cannot be successfully captured or controlled by a trenching system may, however, necessitate the
installation of a leachate collection system for leachate control and containment. Operating the drains
only after disposal and the onset of desiccation will extend the life of the underdrain and improve the
Performance/Effectiveness. Performance of liner systems relates to the ability of the liner (working
as a system with the compacted subgrade, dredged material, lateral control measures, and covers) to
control the movement of leachate so as to meet or exceed design requirements. The overall effectiveness
of liner systems within CDFs thus encompasses an evaluation of the performance of the entire CDF
system. It is likely that a number of different combinations of liner materials, lateral controls, and cover
materials will provide adequate performance. The design should thus use the lowest cost combination of
reliable and implementable control means that meet performance objectives. The Hydrologic Evaluation
of Leachate Production and Quality (HELPQ) model (Aziz and Schroeder 1999) provides a means to
assess leachate quantity and quality for various combinations of liner and cover systems.
Reliability. Reliability of liner systems refers to the ability of the liner to perform as intended
throughout its design life. The reliability of individual systems is a function of material manufacture,
subgrade preparation, installation techniques, experience of construction personnel, projected loads,
leachate compatibility, and control of desiccation. In effect, increased liner reliability implies a reduction
1998) provides an overview of the risk management approach to dredged material operations.
Monitoring. Monitoring landfill leachate provides a means to assess the relative reliability of liner
systems. If it is determined that monitoring is required, then several methods should be considered,
including installation of groundwater monitoring wells to detect potential contaminant migration in the
subsurface or installation of secondary leachate collection systems to collect leachate that may pass
through the liner system. However, effective monitoring is difficult to perform and can be expensive.
SUMMARY: This technical note presents technical guidance for CDF liner design to enhance
containment of pollutants. The guidance is summarized in the points below.
Liner materials that isolate the contaminated material in CDFs from the environment must be
properly designed and constructed.
Liner materials must also be characterized from physical, chemical, and biological standpoints.
Physical characteristics determine the behavior during placement of the liner and long-term
consolidation and stability against chemical and biological degradation.
Selection of an appropriate site is a critical requirement for any CDF construction project. General
considerations include potential leachate pathways (bottom leakage, lateral seepage, vertical leachate
movement during consolidation, surface runoff, and volatile emissions).
Leachate screening considerations (Schroeder 2000) guide designers in selecting the best materials to
adequately control potential leachate pathways. Modeling efforts for leachate transport serve as the
basis for liner design material and thickness requirements. Further, consolidation effects on
permeability and overall leachate generation and movement can be modeled with the Primary
Consolidation, Secondary Compression, and Desiccation of Dredged Fill (PSDDF) model (Stark
1996) and the model Hydrologic Evaluation of Leachate Production and Quality (HELPQ) model
(Aziz and Schroeder 1999), respectively.