A review of non-traditional materials used in environmental cover applications was conducted to determine which of these materials have potential for use in acid rock/acid mine drainage cover systems. The following non-traditional materials were reviewed:
• asphalt covers,
• cementitious covers (polypropylene fibre reinforced shotcrete),
• cement-stabilized coal fly ash grout (Cefill, fly ash mixtures, and geopolymers),
• synthetic liners and covers (geomembranes, spray-on membranes barriers, and geosynthetic clay liners),
• bentonite modified soil barriers (soil-bentonite mixtures, polymer modified soil, and polymer surfactants),
• mine waste (tailings and waste rock), and
• wax barriers.
The above non-traditional cover materials can all be characterized as possessing low hydraulic conductivity. A majority of the case studies reviewed were focused primarily on the nature of the hydraulic barrier (i.e. measuring saturated hydraulic conductivity). This may be adequate for landfill and other similar waste applications where water infiltration alone needs to be controlled. In AMD applications, it is also necessary to prevent the transfer (ingress) of oxygen to the underlaying waste. Also, it is desirable that the cover systems provide a medium for the development of sustainable vegetation.
Most of the non-traditional cover materials reviewed have the potential to be used as hydraulic barriers to control infiltration into acid generating waste piles. Some of the non-traditional cover materials may also be effective oxygen barriers. These materials would include bentonite modified sands, asphalt, spray-on membranes and wax. Other materials such as Cefill, geosynthetic clay liners and soil modified barriers can also be effective oxygen barriers if they are maintained in a saturated state.
The most attractive non-traditional cover materials for AMD applications are mining wastes (tailings and waste rocks) since these materials are already on site. These materials may need to be modified with bentonite or possibly fly ash to make the tailings and waste rock suitable as cover materials. The use of bentonite modified sand appears to provide one of the best combination of low permeability water barrier and oxygen ingress barrier, however, this material may be more costly than using mining wastes. Many of the remaining non-traditional cover materials have potential uses especially as a redundant hydraulic barrier in multi-layer cover systems using capillary barriers and capillary breaks.
