Discussion of "Variations in moisture content for a soil cover over a 10 year period"

Abstract

The authors have presented a very interesting paper on the in situ behaviour of a soil cover constructed on a large waste rock pile. As this type of cover is relatively new (at least in the mining industry), few investigations with such extensive field data have been reported in the literature. This type of information is quite useful to those who must design cover systems, as it brings an improved understanding of how such a complex system responds to actual field conditions. The discussers (and their collaborators) have also been involved in somewhat similar projects (e.g., Ricard et al. 1997, 1999; Bussiere et al. 2003c; Dagenais et al. 2005), and the results obtained from the corresponding monitoring and modelling programs have revealed similar trends. Many of the main findings presented by the authors are in close agreement with those gathered by the discussers. The results confirm that under actual field conditions, cover system behaviour is intrinsically dynamic, and rarely (if ever) reaches a steady state under a relatively humid climate common in many parts of Canada. This means that the monitoring program and modelling work should focus on the transient conditions that dominate the cover response (Aubertin and Bussiere 2001). Aubertin et al. 106 The authors’ results also show that a cover with capillary barrier effects (CCBE) can induce the favourable conditions required to control the production of acid rock (or mine) drainage (ARD or AMD). The authors should also be complimented for reporting on a diversity of performance indicators, and for pointing out some of the challenges and difficulties encountered when monitoring the in situ behaviour of a CCBE on a large scale, and in the interpretation of the data. In the following paragraphs, the discussers would like to draw attention to some complementary issues that may be of interest but were not addressed in detail in the authors’ paper (however, the authors have probably considered these issues elsewhere). As mentioned by the authors in their paper, and also pointed out in a recently published companion article (Fredlund and Wilson 2006), soil covers seldom behave as a one-dimensional system. Except for the relatively flat surfaces encountered on some tailing stacks, the actual flow of water in a layered cover usually involves both vertical and lateral movements within and between the different soil layers. In this regard, previous work by Aubertin et al. (1997) and Bussiere et al. (2000, 2003b) has shown, using numerical simulations and in situ observations, that the difference in elevation between the bottom and top portions of an inclined cover can induce a significant suction gradient. This in turn can create a variation in the moisture distribution in the cover along the slope. In many instances, there is a tendency for water to accumulate at lower elevations, with increasing desaturation towards the top of the cover (Aubertin et al. 1997; Bussiere et al. 2000). This is an important aspect of the behaviour of CCBEs that may not have received enough attention in the past. For inclined cover systems, the local degree of saturation along the slope depends on many factors, including the water retention curve of the materials (particularly the air entry value (AEV) of the moisture retaining layer), the thickness of the layer(s), the water budget components and sequence, the slope angle, length, and other geometric features such as benches (Bussiere et al. 2003b). It would thus be interesting if the authors could use available field data (or new data gathered in the future) to investigate how the slopes may influence the cover behaviour at the Equity Silver mine site in Houston, British Columbia. This aspect is important because, as the authors pointed out, a CCBE that aims to control the production of ARD by reducing the oxygen flux must rely on a highly saturated layer to act as an oxygen barrier. Limiting oxygen penetration into reactive mine wastes has in fact been shown to be an effective control method that is well suited to relatively humid climates, particularly on flat areas (Bussiere et al. 2003b, 2003c; Dagenais et al. 2005). The cover geometry may, however, create additional difficulties in maintaining an effective oxygen barrier along the entire sloping area, because the suction that develops at higher elevation can in some cases exceed the AEV of the material used for the moisture-retaining layer. The conditions at the site described by the authors do not seem to have created such situations (based on the properties given in the paper), but it is not clear from the presented data if (and how) the