Étude expérimentale des propriétés hydrogéologiques des roches stériles à une échelle intermédiaire de laboratoire

RESUME Les grandes quantites de steriles miniers generees annuellement par l’industrie miniere representent des risques pour l’environnement. Le stockage des roches steriles dans les haldes peut conduire a de graves problemes causes par le drainage minier acide (DMA) ou le drainage neutre contamine (DNC). Par consequent, des etudes systematiques sur les steriles miniers sont necessaires afin d’elaborer des methodes de stockage ecologiques et securitaires. Le projet de recherche presente dans ce memoire avait pour objectif d’etudier les principales caracteristiques hydrogeologiques de steriles miniers a partir d’une approche experimentale a l’echelle intermediaire en laboratoire. Le but specifique du projet etait la determination de la conductivite hydraulique saturee des echantillons de roches steriles et de leur courbe de retention d’eau. On a aussi evalue l’effet du deplacement des particules fines, lors de l’ecoulement de l’eau, sur les proprietes hydriques des steriles. Les essais ont ete realises sur les echantillons preleves sur une des haldes a steriles de la mine Lac Tio (RTFT, Havre-Saint-Pierre, Quebec). Selon le systeme unifie (ou ASTM) de classification des sols, les echantillons testes (diametre ≤ 50 mm) correspondent a un sable bien gradue (SW), avec diverses proportions de particules fines, de gravier et de cailloux. Les proprietes hydrogeologiques de ces steriles ont ete determinees a l’aide d’essais de permeabilite, de traceur, d’infiltration et de retention d’eau dans des grandes colonnes instrumentees. Ces essais ont ete menes sur des echantillons (specimens) dont la granulometrie a ete tronquee a des dimensions preselectionnees (de 5 a 50 mm). Les resultats des essais en colonne ont montre que la conductivite hydraulique saturee des steriles, mesuree en laboratoire a echelle intermediaire, est peu influencee par la presence de differents pourcentages de graviers et de cailloux. La valeur de ksat des steriles varie autour de 10-1 cm/s pour les diverses fractions granulometriques testees. Les valeurs obtenues experimentalement ont aussi montre qu’a l’echelle du laboratoire, la conductivite hydraulique saturee des steriles n’est pas influencee significativement par la migration des particules, qui se produit surtout pour des gradients hydrauliques eleves (i ≥ 2-3). ----------ABSTRACT The mining industry produces large quantities of waste rock which are usually stored in piles on the surface. Waste rocks often contain reactive minerals that may give rise to a contaminated effluent. It is essential to define the hydrogeological properties of these wastes to assess the environmental risks associated with surface disposal. The main purpose of this project was to assess the hydrogeological properties of waste rocks by means of intermediate scale laboratory tests. More specifically, the project aimed to determine the saturated hydraulic conductivity (ksat) and the water retention curve (WRC) of waste rock samples. The effect of fine particles displacement that may occur when water percolates through widely graded materials was also evaluated. The tests were conducted on samples taken from one of the waste rock piles at the Lac Tio mine (RTFT, Havre-Saint-Pierre, Quebec). The samples (diameter ≤ 50 mm) correspond to widely graded sand with diverse proportions of fine particles, gravel and pebbles. The hydrogeological properties of the waste rocks were determined by means of permeability, tracer, infiltration and drainage tests in large instrumented columns. The tests were conducted on samples having a particle size that was truncated to the preselected maximum sizes (from 5 to 50 mm). The results of the tests showed that the saturated hydraulic conductivity ksat of the waste rocks, measured in the laboratory, is not significantly influenced by the presence of various percentages of gravels and pebbles. The value of ksat varies around 10-1 cm/s for the samples with different particle size distributions. At the laboratory scale, the experimental values of ksat is not significantly influenced by the movement of fine particles, which often occurs at high hydraulic gradients (i = 2-3). The comparison of the test results with ksat values obtained from predictive models showed significant differences; the measured values are much larger than the latter. The saturated hydraulic conductivity of waste rocks seems to be controlled by a system of interconnected macropores, which act as preferential water flow paths. The presence of such preferential paths in the waste rocks was partially confirmed by the results of tracer and infiltration tests. Alternative models are proposed to predict the value of ksat for these waste rocks.