Abdelkabir Maqsoud

An Investigation of Factors that Influence the Water Diversion Capacity of Inclined Covers with Capillary Barrier Effects

Covers with capillary barrier effects (CCBEs) can be used as an alternative to more traditional covers that rely on materials with a low saturated hydraulic conductivity. A typical CCBE includes a fine-grained soil layer placed on a coarser material. Other layers can also be added to help the cover play its role(s) efficiently. In a cover built on a flat area, the capillary barrier effect at the interface between the fine and coarse materials allows the finer soil layer to store incoming water, which can later be released by evaporation. Such Store-and-Release covers can be quite convenient in arid and semi-arid conditions. In sloping areas, a CCBE also acts as a lateral water diversion system. Part of this diversion occurs along the sloping interface between the two superimposed soils. It can contribute significantly to the reduction of water percolation deeper into the underlying wastes. However, inclined CCBEs are more complex, as they are influenced by many factors that are not yet fully understood. In such layered covers, moisture is not evenly distributed along the length of the slope. Under some conditions, the moisture-retaining layer can reach a critical degree of saturation at a certain down dip location, which increases infiltration of water into the coarse material and reduces the cover efficiency. This paper presents some of the main results of an ongoing investigation of inclined CCBEs that includes testing on a physical model, field work on large scale covers, and numerical analyses of various cover scenarios. Emphasis is placed here on numerical simulations to assess some of the influence factors that affect the diversion capacity and moisture distribution. The numerical results show how the diversion capacity is affected by the saturated hydraulic conductivity of the fine-grained soil, by the precipitation rate and duration, and by the layer thickness. The findings presented here can be useful for the design of inclined CCBEs.

Predicting Hysteresis of the Water Retention Curve from Basic Properties of Granular Soils

The water retention curve (WRC), which represents the relationship between volumetric water content (θ) and suction (ψ), is required to analyze the hydro-geotechnical response of unsaturated soils. The laboratory (or field) determination of the WRC can however be time consuming and difficult to conduct. A practical alternative, particularly useful at the preliminary stages of a project, is to estimate the WRC using a predictive model based on basic geotechnical properties that are easy to obtain. One common limitation of such predictive models is due to hysteresis effects, which are not taken into account by most of these models. The authors present in this paper an extended version of the Modified Kovács (MK) predictive model that incorporates hysteresis of the WRC along different paths, including the main wetting and drying curves and the wetting and drying scanning curves for granular soils. The model formulation is presented, and predictions are compared to experimental data obtained on different granular soils. The results show a good agreement for the main and scanning curves.

Tracer tests to evaluate hydraulic residence time in limestone drains: Case study of the Lorraine site, Latulipe, Québec, Canada

Acid mine drainage (AMD) remains one of the major environmental problems for the mining industry. When AMD is produced on a mine site, passive treatment techniques such as limestone drains (LDs) can be used to improve water quality, particularly when the effluent is relatively small. In 1999, LDs were installed in combination with a cover with capillary barrier effects to rehabilitate the abandoned acid-generating Lorraine mine site in Quebéc, Canada. However, the quality of the water exiting the LD does not meet the local regulations (even if a significant improvement has been observed). To better understand the behaviour of the drains, the hydraulic residence time (HRT) was evaluated using various tracer tests. Tests’ results indicate that the HRT in the Lorraine LDs is close to the minimum value targeted at the design stage but is significantly different than those estimated from the geometrical characteristics and porosity of the drains and the water flow discharge.

Comparison between the predictive modified Kovács model and a simplified one-point method measurement to estimate the water retention curve

ABSTRACT The modified Kovács (MK) model can predict fairly well the water retention curve (WRC) of different types of soils, using basic geotechnical properties. Considering its relative simplicity and its ability, a study was initiated to compare this model to an alternative predictive approach involving a one-point measurement method (OPMM). The comparison result indicates that there is generally a good agreement between volumetric water content values predicted using the OPMM and the MK model and those obtained from direct measurements. However, the OPMM typically leads to a better correlation than the predicted values obtained with the MK model for cohesive fine-grained soils because shrinkage induced by suction is not taken into account. Hence, the MK model is deemed as a valuable tool that gives realistic estimates of the WRC using easy to obtain basic geotechnical parameters, when there is no direct measurement available for a point on the WRC, and this predictive model can be particularly useful at the preliminary phase of a project.