Geosynthetics and coal combustion residuals

According to the World Bank Group, in 2015 coal produced 41% of the world’s energy and about 34% of the energy production in the United States. While global warming and pollution are serious challenges according to most scientific evidence, and coal burning is cited as one cause of both, there are many staunch advocates of the use of coal as well (e.g., business interests and unions). It is likely that a meaningful amount of both deep mining and strip mining of coal will continue. In this regard, there are three residual products after the coal has been mined and then burned for energy, namely: (1) coal spoil tips, (2) dry-placed coal combustion residuals (CCRs) and (3) slurry-placed CCRs.

Enormous coal spoil tips, essentially piles of waste rock from coal mining, are found worldwide (Figure 1). When uncovered (as they usually are), acidic mine drainage occurs, erosion is commonplace, spontaneous combustion has occurred and the tip itself can be unstable. Notable failures have occurred in Aberfan, Wales, United Kingdom (144 people killed), and Coed-Ely, also in Wales (major creep slide). Our analysis of each shows that water within the mass was instrumental in both failures, and that no dewatering or stabilizing buttressing berms were involved.

Other than mine spoils, the composition of CCRs after burning consists of bottom ash, fly ash, flue gas desulfurization material and boiler slag. The U.S. Environmental Protection Agency (EPA) estimates that in 2017 there were 110 million tons (100 million tonnes) disposed of at 470 sites scattered across 25 U.S. states. Furthermore, dry CCR disposal currently occurs at more than 310 active on-site landfills, averaging more than 120 acres (49 ha) in size with an average depth of more than 40 feet (12 m), and wet CCR disposal occurs at more than 735 active on-site surface impoundments, averaging more than 50 acres (20 ha) in size with an average depth of 20 feet (6 m). Each type will be described.

Regarding dry-placed CCRs, they are usually sited directly on the ground surface, and yet failures have occurred. The most recent failure in this category was in Milwaukee, Wis., on Oct. 31, 2011, involving 706 million cubic feet (20,000 m3) of power plant ash.

Regarding slurry-placed or wet CCRs, they are necessarily contained by a perimeter soil berm. The disposal sites and certainly the berms can and should be engineered, but several notable failures have occurred, including berm failures in Spain, Hungary, Tennessee and North Carolina. It is the 2008 TVA Kingston Fossil Plant failure near Harriman, Tenn. (Figure 2), of 1.45 billion cubic feet (4.1 million m3) of slurried ash that stimulated the EPA and then Congress to pass the Final Rule for “Disposal of CCRs from Electric Utilities” in 2015. As described by Ramsey (2015), this 450-page rule contains a strong endorsement for geosynthetic composite liners and related geosynthetic materials.

This brief discussion of these three types of coal waste materials is designed to appreciate the myriad opportunities for proper siting and contaminant strategies. Many of these strategies utilize geosynthetics, in whole or in part. At a minimum, they are as follows:

1. Single composite geomembrane/geosynthetic clay liner (GCL) systems
2. Double composite geomembrane/GCL systems
3. Soft foundation soil stabilization with prefabricated vertical drains (aka, PVDs or wick drains)
4. Use of deep foundations with high-strength basal reinforcement (geotextiles or geogrids) when addressing poor soil foundation conditions
5. Geotextile- and geogrid-enclosed sand or gravel columns for support systems
6. Mechanically stabilized earth (MSE) walls and slopes using geogrids and geotextiles for restraining berm stabilization and buttressing
7. Exposed and covered geomembrane cover systems above both spoil tips and dry-placed CCRs
8. Geomembrane cover systems above wet (slurried-placed) CCRs
9. Geogrid or geotextile support systems beneath geomembranes covering wet CCRs
10. Geotextile tubes for containing, dewatering of and decontamination of wet CCRs

Geosynthetics companies should certainly continue to position themselves as key players in the safe and environmentally acceptable disposal practices for coal-related waste materials.

References

Ramsey, B. (2015). “The status of coal ash regulations.” Geosynthetics, 33(2), 38–42.