Mining subsidence prevention has not been systematically addressed in Estonia. Despite the presence of numerous shallow and hazardous old mines, including some in urban areas, there are only a few practical examples of land stabilization. This article provides an overview of stabilization methods commonly used elsewhere. By considering the specific characteristics of shallow Estonian mines and existing infrastructure, we propose injectable backfilling through treatment boreholes as the most applicable land stabilization method in Estonia. The backfill typically consists of locally available byproducts or waste materials. We evaluate how Estonian oil shale ash, a residue from power and oil production, compares to the properties required for effective backfilling. Some properties, such as self-cementation, make it suitable for use in backfills. However, it also exhibits less desirable features – high water demand and fast setting. Despite extensive research on oil shale ash, certain material properties that critically impact its usability in backfills – such as its pumpability over longer distances – remain to be fully determined.
1. Karu, V., Valgma, I., Kolats, M. Mine water as a potential source of energy from underground mined areas in Estonian oil shale deposit. Oil Shale, 2013, 30(2S), 336–362.
https://doi.org/10.3176/oil.2013.2S.12
2. Vassiljev, J., Aarniste, M., Rebane, K., Urtson, K. Põlevkivi altkaevandatud alade varingute uuring. Lõpparuanne. Tallinn, 2018.
https://fond.egt.ee/fond/egf/8911
3. Reinsalu, E., Valgma, I. Geotechnical processes in closed oil shale mines. Oil Shale, 2003, 20(3S), 398–403.
https://doi.org/10.3176/oil.2003.3S.14
4. Karu, V. Stability problems in undermined areas. In: 8th International Symposium “Topical Problems in the Field of Electrical and Power Engineering. Doctoral School of Energy and Geotechnology II”, January 11–16, 2010, Pärnu, Estonia (Lahtmets, R., ed.). Elektriajam, 2010, 134–137.
5. Väizene, V., Valgma, I. Methods for digitizing mine plans and data of mined out areas. In: Closing Conference of the Project “Doctoral School of Energy and Geotechnology II”, January 12–17, 2015, Pärnu, Estonia (Zakis, J., ed.). Tallinn University of Technology, 241–244.
6. Whaite, R. H., Allen, A. S. Pumped-Slurry Backfilling of Inaccessible Mine Workings for Subsidence Control. Vol. 8667. United States Bureau of Mines, 1975.
7. Roy, R., Chakraborty, S., Bisai, R., Pal, S. K., Mishra, S. Gravity blind back-filling of abandoned underground mine voids using suitable mix proportion of fill materials and method of filling. Geotech. Geol. Eng., 2023, 41, 1801–1819.
https://doi.org/10.1007/s10706-022-02371-8
8. Kawalec, J., Grygierek, M., Koda, E., Osiński, P. Lessons learned on geo-synthetics applications in road structures in Silesia mining region in Poland. Appl. Sci., 2019, 9(6), 1122.
https://doi.org/10.3390/app9061122
9. Gray, R. E. Subsidence over abandoned mines: US experience. In: Surface Subsidence Engineering: Theory and Practice (Peng, S. S., ed.). CSIRO Publishing, Clayton, Victoria, 2020, 131–160.
10. Watson, P. D., Dennehy, J. P., O’Neill-Gwilliams, S., Parry, D. N., Fraser, I. W. Ground treatment and remediation of workings. In: Abandoned Mine Workings Manual (Parry, D. N. and Chiverrell, C. P., eds). Vol. C758D. CIRIA, London, 2019, 354–414.
11. Sevastjanova, A. Научно-технический отчет по теме «Разработать техно-логии заполнения выработаванного пространства сланцевой шахты». Silbet, Estonia, 1988. https://fond.egt.ee/fond/egf/4466
12. Valgma, I., Kattel, T. Low depth mining in Estonian oil shale deposit – Abbau von Ölschiefer in Estland. In: Kolloquium Schacht, Strecke und Tunnel 2005, April 14–15, 2005, Freiberg, Germany. TU Bergakademie, Freiberg, 2005, 213–223.
