Disposal in underground mine under the water level (during mine closure)
Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi
Underground backfilling is a method where mine waste or waste plus additives is used to fill void openings created by underground mining. Nowadays backfilling is usually conducted throughout mine operations, but it may also be implemented for the remediation of abandoned mine sites. Backfill material can be waste rock, tailings or water treatment sludge, and may include additives such as cement or other modifiers to increase strength and chemical stability. During mine closure backfilling is conducted to increase the mine workings’ structural stability and to decrease the environmental impacts of the waste by limiting oxidation. This is an especially relevant consideration if the waste is sulphur rich, as underground disposal can efficiently limit acid generation and metals release (Masniyom 2009). While backfilling waste supports mine cavities and reduces the surface area required for the waste impoundments, it may not entirely eliminate subsidence. If the waste material has been exposed to atmospheric oxygen (or dissolved oxygen in the vadose zone) and rainwater, it may undergo sulphide oxidation and trigger acid mine drainage (AMD).
Underground backfill disposal can be an advantageous method for minimizing environmental impacts of mining, especially when mine waste materials are a potential source of acid mine drainage (AMD), when the mine site is located in an ecologically sensitive area, and when mine site topography makes surface disposal difficult. This method is especially efficient when disposal in mining cavities is below the water table, as it provides a water cover for the waste material. Underwater disposal reduces the air transfer into the waste material and thereby limits the supply of the oxygen to sulphide minerals. Because of the protection provided by the saturated zone, the oxidation of sulphides almost completely stops and the generation of AMD is reduced (Lottermoser 2007).
To minimize the time the sulphides are exposed to oxidation, the backfilling process should be carried out rapidly after ore processing. Oxidation should be avoided both in the waste material and on the walls of the mine cavities. If the tailings are not saturated with water, the oxidation front in the tailings will gradually move down towards the groundwater table. In the past only the coarsest part of the tailings material was used together with a small amount of hardening agent (cement, blast furnace slag, fly ash) for backfilling. New paste backfill technologies make backfilling possible to use at almost all mines, dependant only upon profitability (INAP 2009, EC 2009).
Other Closedure pages related to the use of backfill method include:
- Underwater disposal in underground mines (during mine operation)
- Subaqueous in-pit disposal
- Disposal in underground mine (during mine closure)
- Backfilling in water management
Description of the method
For the backfill technique waste material is placed in underground voids created by the extraction of ore. This disposal method has several benefits that are especially apparent when evaluating the environmental impacts of waste materials across different disposal techniques. This method is a good option when disposal occurs below the water table, where the material will eventually become saturated. Underground backfilling systems can be cyclic, where the fill is placed in successive lifts (as in cut-and-fill mining sequences), or can be conducted by delayed filling, where the entire stope is filled in one operation.
The common backfill methods include a) hydraulic fill, which relies on water to transport slurry through pipelines, b) paste backfill, in which backfill is emplaced as a high density mixture of solids, or c) rock backfill (e.g. stone, gravel, soil), which relies on mechanical methods such as conveyors, mobile equipment or gravity to emplace rock-sized aggregates. The rock backfill method requires material processing using mechanical forces such as crushing, sieving and mixing. Paste backfilling requires producing a tailings-cement paste mixture before final underground disposal. If the cement used in the paste mixture technique contains neutralizing minerals, it may also provide limited acid buffering capacity to the waste (Lottermoser 2007).
Backfill methods used at a particular mine vary considerably from mine to mine and depend on several factors such as environmental concerns, availability of materials, the mining method, and backfill schedule. It can be difficult to fill all the space between the backfill body and the roof of the voids due to slurry volume loss during dewatering. Subsequently, (multiple backfilling procedures can be required at one site. Because different backfill methods use different portions of the mine waste, the utilization efficiency of tailings may be low, and result in large quantities of waste material which cannot be disposed of underground.
The quality of groundwater in an underground mine depends on the composition and reactivity of the wall rocks, fractures, and waste material, as well as contact time. The primary source of groundwater in deep mines is the regional groundwater system, and in shallow mines the local groundwater system. As the water table rebounds after the filling of the underground workings, sulphide oxidation products present on the mine walls and backfilled waste are flushed, resulting in a release of sulphur compounds, metals, and acidity to the mine water. These releases may be partially buffered by alkalinity from carbonates and other minerals in the mine waste. After the initial flushing of oxidation products, if the groundwater is not oxygen rich, the inundation of sulphide minerals will prevent further sulphide oxidation. After backfilling the groundwater will eventually return to approximately the level it was before mining. However, water levels in underground mines can show considerable fluctuation due to variability in meteoric conditions and local groundwater pumping (INAP 2009). While permanent saturation reduces or eliminates sulphide oxidation and ARD, saturated backfill may not have the desired strength and stiffness for ground control requirements. As a result, in some cases cementing admixtures may be a more appropriate backfill solution for reducing emissions than the use of water cover.
