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Limiting leachate

Geosynthetic manufacturer Naue Fasertechnik has supplied its needle punched Bentofix geosynthetic clay liner as the sealing element on a large effluent lagoon for a paper mill in Red Rock on the shores of Lake Superior in Ontario, Canada.

With 12m of silty-sand glacial till underlying the site, the floor of the 650,000m3 lagoon had adequate natural protection, so the GCL was just used for the 58,000m2 of embankments and keyed into the floor, 6m from the toe of the slope.

One of the most important issues for client Domtar Packaging was the possible leaching of effluent from the lagoon into the lake, as the International Joint Commission on the Great Lakes has designated the area as a demonstration case for zero discharge. The challenge was to meet these requirements but also to build a lagoon cost effectively within the confines of the mill site.

The GCL had to have sufficiently low hydraulic conductivity to prevent contaminants migrating from the lagoon, as stipulated by the Ontario Ministry of the Environment and Energy. Tests were carried out for the Red Rock project, using the mill effluent as the hydrating fluid and the test permeant. Results showed that acceptable permeability values lay in the range of 5.5x10-12m/s to 6.5x10-12m/s. The material also had to be stable on the 8m high 2.5:1 embankment slopes.

The designers decided to use reinforced Bentofix, with non woven geotextiles either side to provide the most stable design for the site. The selected soil had an internal failure angle of 36 and 31 against the GCL; the internal shear value of the GCL was measured at 40, deemed sufficient for the specified slopes.

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