Single soil layer

Päivi M. Kauppila, Geological Survey of Finland, P.O. BOX 1237, FI-70211, FINLAND, e-mail: paivi.kauppila(at)gtk.fi

Introduction

A single-layer soil cover of natural soil is the simplest and least expensive dry cover type. It is composed of a single mineral soil layer placed directly over the mine waste. The purpose of this cover type is to 1) minimize dusting from the waste, 2) prevent surface erosion of the waste and, 3) to act as a growth media for the vegetation. It will also decrease infiltration into the underlying waste, but it will not prevent infiltration as a whole unless applied in a dry climate where evapotranspiration exceeds precipitation. Effectiveness of the cover layer to act as a barrier to oxygen transport depends on the moisture retention capacity of the material. (MEND 1994, EC 2009, INAP 2009, Kauppila et al. 2013)

Description of the methodology

Single-layer soil cover is a cap-and-cover solution typically constructed of natural earth materials, such as till, clay or coarse gravel/rock, available close to or at the mine site (MEND 1994, Lottermoser 2007, EC 2009). In Finland, till is usually more easily available at or near the mine sites than clay.

There are basically two different types of single-layer soil covers: covers that are vegetated (consists of till or clay) and, covers that consist of coarse gravel or riprap and do not support vegetation growth. The main purpose of the latter is the erosion-protection of the mine waste. (MEND 2004, Lottermoser 2007, INAP 2009)

Figure 1. Structures of a single-layer soil cover. a) Till/clay cover, and b) Coarse gravel/rip rap cover (modified after Tremblay & Hogan 2001, Lottermoser 2007).

The typical layer thickness of the single-layer till/clay cover is 0.5-1.5 m (EC 2009). The thickness depends on the waste type and site-specific requirements. In minimum, the thickness should be thick enough to support vegetation. Therefore, waste materials containing toxic compounds require thicker cover layer than those with less hazardous properties to prevent the vegetation from dying upon rooting into the covered waste.

The waste material is covered with soil material after removal of ponded water on the surface of the waste and drying of the waste material to allow access of earth-moving machines on the waste pile surface. The layer can be left uncompacted or it can be compacted to reduce the hydraulic conductivity and to increase the saturation degree of the cover material. Compaction is made in layers. (Lottermoser 2007, EC 2009)

Appropriate applications

Single-layer soil covers are best suited for covering of wastes that are not reactive and for which the main closure object is to control dust emissions, protect the waste from erosion and to develop a stable landform. These wastes include non-acid generating wastes and wastes in which the long-term solubility of potentially harmful substances is minimal or manageable by conducting the water for treatment. (Kauppila et al. 2013)

Single-layer soil covers are not suitable for covering of acid-producing wastes as they are not effective in reducing acid mine drainage. These types of covers do not fully prevent access of water or oxygen into the waste. (Höglund et al. 2004, INAP 2009) Infiltration of water can be reduced by using materials with low hydraulic conductivity (e.g. O’Kane & Wels 2003). In addition, soil layers at or near water saturation, such as clay layers, reduce also oxygen flux into the waste since diffusion of oxygen is slower in water than in air, thus decreasing the oxidation rate of the wastes (MEND 1994).

Table 1. Advantages and disadvantages of single-layer soil covers (EC 2009, INAP 2009, Kauppila et al. 2013).

Performance of a single-layer soil cover to reduce AMD can be increased by extending the water table in the waste above the tailings, into the cover itself. However, this is difficult to maintain at most of the mine sites. This type of approach is e.g. for tailings deposited into a valley, in which water saturation is possible to maintain by limiting discharge from the saturated zones of the impoundment. This technique has been applied e.g. in Kristineberg, Sweden. (Höglund et al. 2004)

Performance

The soil covers may require long-term maintenance to monitor the solidity of the cover, since they are vulnerable to vegetation, animal and human activity including vehicle traffic, and may be prone to erosion. Clay layers in particular are prone to cracking and other deterioration due to e.g. freeze/thaw, desiccation, burrowing, root penetration or erosion, providing access to oxygen into the waste material. (MEND 1994)