13. Valgma, I., Kolats, M., Karu, V. Streki toestamine põlevkiviaherainebetooniga. In: Maapõue kasutamise arengud (Västrik, A., Reinsalu, E., Vesiloo, P., Pastarus, J. R., Kõpp, V., Sein, O., Soosalu, H., Viilup, H., Tohver, T., eds). Tallinna Tehnikaülikooli Kirjastus, Tallinn, 2010, 33–38.
14. Walker, J. S. State-of-the-Art Techniques for Backfilling Abandoned Mine Voids. Information circular/1993. Vol. 9359. United States Department of the Interior, Bureau of Mines, 1993.
15. Brook, D. Reclamation of abandoned underground mines in the United Kingdom. In: International Land Reclamation and Mine Drainage Conference and Third International Conference on the Abatement of Acidic Drainage: Abandoned Mine Lands and Topical Issues. Vol. 4. United States Department of the Interior, Bureau of Mines, 1994, 163.
https://doi.org/10.21000/JASMR94040163
16. Zięba, M., Kalisz, P., Grygierek, M. The impact of mining deformations on road pavements reinforced with geosynthetics. Arch. Min. Sci., 2020, 65(4), 751–767.
https://doi.org/10.24425/ams.2020.134145
17. Workman, J. L., Thompson, J. Blasting for Abandoned-Mine Land Reclamation (Closure of Individual Subsidence Features and Erratic, Undocumented Underground Coal-Mine Workings). Final Report. Calder and Workman, Inc., Washburn, USA, 1991.
https://www.osti.gov/biblio/5788957
18. Potvin, Y., Thomas, E., Fourie, A. Handbook on Mine Fill. Australian Centre for Geomechanics, 2005.
19. Moser, A. K. State of the Art of Backfill Technology in Underground Mining Excavations. Master’s thesis. Montanuniversität Leoben, Austria, 2015.
20. Sivakugan, N., Veenstra, R., Naguleswaran, N. Underground mine backfilling in Australia using paste fills and hydraulic fills. Int. J. Geosynth. Ground Eng., 2015, 1, 18.
https://doi.org/10.1007/s40891-015-0020-8
21. UK Quality Ash Association. Technical Data Sheet 3. Pulverised Fuel Ash for Grouting, 2006.
22. Giacinto, J. F., Petzrick, P., Rafalko, L. G. Cost optimization for mine void stabilization projects: a deep mine case study. In: 2007 World of Coal Ash (WOCA), May 7–10, 2007, Covington, USA, 13.
23. Johnson, D. Cementitious grouts – standards update 1999. In: Specialist Techniques and Materials for Concrete Construction: Proceedings of the International Conference Held at the University of Dundee, September 8–10, 1999, Scotland, UK (Dhir, R. K., Henderson, N. A., eds). Thomas Telford, Dundee, 1999, 41–48.
https://doi.org/10.1680/stamfcc.28258.0004
24. Scott, J. T. Sustainable alternatives to PFA in Scotland. In: Procedia Environmental Science, Engineering and Management, 4, 2017, 301–307.
25. Colaizzi, G. J., Whaite, R. H., Donner, D. L. Pumped-Slurry Backfilling of Abandoned Coal Mine Workings for Subsidence Control at Rock Springs, Wyo. Information Circular 8846, United States Department of the Interior, Bureau of Mines, 1981.
26. Stone, D. The evolution of paste for backfill. In: Mine Fill 2014. Proceedings of the Eleventh International Symposium on Mining with Backfill. Australian Centre for Geomechanics, Perth, 2014, 31–38.
https://doi.org/10.36487/ACG_rep/1404_0.3_Stone
27. Udd, J. E. Backfill research in Canadian mines. In: Innovations in Mining Backfill Technology (Hassani, F. P., Scoble, M. J., Yu, T. R., eds). CRC Press, Boca Rotan, Florida, 1989.
https://doi.org/10.4095/325861
28. Behera, S. K., Mishra, D. P., Singh, P., Mishra, K., Mandal, S. K., Ghosh, C. N., Kumar, R., Mandal, P. K. Utilization of mill tailings, fly ash and slag as mine paste backfill material: review and future perspective. Constr. Build. Mater., 2021, 309, 125120.
https://doi.org/10.1016/j.conbuildmat.2021.125120
29. Sheshpari, M. A review of underground mine backfilling methods with emphasis on cemented paste backfill. Electron. J. Geotech. Eng., 2015, 20(13), 5183–5208.
