0 cm).

7 These results show the importance of k sat on the diversion capacity of this type of inclined CCBE. Figure 2. Evolution of the DDL point location (z 8 ) during the 28 days of modelling for precipitation rates of 0 and 20 cm/day and for cover thicknesses of 2 and 0 cm..2 z8 position (m) Ksat=*0- m/s Ksat=*0-6 m/s Ksat=*0- m/s Ksat=*0-8 m/s Precipitations (cm) Figure. Location of the DDL point (z 8 ) after days of precipitation for different cover s material having different k sat ; an increasing z 8 corresponds to a smaller length of the capillary barrier. 4.6 Influence of Cover Thickness Numerical simulations were performed on two covers having two different thicknesses: 2 and 0 cm. For these simulations, two P values (of 0 and 20 cm/day) were applied as boundary conditions; a k sat of 0-6 m/s was used for the cover material. The location of the DDL point (z 8 ) versus time for the studied conditions is shown in Figure 2. For a P value of 0 cm/day, the z 8 locations are around 0.2 m and.6 m for cover thickness of 2 and 0 cm respectively. When the precipitation rate is increased to 20 cm/day, the z 8 moves to an elevation of 2 and 0. for a thickness of 2 and 0 cm, respectively. One can see that higher values of z 8 are obtained for higher P values, and lower cover thicknesses. This means that increasing the thickness of the cover can help reduce the water infiltration by increasing the length of the capillary barrier. z8 position (m) cm/day - 2 cm of silt 0 cm/day - 0 cm of silt 20 cm/day - 2 cm of silt 20 cm/day - 0 cm of silt. SUMMARY AND CONCLUSIONS An inclined cover with capillary barrier effects can be used to limit water infiltration, particularly in arid and semi-arid climates. One key component of this type of cover is the location of the DDL point (or the effective length of the capillary barrier L cb ) during precipitation events. The main objective of this study was to investigate the diversion capacity (expressed by the DDL point location or the L cb ) of inclined cover systems in which capillary barrier effects exist. To do so, laboratory tests in a unique physical model were performed. The main findings of the laboratory study are: An inclined capillary barrier has the ability to divert water; water flows at the interface between coarseand fine-grained soils until a certain point down dip (DDL point) is reached. At this location, the finegrained soil is sufficiently wet so the pressure applied to the coarse-grained soil becomes greater than the WEV; water ingress then becomes possible. The disappearance of capillary barrier effects at the DDL point is gradual and does not occur at a pinpoint location as postulated by analytical solutions; Precipitation rate and slope angle of the layered system affect the diversion capacity of an inclined capillary barrier. The laboratory study helped better understand the diversion capacity of inclined capillary barriers, but can not be considered representative of most real world situations. A numerical study (of more than 0 different scenarios) based on an typical field case was performed to quantify the impact of different parameters on the performance of a CCBE built to limit water infiltration under a semi-arid climate. The main conclusions of the numerical study are: A critical precipitation rate P exists, above which the diversion capacity of an inclined CCBE is reduced; beyond that, the DDL point moves upward along the slope with an increased P value). Steady-state conditions are not reached even after days of precipitation The saturated hydraulic conductivity k sat of the cover material is a critical factor on the diversion capacity of an inclined CCBE. Decreasing k sat increases the effective length L eff. Increasing the thickness of the cover may help to reduce the infiltration of water Time (s) This study also showed that it is essential to evaluate the position of the DDL point for critical precipitation rates to design an efficient CCBE used to divert water. To do so, 2D transient modelling is necessary. The objective of the calculations is to insure that the DDL point would stay beyond the bottom of the layered cover, even for critical climatic conditions. This is required to prevent water infiltration into the wastes.