Tailings deposition below the water table provides an ideal oxygen transport barrier and reduces or prevents acid generating reactions in sulphides. Even so, groundwater outflow from the backfilled cavities may require treatment before discharge into the environment. At a minimum, groundwater should be monitored during mine closure when oxygen and water levels are balancing (INAP 2009). If tailings material has a very high acid-generating capacity, it may require the addition of neutralizing agents prior to, or during backfilling (Lottermoser 2007). Groundwater quality must be considered throughout mining and backfill operations, as it can also be impacted by chemicals introduced during mining activities (e.g., diesel, nitrogen from blasting residuals, grout, lime dusting) (INAP 2009).
Development stage, links to cases
Paste filling by mixing cement with tailings is the most common modern backfilling method. However, cemented rock fill is still the preferred system of some operators. New promising methods used in underground backfill limit ARD and metal leaching by reducing waste permeability, and increase system alkalinity (Moran et al. 2013). Backfilling is also becoming more desirable, as mines progress deeper underground and ground stabilization is essential.
At the following mine sites, underwater disposal of mine wastes in underground mines has been used as part of remediation after mine closure:
- Copper basin, Polk County, Tennessee, USA.
- Tar Creek Superfund Site, Ottawa County, Oklahoma, USA
- McNeely Green Reclamation Tar Creek Superfund Site, Ottawa County, Oklahoma, USA
The backfill technique is suitable for the disposal of both waste rock and tailings. Extra sealing structures are needed at sites where oxygen-rich groundwater can enter mine voids through rock fractures.
- Reduces waste storage and disposal rehabilitation (revegetation or landscaping), and surface area requirements
- Subaqueous placement below the ground water table limits waste interactions with the hydrosphere and biosphere
- Better protection against water and wind erosion compared to above ground disposal, so it is less susceptible to erosion or leaching
- Non-existent or less air pollution or dust problems (less health problems for local community)
- Oxidation problems are unlikely as well as ARD when conditions are anaerobic
- Waste storage better concealed, reducing sulphur oxidation and increasing radiological safety of the tailings with radionuclides
- Mechanical failure is unlikely, with no major risk to public compared to the tailings impoundments with dam structures (no tailings dams failures or erosion)
- Waste material is sheltered from effects of seasonal weather and long term climatic variability
- Underground backfilling reduces the potential for subsidence providing better regional support, and hanging wall stability. Additionally, it enables pillar recovery, and increased extraction opportunities for future demand
- Often decreases the potential for surface water pollution
- Usually easy to implement
- Permanent waste storage with low maintenance. (Lottermoser 2007, INAP 2009, MEND 2009)
- Site specific method
- Storage capacity finite although should be adequate for the purpose
- Water cannot be treated easily, nor reclaimed due to difficult accessibility
- Pollution control needs good planning, proper waste characterisation, and deep monitor wells
- Requires a lot of underground data and modelling such as groundwater flow directions, fault and fracture investigations
- Damaged rock zones in underground workings can cause preferential pathways for mine water to mix with groundwater resulting in the need for groundwater treatment
- In the event that future demand rises, waste is more difficult to access than in impoundments above the surface
- Potentially soluble radionuclides in mine waste have to be taken into account during backfill method design to avoid groundwater contamination
- Unoxidized material in saturated impoundment has potential to oxidize as the result of fluctuations in water table (i.e. the water table falls). Potential leaching of metals due to water-rock reactions and subsequent mobilization of contaminants into groundwater and potentially surface water
- Cement in the slurry can be flushed away by water during the dewatering process, which can have a negative environmental impact and decrease the strength of the backfill body
- Prevents/complicates the extraction of the potential remaining ore in the ore body. (Lottermoser 2007, INAP 2009, MEND 2009)
Usually it is not possible to dispose of all extractive waste as backfill, as waste material volume generally increases after the blasting, excavation, and mill processing. The quantity of material used for backfill can be calculated by taking into account the capacity of the cavities, weight and density of the material, water requirements, etc.
After mine closure there are no maintenance needs beyond quality control monitoring, which should be conducted until mine voids are filled with water and a water balance is achieved. Groundwater quality and flow and geotechnical conditions should be monitored. Common monitoring instruments used during backfill operations include flow meters, piezometers, extensometers, strain cells, pressure cells and load cells.