The main costs in the use of covers are the cost of construction materials and the cost for the use of heavy equipment during cover construction. Costs for the cover depend on the availability of the materials, the size of the covered area and the thickness of the cover layer. The single-layer soil cover is usually the least expensive dry cover type. Cost estimates presented in literature range in $20,000-60,000 /hectare and $0.03-0.10 t/waste rock. (Robertson & Shaw)

Design requirements

Designing of a single-layer soil cover requires information

• on the characteristics of the mining waste,
• on the climatic conditions at the mine site (frost/thaw effects, precipitation),
• on the characteristics and availability of cover materials within the mine site or close-by, and
• of the root penetration depth of the flora to be planted on top of the cover to ensure that the cover will be adequate and will function as planned. (INAP 2009, MEND 2004, Kauppila et al. 2013)

Other things to consider:

• The materials to be used should support vegetation growth (i.e. should have sufficient water-holding capacity) and be resistant to erosion.
• Permeability of the cover depends on the grain size of the cover material and the level of compaction, and diffusion of oxygen on the moisture content of the material.
• Construction requires removal of the ponded water from the surface of the waste and drying of the waste material to allow access of earth-moving machines on the waste pile surface.
• Solidity of the cover layer may require monitoring. (MEND 2004, EC 2009, INAP 2009, Kauppila et al. 2013)

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 Accessed 28^th November 2013

Höglund, L.O. (Ed.), Herbert, R. (Ed.), Lövgren, L, Öhlander, B., Neretnieks, I., Moreno, L., Malmström, M., Elander, P., Lindvall, M. & Lindström, B. 2004. MiMi – Performance assessment Main report. Mimi 2003:3. ISBN: 91-89350-27-8. 345 p.

INAP 2009. The GARDGuide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/

Kauppila, P., Räisänen, M.L. & Myllyoja, S. (Eds) 2013. Best Environmental Practices in Metal Ore Mining. The Finnish Environment 29en/2011. Helsinki, Finnish Environment Institute. ISBN: 978-952-11-3942-0. 219 p. https://helda.helsinki.fi/handle/10138/40006

Lottermoser, B.G. 2007. Mine wastes – Characterization, Treatment, Environmental Impacts 2nd ed. Springer. 304 p.

MEND 1994. Evaluation of alternate dry covers for the inhibition of acid mine drainage from tailings. MEND Project 2.20.1. Canadian centre for Mienral and Energy Technology (CANMET).

MEND 2004. Design, construction and performance monitoring of cover systems for waste rock and tailings. MEND 2.21.4. Volume 1. Summary. 82 p. http://mend-nedem.org/wp-content/uploads/2.21.4a.pdf Accessed 28^th November 2013.

MEND 2012. Cold Regions Cover Systems Design Technical Guidance Document. MEND Report 1.61.5c. http://mvlwb.com/sites/default/files/mvlwb/documents/Cold%20Regions%20Cover%20System%20Design%20Technical%20Guidance%20Document%20(MEND%20Report%201.16.5c).pdf Accessed 28^th November 2013

O’Kane, M. & Wels, C. 2003. Mine Waste Cover System Design – Linking Predicted Performance to Groundwater and Surface Water Impacts. Proceedings of 6 th International Conference for Acid Rock Drainage, Cairns, Qld., Australia, July 2003. http://www.okc-sk.com/wp-content/uploads/2012/02/MineWaste.pdf Accessed 28^th November 2013.

Robertson, A. & Shaw, S. Covers for mine waste disposal facilities. Robertson GeoConsultants Inc. & Mesh Environmental Inc. www.infomine.com Accessed 28^th November 2013

Trembley, G.A. & Hogan, C.M. 2001. MEND Manual, Volume 4. Prevention and Control. MEND Report 5.4.2d.