8 6. AKNOWLEDGMENTS Funding for this work came from the industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management ( and from an NSERC discovery grant (individual grant, B. Bussière).. REFERENCES Apithy, S.A. (200). Étude du comportement de couvertures à effets de barrière capillaire placées sur des haldes à stériles en climat semi-aride. Master thesis, Civil, Geological, and Mining Engineering Department, Ecole Polytechnique de Montréal. Aubertin, M., Chapuis, R.P., Bouchentouf, A. and Bussière, B.. Unsaturated flow modeling of inclined layers for the analysis of covers. Proceedings of the 4 th ICARD, Vancouver, B.C., 2 : -46. Aubertin, M., Bussière, B., Aachib, M. and Chapuis, R.P. 6. Une modélisation numérique des écoulements non saturés dans des couvertures multicouches en sols. Hydrogéologie, : -. Aubertin, M., Chapuis, R.P., Aachib, M., Bussière, B., Ricard, J.-F. and Tremblay, L.. Évaluation en laboratoire de barrières sèches construites à partir de résidus miniers, MEND Report a. Barbour, S.L. and Yanful, E.K. 4. A column study of static nonequilibrium fluid pressures in sand during prolonged drainage. Canadian Geotechnical Journal, :2-0. Bussière, B.. Étude du comportement hydrique de couvertures avec effets de barrière capillaire inclinées à l'aide de modélisations physiques et numériques. Ph.D. Thesis, Mineral Engineering Department, École Polytechnique de Montréal. Bussière, B., Aubertin, M. and Chapuis, R.P The behavior of inclined covers used as oxygen barriers. Canadian Geotechnical Journal, 40 : 2-. Bussière, B., Aubertin, M. and Chapuis, R.P A laboratory set up to evaluate the hydraulic behavior of inclined capillary barriers. Proceedings of the International Conference on Physical Modelling in Geotechnics, St.Jonh s, Newfoundland, -6. Bussière, B., Aubertin, M., Morel-Seytoux, H.J. and Chapuis, R.P. 8. A laboratory investigation of slope influence on the behavior of capillary barriers. Proceedings of the th Canadian Geotechnical Conference, Edmonton, 2 : Bussière, B., Aubertin, M., Aachib, M., Chapuis, R.P. and Crespo, R.J.. Unsaturated flow modelling of covers for reactive tailings. CAMI, Proceedings of the Third Canadian Conference on Computer Applications in the Mineral Industry, Montréal, Chapuis, R.P., Chenaf, D., Bussière, B., Aubertin, M. and Crespo, R A user's approach to assess numerical codes for saturated and unsaturated seepage conditions. Canadian Geotechnical Journal, 8(): Chapuis, R.P., Crespo, J.R., Chenaf, D. and Aubertin, M.. Evaluation of a groundwater F.E.M. software for steady and unsteady state conditions. Proceedings of the 46 th Canadian Geotechnical Conference, Saskatoon, 6-0. Fredlund, D.G. and Xing, A. 4. Equations for the soilwater characteristic curve. Canadian Geotechnical Journal, : 2-2. Frind, E.O., Gillham, R.W. and Pickens, J.F. 6. Application of unsaturated flow properties in the design of geologic environments for radioactive waste storage facilities. Proceedings of the st International Conference on Finite Elements in Water Resources, Princeton, N.J., Gray, W.G., Pinder, G.F. & Brebbia, C.A. (eds),.-.6. GEOSLOPE International 6. SEEP/W User's Guide. Morel-Seytoux, H.J. 4. Steady-state effectiveness of a capillary barrier on a sloping interface. 4 th Hydrology Days, Ed. Hubert J. Morel-Seytoux, Hydrology Days Publications, -46. Morel-Seytoux, H.J. 2. The capillary barrier effect at the interface of two soil layers with some contrast in properties. HYDROWAR Report 2.4, Hydrology Days Publications, Selby Lane, Atherton, CA , 0 p. Morris, C.E. and Stormont, J.C.. Capillary barriers and Subtitle D covers: estimating equivalency. Journal of Environmental Engineering, 2: -0. Oldenburg, C.M. and Pruess, K.. On numerical modeling of capillary barriers. Water Resources Research, 2 : Rasmusson, A. and Erikson, J.C. 86. Capillary barriers in covers for mine tailings dumps. Report 0, The National Swedish Environmental Protection Board. Richards, L.A.. Capillary conduction of liquids through porous medium. J. Physics. : 8-. Ross, B. 0. The diversion capacity of capillary barriers. Water Resources Research, 26 : Steenhuis, T.S., Parlange, J-Y. and Kung, K-J.S.. Comment on The diversion capacity of capillary barriers by Benjamin Ross. Water Resources Research, 2 : Stormont, J.C. 6. The effectiveness of two capillary barrier on a 0% slope. Geotechnical and Geological Engineering, 4 : Stormont, J.C.. The effect of constant anisotropy on capillary barrier performance. Water Resources Research, :8-8. Warrick, A.W., Wierenga, P.J. and Pan, L.. Downward water flow through sloping layers in the vadose zone: analytical solutions for diversions. Journal of Hydrology, 2: 2-. Wing, R. and Gee, G. 4. Quest for the perfect cap. Civil Engineering, October, 8-4. Woyshner, M.R. and Yanful, E.K.. Modelling and field measurements of water percolation through an experimental soil cover on mine tailings. Canadian Geotechnical Journal, 2 : Yanful, E.K. and Aubé, B.. Modelling moistureretaining soil covers. Proceedings of the Joint CSCE-

9 ASCE National Conference on Environmental Engineering, Montréal, : -80. Zhan, G., Aubertin, M. Mayer, A., Burke, K. and Mcmullen, J Capillary cover design for leach pad closure, SME Transaction 200, : 04-0.