Impermeable basal structure - with natural materials/soils
Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi
Natural soils can provide sufficient protection and be as reliable as comparable synthetic structures in a basal structure in a waste facility. Well-designed natural soil layers reduce the transportation of effluents from the waste area and decrease the concentration of constituent levels in the effluents through dilution and attenuation. However, synthetic layers are typically considered to be more suitable for non-inert mining waste than soil layers due to the alleged impermeable features of the synthetic liners.
Description of the technology
Soil layer can be used as a single layer or as a part of basal layer system. For example, a compacted clay liner can be the only liner or as part of a composite or double liner system. Compacted clay liners can be designed and used also as hydraulic barriers due to their low hydraulic conductivity. This requires effective quality control methods, efficient soil investigations and suitable construction practices. Increasing amount of fines (silt and clay) in soil usually decreases hydraulic conductivity. Common additive to amend hydraulic conductivity of soils is clay, e.g. sodium or calcium bentonite. Bentonite is blended with soil material especially when the soil layer does not contain sufficient percentage of clay. Soil layers (especially in-situ peat layers) have been used as a basal structure in several mine waste facilities in Finland (e.g. at the Hammaslahti and Luikonlahti mine sites). Fine-grained till is also widely used material in basal structures in mine waste facilities due to its slow hydraulic conductivity. EPA (2012) states that typical soil liner material contains at least 30% fines and can contain up to 50% gravel by weight. Soil layers used in basal structures of mine waste facilities have typically had minimum thickness of 0.5 m.
Soil layers are often placed in as parallel lifts or horizontal lifts when constructing a basal structure of waste disposal unit (see Figure 1 in Subsoil base). With the parallel lifts, liner material is compacted up and down the slopes and the bottom of the impoundment. With the horizontal construction, the liner material is compacted in a series of lifts on the inside slopes of the impoundment. Vertical approach opens up more pathways for seepage between lifts if adequate bonding is not present. Hydraulic conductivity can be measured in the horizontal (kh) and vertical (kv) 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. In liners constructed in parallel, vertical permeability controls the water flow and therefore conductivity (kv) should be less than 1×10-7 cm/s. Both components of permeability are critical for liners constructed in horizontal lifts and therefore both horizontal (kh) and vertical (kv) conductivity should be less than 1×10-7 cm/s (OhioEPA 2004).
Development stage, links to cases
Cases where basal structure of a mine waste facility is based on soil layer can be found in several old mine sites across the Finland, e.g. at Hammaslahti and Luikonlahti mine sites. Some of these basal structures are practically close to impermeable due to the compaction of the soil layers, e.g. peat and gyttja, underneath the waste.
- Construction and maintenance costs are usually smaller compared to the synthetic liners. If the facility area can utilize in-situ soils, costs are even smaller.
- Not as delicate in the construction stage as the more fragile synthetic liners.
- Often simple construction stage, no extra skills are needed by employees
Disadvantages and limitations
- Soils seldom have any significant tensile strength
- It may be difficult to compact a soil layer dense enough to create an impermeable layer
- There should not be any interaction between the disposed waste and the underlying soil layer
- In-situ soil layers are usually heterogeneous (particle size varies from area to area) and include cracks and root holes. This can be improved by spreading an extra soil layer on top of the in-situ soil layer.
- Hydraulic conductivity can change due to the dissolution of soil minerals when in contact with acidic/basic solutions, or to the changes in structures of clay minerals.
- Channels and cracks can lead to an increased hydraulic conductivity in long-term, even pipes and streams can occur. Therefore, it is important to characterize the waste and predict also the possible long-term impacts causing interaction between the liner and waste particles.
Properties and capacity
When choosing a suitable soil layer or a combination of materials for a basal structure there are many things to consider:
- Selected soil material has to be suitable (in quality and quantity), has sufficient thickness and it should contain enough fines or peat (e.g. in-situ peat layer) to decrease the hydraulic conductivity.
- Preparation and compaction of foundation material have to reach required bearing strength which depends on the estimated waste amount (weight)
- Soil layers have to be properly placed and compacted
- The completed structure needs to be properly protected before, during and after construction to avoid any degradation.
Only monitoring is required. No further costs, if the structure works as designed. It is very expensive to remove waste from the facility, store it temporarily, repair the liner and restore the waste back to the facility or relocate the waste material into another, newly designed facility.
Environmental cost aspects
In-situ soil layers (when suitable for the purpose and sufficient in depth) or suitable soil materials available at the site are typically more cost-efficient to use than the transported soils or synthetic liners.
