Subsoil base

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

Introduction

Permeable basal structure at the waste facility can provide sufficient protection when waste has little or no potential to generate contaminative leachates and initial concentrations of harmful substances in the waste are low. In practice, this means that the waste is inert. In addition, permeable base is applicable if hydrologic setting provides sufficient protection, constituent concentrations in the receiving water body are higher than in the expected leachate, and if the local legislation accepts permeable basal (and dam) structures for waste facilities. Although the waste is inert, the soil in the basal structure has to be suitable for construction and provide a firm foundation for the waste facility. (Kauppila et al. 2013)

Description of the technology

Soil layers are often placed in as parallel lifts (bathtub) or horizontal (stair step) lifts when constructing a basal structure of waste disposal unit. With the parallel lifts, liner material is compacted up and down the slopes and bottom of the impoundment. Water flow through the recompacted soil layers can occur through the soil material and lift interfaces. With the horizontal construction, the liner material is compacted in a series of lifts on the inside slopes of the impoundment. Figure 1 illustrates these two approaches. In parallel constructions, fewer lifts are often required but steep slopes can be a limiting factor (3:1 or less) and result in the shearing of the soil while equipments tend to slide on the slope. Hydraulic conductivity can be measures in the horizontal (k_h) and vertical (k_v) directions, although the rules do not distinguish between horizontal and vertical permeability. The horizontal permeability is usually 2-10 times greater than the vertical permeability. (OhioEPA 2004).

Figure 1. A. Liners are constructed parallel to the slope/dam. B. Liners are constructed in horizontal lifts against a slope/dam. Arrows are indicating the main directions of water flow from the disposal facility (modified after OhioEPA 2004)

Appropriate applications

Advantages:

• Simpler water balance management
• Enables different dam structures

Disadvantages:

• Only suitable for inert wastes
• Groundwater monitoring is usually essential
• Impact on soil chemistry over the longer time must be estimated

Performance

Typical hydraulic conductivity value of in situ soil varies between 1×10^-5 and 1×10^-8 cm/sec (Hutchison & Ellison 1992).

Maintenance needs

No specific maintenance needs

Design requirements

Extensive characterization of waste material and soil mapping of the disposal area is essential to ensure permeable structure is suitable for particular waste type at the planned location.

Requirements for the materials and appliances

Natural soil should be uniform enough over the entire area requiring the liner to avoid unwanted load pressure changes and unexpected differences between hydraulic conductivity.

Monitoring / control needs

Selected basal structure should guide the selection of rehabilitation method for the waste area. Monitoring of groundwater and drainage water quality is recommended to ensure that the selected basal structure is adequate.

References

Hutchison I.P.G. & Ellison, R.D. (Eds) 1992. Mine Waste Management – A resource for mining industry professionals, regulators and consulting engineers. Lewis Publishers. 635 p.

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.

OhioEPA 2004. Constructing Recompacted Soil Liners and Soil Barrier Layers. Guidance Document #0692. November 3, 2004. State of Ohio Environmental Protection Agency. 2 p.http://www.epa.ohio.gov/portals/34/document/guidance/gd_692.pdf