Environmental cost aspects
Backfilling is an attractive method from an economic and environmental standpoint and is justifiable when it reduces risk to the environment and humans (both employees and local inhabitants) without greatly affecting the profitability of the operation. Backfilling reduces the potential for subsidence, increases underground safety and eliminates the risk of dam failures. Therefore it is advisable to conduct backfilling throughout the course of mine operations.
In some cases, environmental considerations may significantly narrow the range of waste disposal options, and the application of backfilling techniques may be increasing in response to environmental concern and rehabilitation costs. Disposal costs can also vary significantly depending on the future planned use of the waste area/mine site. If the ore is transported a great distance from the underground mine to the mill, it might not be feasible to return the mine waste back to the voids. The cost of backfill typically represent between 10-20% of the mine’s expenses, of which the cost of cement represents up to 75%. The capital cost of a paste fill plant is approximately twice the cost of a conventional hydraulic fill plant of the same capacity. However, paste fill remains a popular option because it uses unclassified tailings and less water than the conventional method (Fernberg 2007).
Capital and operating costs include the following considerations (Masniyoum 2009):
- Modifications of temporary waste area before and after backfill process
- Re-opening cost, earth moving etc.
- Pipelines and drop holes,
- Conveyors and sumps
- Monitoring instrumentation
- Backfill preparation, additives
- Drainage systems and pumping
- Pumping costs if the underground voids are already flooded
- Possible water treatment due to pumping
- Backfill barricades and retaining bulkheads
Site specific data needs
Backfilling is possible only at mine sites where underground mining is the dominant method. When considering backfill disposal methods the following factors should be investigated:
- Geological conditions (dimensions of the orebody and its dip, host rock, fractures and other physical and mechanical properties, groundwater levels and groundwater quality)
- Mine capacity (mining method, operations schedules, void space)
- Backfill material specification/characterization (sludge properties like viscosity)
- Suitable backfilling techniques available (site availability, access, transportation, placement, material preparation)
- Historical experience based on similar cases at the same mining conditions
The results of the field exploration and laboratory test programs should provide adequate information on the engineering properties and compaction characteristics of the materials available. Backfill material should go through a complete inventory, including characterization of physical and drainage properties, geochemical composition, and potential interaction of different materials. Cost can be a prohibitive factor in the consideration of backfilling as a mine waste disposal method. The technology is otherwise relatively easy to implement regardless of the geometry and the depth of the mineral deposit and the mining method utilized.
Requirements for the materials and appliances
Both tailings and waste rock can be used as a backfill material for structural support in underground mines. Backfilling material and method must be in accordance with local environmental requirements and permits. Subsequently, the implementation of backfilling may be prohibited on the basis of national or regional regulatory concerns (such as groundwater contamination). Overall, backfilling material has to fulfil hydraulic, mechanical and environmental requirements, i.e. relevant physical and chemical characterizations should be made. Important factors to consider during backfill material characterization include:
- Particle shape and size distribution
- Water content and saturation
- Weight, density and specific gravity
- Void ratio and porosity (air content)
- Particle friction and cohesion
- Permeability and percolation rate
- Effect of fines and cement
- Reducing bacteria which prevents oxidation
These properties effect consolidation, strength, compaction, stiffness, ground support and liquefaction characteristics of backfill material. Fines content, cement content and slurry density are essential factors in backfill stability and economic viability (Masniyom 2009). Some additional processes to enable the use of extractive waste as backfill material include: dewatering, desulphurization, and effluent treatment (MEND 2009).
Backfilled material can be cemented, depending on mining methods and the required outcome. Common backfill binders are Portland and slag cement, fly ash, filter dust, gypsum, infertile overburden, residues from mineral processing, natural pozzolans and waste glass. Other additives that may be included in the backfill include accelerators, retarders, flocculants and dispersants. Binder additives can be combined to provide the safest and most cost-effective backfill mixtures. Higher backfill strengths are achieved by adding more binders. However, increased additive content also increases the cost of implementation. Water is required for the transport of backfill materials into void space and for hydration of binders. Waste rock or tailings must be sized to meet the transport requirements. The maximum aggregate size for transportation by pipeline is less than 1/4 of the diameter of the pipe. In hydraulic transportation, this usually means aggregate sizes less than 60 mm, while aggregates up to 30 cm can be transported by conveyor or truck. The pressure gradient for pipeline transportation of the paste backfill has to be high (Masniyom 2009).