30. Shen, B., Poulsen, B., Luo, X., Qin, J., Thiruvenkatachari, R., Duan, Y. Remediation and monitoring of abandoned mines. Int. J. Min. Sci. Technol., 2017, 27(5), 803–811.
https://doi.org/10.1016/j.ijmst.2017.07.026
31. Mahmadoliev, A. K., Makhmudov, D. R., Nurboboev, Y. T., Pardaev, F. S. Reducing the cost of backfill in the Kauldi gold mine. IOP Conf. Ser.: Earth Environ. Sci., 2020, 614, 012058.
https://doi.org/10.1088/1755-1315/614/1/012058
32. Belem, T., Benzaazoua, M. Design and application of underground mine paste backfill technology. Geotech. Geol. Eng., 2008, 26(2), 147–174.
https://doi.org/10.1007/s10706-007-9154-3
33. BRE Building Technology Group. Stabilising Mine Workings with PFA Grouts: Environmental Code of Practice (BR 509). BREPress, 2009.
34. Choudhary, B. S., Kumar, S. Underground void filling by cemented mill tailings. Int. J. Min. Sci. Technol., 2013, 23(6), 893–900.
https://doi.org/10.1016/j.ijmst.2013.11.003
35. De Souza, E., Archibald, J. F., Dirige, A. P. Economics and perspectives of underground backfill practices in Canadian mines. In: Proceedings of the 105th Annual General Meeting of CIM. Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, 2003.
36. Yin, S., Yan, Z., Chen, X., Wang, L. Effect of fly-ash as fine aggregate on the workability and mechanical properties of cemented paste backfill. Case Stud. Constr. Mater., 2022, 16, e01039.
https://doi.org/10.1016/j.cscm.2022.e01039
37. Anastasiou, E. K. Effect of high calcium fly ash, ladle furnace slag, and limestone filler on packing density, consistency, and strength of cement pastes. Materials, 2021, 14(2), 301.
https://doi.org/10.3390/ma14020301
38. Nalbantoğlu, Z. Effectiveness of Class C fly ash as an expansive soil stabilizer. Constr. Build. Mater., 2004, 18(6), 377–381.
https://doi.org/10.1016/j.conbuildmat.2004.03.011
39. Mohan, S., Gandhimathi, R. Removal of heavy metal ions from municipal solid waste leachate using coal fly ash as an adsorbent. J. Hazard. Mater., 2009, 169(1–3), 351–359.
https://doi.org/10.1016/j.jhazmat.2009.03.104
40. Uysal, M., Akyuncu, V. Durability performance of concrete incorporating Class F and Class C fly ashes. Constr. Build. Mater., 2012, 34, 170–178.
https://doi.org/10.1016/j.conbuildmat.2012.02.075
41. McCarthy, M. J., Dhir, R. K. Towards maximising the use of fly ash as a binder. Fuel, 1999, 78(2), 121–132.
https://doi.org/10.1016/S0016-2361(98)00151-3
42. Moreno, N., Querol, X., Andrés, J. M., Stanton, K., Towler, M., Nugteren, H., Janssen-Jurkovicová, M., Jones, R. Physico-chemical characteristics of European pulverized coal combustion fly ashes. Fuel, 2005, 84(11), 1351–1363.
https://doi.org/10.1016/j.fuel.2004.06.038
43. Mohebbi, M., Rajabipour, F., Scheetz, B. E. Evaluation of two-atmosphere thermo-gravimetric analysis for determining the unburned carbon content in fly ash. Adv. Civ. Eng. Mater., 2017, 6(1), 258–279.
http://dx.doi.org/10.1520/ACEM20160052
44. Cao, D., Selic, E., Herbell, J.-D. Utilization of fly ash from coal-fired power plants in China. J. Zhejiang Univ. Sci. A, 2008, 9(5), 681–687.
https://doi.org/10.1631/jzus.A072163
45. EN 196-3:2016 – Methods of testing cement – Part 3: Determination of setting times and soundness, 2016.
46. EN 13395-2:2002 – Products and systems for the protection and repair of concrete structures – Test methods – Determination of workability – Part 2: Test for flow of grout or mortar, 2002.
