United States Environmental Protection Agency Hazardous Waste Engineering Research Laboratory Cincinnati OH 45268 Research and Development EPA/600/S2-87/025 July 1987 &EPA Project Summary Prediction/Mitigation of Subsidence Damage to Hazardous Waste Landfill Covers Paul A. Gilbert and William L Murphy This report describes the causes and effects, prediction methods, and tech- nologies that may be applied for the prevention of subsidence in hazardous landfills. The information should be of assistance to those involved in evalua- ting landfill permit applications. The goal is to help prevent damage to, and resulting leaks through, landfill covers caused by subsidence-induced stresses. This Project Summary was devel- oped by EPA's Hazardous Waste Engi- neering Research Laboratory, Cincin- nati, OH, to announce key findings of the research project that is fully doc- umented in a separate report of the same title (see Project Report ordering information at back). Background Section 3004 of the Resource Conser- vation and Recovery Act (RCRA) of 1976 requires the Administrator of the Envi- ronmental Protection Agency (EPA) to establish standards applicable to owners and operators of hazardous waste treat- ment, storage, and disposal (TSD) facil- ities. Among the standards are require- ments for "treatment, storage, or disposal of all such waste received by the facility pursuant to such operating methods, techniques, and practices as may be satisfactory to the Administra- tor." The implementing regulations for landfill covers are found in 40 CFR 264.310, "Closure and postclosure care," which states that the final cover must be designed and constructed to (1) provide long-term minimization of migra- tion of fluids through the closed landfill; (2) function with minimum maintenance; (3) promote drainage and minimize erosion or abrasion of the cover; (4) accommodate settling and subsidence so that the cover's integrity is maintained; and (5) have a permeability less than or equal to the permeability of any bottom liner system or natural soils present. Monitoring and maintenance, includ- ing necessary cover repairs, are also required throughout the postclosure period. The postclosure period is desig- nated in 40 CFR 264.117 as 30 years after completion of closure. EPA recognizes the need to provide guidance in implementing the cover requirements. This document addresses the fourth requirement listed above regarding settlement and cover subsi- dence. Purpose The final report summarized herein, presents technical guidance directed at predicting, reducing, and preventing landfill settlement and related cover damage by subsidence. The final report is intended to be used by regulatory personnel and by operators of hazardous waste landfills. ------- Scope The information presented in the final report pertains to hazardous waste landfills designed, constructed, and operated within the United States under the RCRA regulations. Landfills con- structed and capped before the passage of RCRA in 1976 may not meet RCRA's relatively stringent waste placement, liquid waste limitations, liner specifica- tions, and leachate collection and control requirements, and thus may not be amenable to the analytical, construction, and remedial guidance presented in the final report. Conclusions and Recommendations Hazardous waste landfills meeting RCRA requirements have physical char- acteristics that influence their potential for settlement and subsidence. Attention to those characteristics can minimize postclosure subsidence damage. Data on physical properties of real and simulated hazardous waste are available to assist the landfill operator or permit- ting agency in assessing long- and short- term settlement potential. Landfill subsidence results from pri- mary consolidation and secondary com- pression of the waste mass, and from collapse of voids or cavities in the fill and around containers by corrosion, oxida- tion, combustion, or biochemical decay of landfilled materials. Rarely, a landfill may be a monofill; that is, it may contain uniform layers of drummed wastes or uniformly placed bulk wastes. More often, the landfill consists of different types of wastes placed nonuniformly across the landfill in layers separated by intermediate covers of soil. The potential for differen- tial settlement is greater in landfills with nonuniform wastes and waste place- ment procedures. Bulk wastes behave differently from containerized (e.g., drummed) wastes in settlement characteristics. Bulk wastes behave relatively predictably, much like soils, becoming increasingly consoli- dated with time, but at a decreasing rate. Containerized wastes may remain rela- tively underformed until the containers degrade and collapse, at which time voids will be created, and consolidation will begin. Settlement by consolidation and secondary compression of bulk waste landfills in which drainage layers are provided will probably be essentially complete before final closure. Compac- tion of waste materials and installation of drainage layers are recommended to lessen the potential for postclosure settlement and cover subsidence. The approximate time required for primary consolidation to occur can be estimated for a waste or soil layer if the liquid limit is known for the material and if the shortest distance to a drainage path (e.g., a drain layer) is known. Time, for any degree of consolidation, can be computed more precisely if the compres- sibility or coefficient of consolidation has been determined for the material. Of the controlling factors, the distance to a drainage path has the most pro- nounced effect on consolidation time for a waste layer. This fact indicates the desirability of including frequent drain- age layers and of removing liquid from the landfill mass so that most of the consolidation will occur before closure. The time required for ultimate settle- ment of containerized (drummed) waste to occur cannot be computed without knowledge of the drum deterioration time. The time cannot be determined, although it is expected to be several years, perhaps several decades, if water infiltration is prevented by an impervious cap and liner system. The void space around drums or other containers in a landfill can be a major contributor to total postclosure settle- ment and should be filled with solidifying agents or a free-flowing backfill to minimize the void component of total settlement. The surest way of avoiding problems associated with postclosure deteriora- tion of drums and the delayed settlement and cover subsidence associated with it may be to ban drums from landfills. Instead, drums can be emptied and crushed or reclaimed. Drum contents can be treated and disposed as bulk waste. Equations for calculating settlement time should be used more to identify operational landfilling and waste treat- ment procedures that will minimize settlement time than to predict precise values from theory. Differential settlement across rela- tively short distances that may occur within subcells comprising a larger landfill cell is more threatening than relatively uniform settlement across longer distances that may occur across targe monofills. For the former, tensions! stresses may be sufficient to cause cracks in the cover resulting in leakage of water into the landfill. Those tensional stresses may not develop over longei distances, but ponding of water mai occur on the cover barrier, weakening it! ability to repel water. Similarly, tensional stresses are antic ipated to cause few or no problems wit)' flexible membrane barriers over large subsidence areas. Locally severe differ ential subsidence can cause strair sufficient to rupture a flexible membrane or otherwise cause its premature failure Two or more central column models for analyzing landfill deformation (settle ment) can be used to predict differentia settlement between columns and there by to determine the effect of differentia subsidence on the final cover. Expressions for analyzing the deflec tion of a beam can be used to identif parameters controlling the deformatioi of a landfill cover subjected to differentia settlement. Once identified, the parame ters can be adjusted by cover design am construction procedures to minimizi distress to cover components. Differential settlement can be mini mized by compacting wastes durini placement, eliminating void space withii the landfill, stabilizing liquids befon placement, and other considerations The length of the cover (represented a a beam) subjected to subsidence can b< reduced by placing wastes as uniform! as possible to provide uniform suppor to the cover. The cover soil component can be made more resistant to distres by compacting the cover barrier soils we of optimum water content. Final cover components will stretc under differential settlement and mus be constructed to withstand tensil strain. The average tensile strain in th cover can be computed, and the maxi mum value of the differential settlemer that can be tolerated by the cover soil can be estimated from that computatior Plastic soils (soils with high plasticit indexes) should be selected for use a cover components to produce a cove resistant to tensile strain. Laboratory investigations by other indicate the flexible membrane liner (FML's) (components of the barrier laye in covers) may fail at lower strains tha would be expected from manufacturer! data. Every effort should be made t reduce differential settlement potentii of the landfill and to design the covt to resist tensile strain. Landfilled wastes should be con pacted or treated where possible 1 reduce potential settlement. Compactic ------- methods include standard compaction techniques, vibratory rollers, and pre- compression (preloading and surcharg- ing). Waste treatment methods include addition of fixative agents to render the wastes permanently less compressible. The stabilization of liquid wastes with pozzolanic materials has been shown to increase compressive strength and lessen settlement potential. Such stabil- ization could be especially beneficial for containerized wastes. Paul A. Gilbert and William L Murphy are with the U.S. Army Waterways Experiment Station, Vicksburg, MS 39180. Robert P. Hartley is the EPA Project Officer fsee below). The complete report, entitled "Prediction/Mitigation of Subsidence Damage to Hazardous Waste Landfill Covers." (Order No. PB 87-175 378/AS; Cost: $18.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Hazardous Waste Engineering Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 ------- cr. U.S.OFFICIALM* United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use S300 EPA/600/S2-87/025 0000329 |>s ------- |