Minimisation / treatment of potential discharges
During underground disposal there is always concern that contaminants may seep from the waste materials to the surrounding groundwater system. Additionally, sulphide oxidation and reactions between aluminium and calcium hydroxide components within the backfill material must be fully understood. These processes influence strength development in the emplaced backfill, or can cause expansion in the cement fraction and subsequent breakup of the backfill. Comprehensive geochemical characterization of the waste material is crucial for the evaluation of backfill design and disposal alternatives, as well as preliminary test work. Material testing has an important role in confirming the suitability of the available combinations for a particular process. Test work can include, for example, large-scale field cells. This kind of test is of great value for determining and predicting the long-term environmental stability of waste materials (Masniyom 2009). An important factor in backfilling is waste compaction. Compacted waste material will have smaller surface area and lower reaction capacity which should decrease leachate of various components to the groundwater. Possible environmental impacts of effluents can be reduced if the waste material is lined with an impermeable soil layer that restricts the water flow into the waste material and decreases the leaching of harmful substances from the waste (e.g. Lottermoser 2007).
Material in tailings is much finer grained than waste rock and has a much higher specific surface area available for oxidation and leaching reactions. High concentrations of metals and metalloids (e.g. As, Cd, Ni, Zn) in disposed backfill may pose risks to the surrounding environment. Impacts from such contaminants depend mostly on ongoing weathering processes (related to water and oxygen exposure), the alkalinity and sulphur content of the backfill material and the acidity of local water sources. Therefore, the proposed backfill material must be characterized during the design stage to assess long-term impacts. Backfilling is not recommended if oxidised groundwater has access to the waste via rock fractures. Groundwater contamination can be reduced by sealing the fractures in the rock walls or by soil filter fillings (sulphide free paste/soil/rock powder).
Short-term impacts to the benthic and aquatic communities need to be assessed on a site-by-site basis. Underground disposal may result in a hydraulic barrier to groundwater flow, thus limiting interaction between backfill and groundwater. Anaerobic conditions can also be maintained or increased by manipulating aquifers to promote bioremediation with various substrate additives such as heterotrophic bacteria (ITRC 2015).
Poorly designed and maintained backfill can be a serious disadvantage to the mine and jeopardise its safety. There has been a trend of increasing environmental concern over the last decade, which is unlikely to end. It is noteworthy that backfill disposal technology may utilize co-disposal concepts to modify backfill geochemistry to provide environmental benefits as well. For example, blending, layering and encapsulation of acid generating tailings in appropriately designed backfill applications may provide significant benefits in mine waste management and environmental impact control.
Monitoring / control needs
Usually the groundwater table rebounds close to the pre-mining level after mine closure. Groundwater levels and quality need to be monitored after mine closure. Weathering of the disposed material can also cause long-term (ten to hundreds of years) impacts on water quality. Controlling contaminant loading to surface and groundwater may require water treatment and water quality monitoring for long periods of time after the mine has been closed. The use of underground mines as final disposal requires detailed site and waste characterization to ensure there is no long term risk to human, animal, or plant life. Despite the need for ongoing monitoring of backfilled sites, monitoring requirements are generally less extensive than at surface waste disposal sites due to greater stabilisation of the water levels and restriction of oxygen access. It is also important to remember that any backfill material placed below the water table becomes part of the aquifer. Therefore backfill sites require groundwater monitoring systems (Lottermoser 2007). Monitoring and controls are needed during the disposal process to ensure design expectations are being met, even when the backfilling is executed as part of mine closure. The physical and chemical stability of the backfill material can vary with time.
Fernberg, H. 2007. Backfilling for safety and profit. In: Mining methods in Underground mining. Atlas Copco Rock Drills AB. 2nd ed. 43-45 p.
INAP 2009. The GARDGuide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/ modified 2014. Read 17.7.2014.
ITRC 2015. Mining Waste Treatment Technology Selection. Web-based Technical and Regulatory Guidance, Technology Overview, Backfilling and Subaqueous Disposal. http://www.itrcweb.org/miningwaste-guidance/to_backfilling.htm. Read 9.1.2015
Lottermoser, B.G. 2007. Mine wastes – Characterization, Treatment, Environmental Impacts 2nd ed. Springer. 304 p.
Masniyom, M. 2009. Systematic Selection and Application of Backfill in Underground Mines. Der Fakultät für Geowissenschaften, Geotechnik und Bergbau der Technischen Universität Bergakademie Freiberg genehmigte. Dissertation zur Erlangung des akademischen Grades Doctor-Ingenieur. 168 p.
Moran, P., Christoffersen, L., Gillow, J. & Hay, M. 2013. Cemented tailings backfill – It’s better, now prove it! SME Annual Meeting Feb. 24 – 27, 2013, Denver, CO. 4 p.
Price, W.M. (ed) 2009. Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. MEND Report 1.20.1, CANMET – Mining and Mineral Sciences Laboratories