47. EN 12350-2:2019 – Testing fresh concrete – Part 2: Slump test, 2019.
48. Clayton, S., Grice, T. G., Boger, D. V. Analysis of the slump test for on-site yield stress measurement of mineral suspensions. Int. J. Miner. Process., 2003, 70(1–4), 3–21.
https://doi.org/10.1016/S0301-7516(02)00148-5
49. Uibu, M., Tamm, K., Viires, R., Reinik, J., Somelar, P., Raado, L.-M., Hain, T., Kuusik, R., Trikkel, A. The composition and properties of ash in the context of the modernisation of oil shale industry. Oil Shale, 2021, 38(2), 155–176.
https://doi.org/10.3176/oil.2021.2.04
50. Rice, G., Miles, N., Farris, S. Approaches to control the quality of cementitious PFA grouts for nuclear waste encapsulation. Powder Technol., 2007, 174(1–2), 56–59.
https://doi.org/10.1016/j.powtec.2006.10.022
51. Dave, N., Misra, A. K., Srivastava, A., Kaushik, S. K. Setting time and standard consistency of quaternary binders: the influence of cementitious material addition and mixing. Int. J. Sustain. Built Environ., 2017, 6(1), 30–36.
https://doi.org/10.1016/j.ijsbe.2016.10.004
52. Snelson, D., Wild, S., O’Farrell, M. Setting times of Portland cement–metakaolin–fly ash blends. J. Civ. Eng. Manag., 2011, 17(1), 55–62.
https://doi.org/10.3846/13923730.2011.554171
53. Zhang, S., Shi, T., Ni, W., Li, K., Gao, W., Wang, K., Zhang, Y. The mechanism of hydrating and solidifying green mine fill materials using circulating fluidized bed fly ash-slag-based agent. J. Hazard. Mater., 2021, 415, 125625.
https://doi.org/10.1016/j.jhazmat.2021.125625
54. Singh, P., Ghosh, C. N., Behera, S. K., Mishra, K., Kumar, D., Buragohain, J., Mandal, P. K. Optimisation of binder alternative for cemented paste fill in underground metal mines. Arab. J. Geosci., 2019, 12(15), 462.
https://doi.org/10.1007/s12517-019-4623-6
55. ASTM D7012-23. Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures, 2023.
56. Siitam, L.-O. Researches and Analysis of Technological Solutions for Oil Shale Underground Longwall Mining with Backfilling of Outmined Areas in Conditions of Narva Opencast Mine. Master’s thesis. Tallinn University of Technology, Estonia, 2016.
https://digikogu.taltech.ee/et/Item/c5e3e29b-3dae-44b1-b62b-2b0344eb04a3
57. Benzaazoua, M., Belem, T., Bussière, B. Chemical factors that influence the performance of mine sulphidic paste backfill. Cem. Concr. Res., 2002, 32(7), 1133–1144.
https://doi.org/10.1016/S0008-8846(02)00752-4
58. Moon, G. D., Oh, S., Choi, Y. C. Effects of the physicochemical properties of fly ash on the compressive strength of high-volume fly ash mortar. Constr. Build. Mater., 2016, 124, 1072–1080.
https://doi.org/10.1016/j.conbuildmat.2016.08.148
59. Barnes, D. I. Understanding pulverised coal, biomass and waste combustion – a brief overview. Appl. Therm. Eng., 2015, 74, 89–95.
https://doi.org/10.1016/j.applthermaleng.2014.01.057
60. Woolley, G. R., Simpson, D. T., Quick, W., Graham, J. Ashes to Assets? Studies of the Usefulness and Environmental Management of Ash from Coal Fired Power Stations. UK, 2000.
