Geotechnical Considerations in Surface Impoundment Management and Closure

摘要

The closure of coal combustion residuals(CCRs)surface impoundments(SIs)will be occurring with an increasing frequency over the next two to seven years as the U.S.EPA Effluent Limit Guidelines(ELGs)are included in NPDES discharge permit requirements and exclude the discharge of CCR sluice water.The purpose of this paper is to help SI owners become cognizant of the important geotechnical concerns that need to be addressed during the planning and execution phases of the project to provide a cost-effective and safe closure with effective long-term performance.The geotechnical considerations that will be discussed in this paper will include:(i)deciding whether to close the SI in-place or by removal,(ii)what type of cover system is appropriate under the U.S EPA CCR Rule(CCR Rule),and(iii)equipment-induced localized”liquefaction”.This paper provides practical information and solutions to the awaiting challenges that SI owners will encounter.Smaller SIs may be more effectively closed by removal,including consolidation into other CCR disposal units,provided a”clean closure”can be achieved.Recent assessments have been completed to evaluate the environmental impacts of closing larger SIs in-place compared to closure by removal that indicate fewer environmental impacts through in-place closure.There are many alternate cover systems that can meet the CCR Rule.The important geotechnical considerations include the hydraulic conductivity of the bottom liner or underlying natural geologic conditions,the slope/configuration of the final cover,available materials,and the hydrologic environment.Equipment-induced localized liquefaction is a well-documented condition.Providing methods to consider improving the subgrade will be provided to reduce the localized liquefaction will be provided.