Mine entrance sealing
Kaisa Turunen, Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, FINLAND, e-mail: kaisa.turunen(at)gtk.fi
Many of the AMD problems associated with old abandoned and closed underground mines derive from inadequate barrier construction between mines and hydraulic interconnection with surrounding watersheds and adjacent mine workings (Skousen et al. 1998). In underground mines, a mine entrance sealing is used as a closure method to prevent and attenuate mine drainage, and to minimise oxidation of disulphides. Sealing may also prevent emission of hazardous gases formed in abandoned mine workings. Moreover, mines are often sealed for safety reasons to prevent unauthorised entrance to hazardous open voids (Scott et al. 1975, Wolkersdorfer 2008). Mine entrance sealing results usually in mine inundation/flooding which is presented in Closedure Mine flooding part.
Description and performance of the technology
The anticipated hydraulic pressure determines the selection, design and construction of the mine seal. Moreover, since the layout of a mine (an adit or a shaft mine) affects on the hydraulic pressure, it also determines selection of the sealing type. The mine seals are categorised based on their construction and function (Scott et al. 1975, Wolkersdorfer 2008). Already in the 1920s and 1930s several studies conducted in USA showed that the discharge from the sealed mines tends to be of better quality than that from the unsealed mines. However, since all mine sealings have not always been successful and might even have adverse effects on adjacent waterways (e.g. Mynyd Parys mine site in Wales) only the ability to plug mine openings will not be comprehensive enough as a closure method. Thus there are several aspects what should be considered prior to sealing (Scott et al. 1975, Wolkersdorfer 2008):
- geology (fractures, faults, joints)
- hydrogeology (permeability of the strata, water table, flow paths, hydrodynamics of the site)
- hydrology (climatic variations, extreme storm events)
- geochemistry (soil, bedrock, water interactions)
- mining considerations (adit or shaft mine, connection with other mines, boreholes)
The main reason the seal to fail is the leakages around the seal, due to fissures or fractures in adjacent strata or due to inability to anchor the seal to the roof of the entrance. Grout curtains have been the most successful method to overcome these problems (Scott et al. 1975).
The cost of the sealing depends on the following aspects:
- seal type (the expected hydrostatic head, accessibility)
- size of the opening
- amount of the material needed
- grouting requirements
- amount of site preparation needed
If there is little or no danger to build up of a hydrostatic head, as above the groundwater table, dry seals are recommended. Dry seals prevent the access of oxygen and water into the mine, but are not constructed to withstand hydrostatic pressure. By limiting the water and air entry into the mine, also the oxidation of disulphides is hindered. The dry seal includes an impermeable material and/or structures to be placed in openings or slopes (Fig. 1). It could be constructed by blasting, caving mine portals or building a rock wall, which is then backfilled from the front side. The common materials used for dry seals are concrete, clay, masonry or concrete-fly ash mixture. Since the construction of a dry seal is rather simple and materials are cheap, the costs are fairly low. In addition, dry sealing is fairly long-lived, since the seal does not have to withstand high hydraulic pressure or chemical reactions due to water masses. The main problems with dry seals are fractures, fissures and cracks forming in the surrounding overburden or outcrop, through which the air and water may enter into the mine (Robins 1973, Scott et al. 1975, Skousen et al. 1998).
Figure 1. Possible dry sealings (After Scott et al. 1975, © GTK)
Air trap/wet seal
An air trap seal or a wet seal prevents the access of oxygen, but allows water flow through the seal (Scott et al. 1975), thus raising the water level and inundating the mine workings (Fig. 2). One of the wet seals on the discharge end is usually constructed with concrete blocks with holes or pipes inserted into the block to allow drainage. The main problem of the wet seal has been the clogging of the holes or pipes. As the water drainage decreases due to clogging, the head pressure of the impounded water increases, which may lead into collapse or leakage of the seal (Robins 1973, Skousen et al. 1998).
