Basal structures of waste facilities

Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi

Waste properties, quantity of the waste fractions, and extent of the disposal area affect the selection of the placement, structure (foundation settlements, dam structure design), disposal techniques and water management system of a waste facility. Requirements for underliners in waste areas depend on the local regulations, which vary widely from country to country. Therefore requirements for the soil base and basal structure of the waste area depend for example on the acid formation and neutralizing properties of the waste, potential concentrations of harmful substances in the waste and their solubility in short and long term. (e.g. Kauppila et al. 2013)

The base for mining waste disposal can be a natural soil structure or synthetic (artificial) basal structure on top of the load-bearing natural soil. Selection of the materials to be used in the basal structures takes into consideration also the availability and cost of structural materials, the extent of the waste area, expansion considerations for the facility (e.g. upstream, centreline, downstream construction), and in-situ soil investigations. Soil investigations in the planned waste disposal area play an important role, and typically include information on deposit types and formations, details about hydrology in the waste area (e.g. catchment area boundaries, surface water flow directions) and affect substantially to the selection of the waste area’s location and foundation settlements. (EC 2009, Kauppila et al. 2013)

The basal structure can be completely watertight, slightly permeable or permeable. One principal rule is to have watertight layer as a part of the basal structure when disposing extractive waste that can cause negative environmental impact or have potential of becoming a serious threat to the environment (acid formation properties or the potential concentrations and risk of leaching of harmful substances). Basal structures of tailings can vary from a simple subsoil base to more complex layered structures that contain synthetic materials. Typically, the structure is complex with filter and seepage control layers, geotextile layer, leakage detection and collection layer and extra clay liner and base preparation (sand/gravel), which means the thickness of a basal structure can easily be over 1 meter. Liner system design is usually a critical part for new waste facilities. (Kauppila et al. 2013)

Four main options for basal structures include in-situ soils/no liner, single liner, composite liner and double liner, and there are also multiple alternatives for synthetic liner materials (listed in chapter: Impermeable basal structure with synthetic liners). These structures are evaluated in a more detail in the following  pages:

Permeable basal structure

• Subsoil base

Impermeable basal structure (and dam structures)

• With synthetic liners (plastics, bentonite, bituminous geomembrane)
• With natural materials/soils

Reactive basal structure

• Neutralising base

References

EC 2009. Reference document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009. European Commission. 557 p. http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.pdf