Site specific data needs
Basic soil data about soil types, stratigraphy of the Quaternary deposits and hydrology of soil layers is typically collected already in the baseline study of the mine site. Based on this information, suitable locations for waste disposal areas for different waste types can be located for further investigations. More detailed soil inventory surveys of the soil base on the designed waste disposal site should include inventory surveys and technical investigations of the suitability of the soil. Supplementary surveys include for example stability of foundation soils, quantity of waste fractions and the compatibility of the waste with native soils, waste characterizations, hydraulic conductivity calculations, extent estimations of the disposal area, the potential to recompact existing soils to make sure the proper and suitable basal and dam structures can be constructed. Suitable soil investigation methods include research trenches and drillings or geophysical measurements such as ground-penetrating radar and seismic measurements (EC 2009, EPA 2012, Sivonen & Frilander 2001).
Requirements for the materials and appliances
According to the guidebook of the Best environmental practices in metal ore mining (Kauppila et al. 2013) examples of the investigations for the waste disposal areas include:
- thickness of overburden,
- depth and topography of bedrock surface,
- stratigraphy of the quaternary deposits (changes in soil type and the thickness of the various soil type layers),
- height of the groundwater table measured from the surface of the ground (+ estimate of seasonal changes in heights) and groundwater flow directions,
- catchment boundaries (watersheds) for surface water and groundwater, and
- topography of ground surface and natural flow direction for surface water.
The quality and quantity of the needed material is equally important when protecting the surrounded nature. The soil characterizations and investigations should include all the main properties of soil and designed soil layers, including:
- particle size and size distribution (content of fines <0.06 and clay 0.002 mm etc.),
- hydraulic conductivity/permeability (vertical and horizontal fluctuation throughout the base soil of the waste area),
- water content,
- plasticity and compaction characteristics,
- vapour transmission,
- bearing capacity/subsidence properties (including shear strength/consolidation measurements)
- impact resistance, and
- cracking properties and frost characteristics (especially if consist of clay material).
These two lists of properties provide the tools to assess the suitability of the site and the materials. Soil layers can require also pre-processing, which can include homogenization of the soils, water content adjustment, removal of oversized particles, powdering of lumps, and addition of fines (e.g. bentonite). Construction quality assurance and quality control of compacted soil layers should be implemented in a responsible manner by the manager of the waste facility. Preparation and compaction of foundation material should also have the required bearing strength. During construction, care should be taken to place and compact the chosen material properly. In addition, the compacted, completed structure/in-situ layer should be properly protected before, during and after construction to avoid any damages.
Minimisation / treatment of potential discharges
Especially the clay materials have to be protected against freezing and desiccation in the construction stage. Clays shrink when they dry and this can cause cracks deteriorating impermeability of the liner. It is recommended to conduct a small-scale testing pad after the design, in which the soil, equipments, and construction methods are verified.
If the extractive waste is acid generating and/or contains potentially soluble harmful substances, then the basal structure should (Kauppila et al. 2013):
- promote the stability of waste disposal
- minimize the environmental impacts of the disposal,
- slow down the chemical alteration of the waste,
- prevent oxygen-rich groundwater from accessing the waste,
- guide the selection of rehabilitation method for the waste area.
Monitoring / control needs
The performance of the basal structure is controlled using monitoring.
EPA 2012. Guide for Industrial Waste Management. Protecting Land, Ground Water, Surface Water, Air. Building Partnerships. United States Environmental Protection Agency. http://www.epa.gov/osw/nonhaz/industrial/guide/. Visited in 26.6.2014
EC 2009. Reference document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009. European Commission (EC). 511 p. http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.pdf. Visited in 26.6.2014.
Kauppila, P., Räisänen, M.L. & Myllyoja, S. (Eds) 2013. Best environmental practices in metal mining operations. The Finnish Environment 29en/2011, Environmental Protection, Finnish Environment Institute (SYKE). The publication in English is available only on the internet: www.syke.fi/publications (available also in Finnish). 219 p.
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OhioEPA 2004. Construction Recompacted Soil Liners and Soil Barrier Layers. Guidance Document #0692. November 3, 2004. State of Ohio Environmental Protection Agency. 2 p.
Rantamäki, M., Jääskeläinen, R. & Tammirinne, M. 1979. Geotekniikka. Otatieto 464. Oy Yliopistokustannus/Otatieto. Helsinki. 307 p.
Sivonen, M & Frilander, R. 2001. Patoturvallisuuden toteutuminen Suomen jäte- ja kaivospadoilla. Suomen ympäristökeskus. Suomen ympäristö 462, ympäristönsuojelu. Helsinki. 123 p.