Figure 2. A) An air trap/wet seal. B) A partial air trap/wet seal (After Scott et al. 1975, © GTK)
Due to several failures of wet seals, hydraulic seals were invented. Hydraulic seal works as a watertight underground dam (Skousen et al. 1998). It includes placing a plug or a bulkhead to prevent water discharges as the mine is flooded (Fig. 3). The hydraulic seal excludes also the air from the mine and thus also the oxidation of disulphides (Scott et al. 1975). To avoid water movement in fissures and fractures around the seal, the adjacent strata is sealed further by pressure grouting. A hydraulic seal is built on the entrance where mine discharges flow from the mine and thus it prevents the water from draining into downstream watersheds (Wolkersdorfer 2008). Since the hydraulic seal generates a hydrostatic pressure as the mine floods, these seals should be designed to withstand pressure to prevent pressure blow-outs (Robins et al. 1975, Scott et al 1975). However, the seal should not be the only determining factor in inundating the workings and controlling the water pressure, but additionally underground barriers should be installed (Skousen et al. 1998). In addition, the seal should include an emergency discharge boreholes and a complete monitoring system. As the water table rises to its maximum level in workings, the emergency boreholes will fill by gravity and discharge thus preventing any unbearable water pressure increase. If the boreholes are constructed to withstand the maximum flow rates, it is fairly low cost and easily maintained system. Moreover, through boreholes monitoring is easily carried out (Scott et al. 1975). The hydraulic sealing usually results in mine inundation/flooding which is presented in Closedure Mine flooding part.
Figure 3. Double bulkhead hydraulic seal (After Scott et al. 1975, © GTK)
Scott et al. 1975 divided the hydraulic seals further into nine types based on the sealing technology: double or single bulkhead, permeable limestone, gunite seal, clay seal and grout bag systems. In the double bulkhead system, an impermeable seal is placed by injecting concrete or grout between two bulkheads. The bulkheads can be constructed from concrete and grouted coarse aggregate. These systems can be built also on inaccessible mine openings, since the material can be placed also via drill holes or pipelines. To prevent leakages around the seal contacts, grouting is recommended. The construction of the single bulkhead systems is similar to double bulkhead seals, but with only one bulkhead. The water tightness and the ability to withstand hydrostatic water pressure have been the main problem with single bulkhead systems. Thus, especially in high pressure areas the double bulkhead systems should be applied instead of single bulkhead systems. Permeable limestone systems are suitable in opening where acidic mine water may flow into the mine. The seal is constructed by placing alkaline aggregates in the mine opening. As the acidic water flows through the seal, it reacts with the alkaline material in the seal, is neutralised and the precipitates that are formed fill the open voids, thus decreasing the permeability of the seal. The main problem with the seal has been leakages through or around the seal, as the precipitates have not been able to either withstand the water pressure or to plug the pores in the seal. Moreover, the permeable aggregate seals can be built only on accessible mine openings unlike e.g. single and double bulkhead systems. The gunite (or shotcrete) seal systems are constructed by conveying the gunite layers through a hose and pneumatically projected in the mine opening until it is completely filled. For proper placement of the gunite layers, a wooden bulkhead is constructed on the inner side of the seal. The gunite systems tend to form a wedge shape seal which is able to withstand higher hydrostatic pressures. However, as with other seal types also gunite systems require fairly solid adjacent strata. Thus grouting may be required. Moreover, to ensure watertight conditions, the roof, walls and floor should be cleaned and shaped before placing the gunite. Clay seals are constructed by layering and compacting the material in openings. The opening should be first cleared of all debris to allow the clay to fill all the cracks and voids along the adjacent strata. To prevent the erosion of the seal, the seal should be backfilled with earth. Clay seals are suitable in areas with low hydraulic pressure. Grout bag systems are constructed by placing expandable grout containers in the mine opening. These seals are constructed gradually. As the cement slurry inside the first containers hardens enough, further rows are placed above the first ones until the opening is filled with the containers. Grout bag systems have failed due to leakages and poor bonding capacity between the bags and the mine opening surface (Scott et al. 1975).
Scott, L.R., Hays, R.M., Baker, M. 1975. Inactive and abandoned underground mines: water pollution prevention and control. Environmental Protection Agency (EPA). Office of Water Planning and Standards. pp. 338. EPA-440/9-75-007
Robins, J. 1973. Gas requirements to Pressurize Abandoned Deep mines. Environmental Protection Agency (EPA). Environmental Protection Technology Series. EPA-670/2-73-054
Skousen, J., Rose, A., Geidel, G., Foreman, J., Evans, R., Hellier, W. 1998. A Handbook of Technologies for Avoidance and Remediation of Acid Mine Drainage. National Land Reclamation Center at Western Virginia University. pp.131.W
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Fundamentals, Tracer Tests, Modelling, Water Treatment. Springer. 465 p.