61. Sear, L. K. A., Coombs, R., Yong, R. N., Thomas, H. R. The use of PFA as a fill material and the environment. In: Geoenvironmental Engineering: Geo-environmental Impact Management (Yong, R. N., Thomas, H. R., eds). Thomas Telford, London, 2001, 33–38.
https://doi.org/10.1680/gegim.30336.0006
62. ASTM C618-23e1. Standard Specification for Coal Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, 2023.
63. ASTM C618-03. Standard Specification for Coal Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, 2003.
64. Valgma, I. Mapping potential areas of ground subsidence in Estonian underground oil shale mining district. In: Environment. Technology. Resources. Proceedings of the International Scientific and Practical Conference, June 25–27, 1999, Rēzekne, Latvia, 227–232.
https://doi.org/10.17770/etr1999vol1.1869
65. Karu, V. Dependence of land stability on applied mining technology. In: 11th International Symposium “Topical Problems in the Field of Electrical and Power Engineering. Doctoral School of Energy and Geotechnology”, January 16–21, 2012, Pärnu, Estonia (Zakis, J., ed.). Tallinn University of Technology, 16–21.
66. Toomik, A. Assessment of the impact of underground mining on ground surface. In: Impact of Oil Shale Mining and Processing on the Environment in North-East Estonia (Liblik, V., Punning, J.-M., eds). Institute of Ecology, Tallinn, 1999, 109–129.
67. Sahu, P., Lokhande, R. D. An investigation of sinkhole subsidence and its preventive measures in underground coal mining. Procedia Earth Planet. Sci., 2015, 11, 63–75.
https://doi.org/10.1016/j.proeps.2015.06.009
68. Backhaus, C., Mroz, A., Willenbrink, B. Coal mine gas from abandoned mines. Pol. Geol. Inst. Spec. Pap., 2002, 7, 33–40.
69. Selberg, A., Viik, M. Relationship between water quality and oil-shale mines in northern Estonia. In: Water Quality Monitoring and Assessment (Voudoris, K., Voutsa, D., eds). InTech-Open Access Publisher, 2012, 391–406.
https://doi.org/10.5772/32151
70. Järvan, A. Endistel põlevkivialadel asuvatele hoonetele kaevandamisega põhjus-ta—tud kahjustuste hindamine. Aruanne nr 2107-118. EKIF OÜ, Tallinn, 2021.
71. Tamberg, I., Hansen, R. Kukruse A-kategooria jäätmehoidla (Kukruse aheraine-mäe) korrastamiseks ettevalmistava projekti koostamine.Teostatavusuuring.AS Infragate Eesti, IPT Projektijuhtimine OÜ, Ministry of Environment of the Republic of Estonia, Tallinn, 2015.
https://docplayer.gr/43621451-Kukruse-a-kategooria-jaatmehoidla-kukruse-aherainemae-korrastamiseks-ettevalmistava-projekti-koostamine-projekti-sfos-kood.html
72. Klaas, H. Stabilization of Post-Mining Areas and Expansion of Land Use Opportunities: A Case Study of the Kiviõli Oil Shale Mine. Master’s thesis. Tallinn University of Technology, Estonia, 2024.
73. Pastarus, J. R., Šommet, J., Valgma, I., Väizene, V., Karu, V. Paste fills technology in condition of Estonian oil shale mine. In: Environment. Technologies. Resources. Proceedings of the International Scientific and Practical Conference, June 20–22, 2013, Rēzekne, Latvia, 182–185.
https://doi.org/10.17770/etr2013vol1.823
74. Tamm, I. Uuring Ubja põlevkivi kaevanduskäikude püsivuse ja maapinna stabiil-suse hindamiseks. Eesti Keskkonna-uuringute Keskus, Tallinn, 2023.
https://www.kik.ee/sites/default/files/uuring_ubja_polevkivi_kaevanduskaikude_pusivuse_ja_maapinna_stabiilsuse_hindamiseks.pdf
75. Lees, H., Järvik, O., Konist, A., Siirde, A., Maaten, B. Comparison of the ecotoxic properties of oil shale industry by-products to those of coal ash. Oil Shale, 2022, 39(1), 1–19.
https://doi.org/10.3176/oil.2022.1.01
76. Uibu, M., Somelar, P., Raado, L.-M., Irha, N., Hain, T., Koroljova, A., Kuusik, R. Oil shale ash based backfilling concrete – strength development, mineral transformations and leachability. Constr. Build. Mater., 2016, 102, 620–630.
https://doi.org/10.1016/j.conbuildmat.2015.10.197
77. Siirde, A., Pihu, T., Konist, A., Järvik, O., Nešumajev, D., Maaten, B., Rannaveski, R., Sulg, M., Kirsimäe, K., Paiste, K., Liira, M., Mõtlep, R., Somelar, P., Leben, K., Paaver, P. Põlevkivituhkade ohtlikkuse uuring. University of Tartu, Tallinn University of Technology, Tallinn, 2019.
http://www.digar.ee/id/et/nlib-digar:399360
78. Irha, N., Reinik, J., Jefimova, J., Koroljova, A., Raado, L.-M., Hain, T., Uibu, M., Kuusik, R. PAHs in leachates from thermal power plant wastes and ash-based construction materials. Environ. Sci. Pollut. Res., 2015, 22(15), 11877–11889.
https://doi.org/10.1007/s11356-015-4459-x
79. Paaver, P., Paiste, P., Liira, M., Kirsimäe, K. Mechanical activation of the Ca-rich circulating fluidized bed combustion fly ash: development of an alternative binder system. Minerals, 2020, 11(1), 3–19.
https://doi.org/10.3390/min11010003
80. Pastarus, J.-R., Otsmaa, M., Šommet, J., Pototski, A., Kuusik, R. Improvement of current mining technology in Estonian oil shale mines. In: Proceeding of the V-Th International Geomechanics Conference, June 18–21, 2012, Varna, Bulgaria. Scientific and Technical Union of Mining, Geology and Metallurgy, Sofia, 2012, 275–279.
81. Lanzerstorfer, C. Fly ash from coal combustion: dependence of the concentration of various elements on the particle size. Fuel, 2018, 228, 263–271.
https://doi.org/10.1016/j.fuel.2018.04.136
82. Usta, M. C., Yörük, C. R., Hain, T., Paaver, P., Snellings, R., Rozov, E., Gregor, A., Kuusik, R., Trikkel, A., Uibu, M. Evaluation of new applications of oil shale ashes in building materials. Minerals, 2020, 10(9), 765–783.
https://doi.org/10.3390/min10090765
83. Raado, L.-M., Kuusik, R., Hain, T., Uibu, M., Somelar, P. Oil shale ash based stone formation – hydration, hardening dynamics and phase transformations. Oil Shale, 2014, 31(1), 91–101.
https://doi.org/10.3176/oil.2014.1.09
84. Fall, M., Benzaazoua, M., Ouellet, S. Effect of tailings properties on paste backfill performance. In: Proceedings of the 8th International Symposia on Mining with Backfill, September 19–21, 2004, Beijing, China, 193–202.
85. Pekrioglu Balkis, A. Properties and performance of a high volume fly ash grout. Mar. Georesour. Geotechnol., 2020, 38(1), 73–82.
https://doi.org/10.1080/1064119X.2018.1552999
86. Leben, K., Mõtlep, R., Paaver, P., Konist, A., Pihu, T., Paiste, P., Heinmaa, I., Nurk, G., Anthony, E. J., Kirsimäe, K. Long-term mineral transformation of Ca-rich oil shale ash waste. Sci. Total Environ., 2019, 658, 1404–1415.
https://doi.org/10.1016/j.scitotenv.2018.12.326
87. Dias, W. P. S., Nanayakkara, S. M. A., Ekneligoda, T. C. Performance of concrete mixes with OPC–PFA blends. Mag. Concr. Res., 2003, 55(2), 161–170.
https://doi.org/10.1680/macr.2003.55.2.161
88. Fatema, N., Bhatia, S. K., Palomino, A. M. Flow behavior of impounded fly ash slurries using flow test. Fuel, 2023, 339, 126906.
https://doi.org/10.1016/j.fuel.2022.126906
89. Rakngan, W., Williamson, T., Ferron, R. D., Sant, G., Juenger, M. C. G. Controlling workability in alkali-activated Class C fly ash. Constr. Build. Mater., 2018, 183, 226–233.
http://dx.doi.org/10.1016/j.conbuildmat.2018.06.174
