Step 6 - Evaluate Site Geometry, Stability, Erosion, and Consolidation

ERDC TN-DOER-R6

December 2004

1x10-5 cm/sec or less when compacted. Additional reductions in hydraulic conductivity may be realized

through modification of clean dredged material with additives, use of clay layers, or employment of

geosynthetic materials and composite liner systems. Liners and their underlying soils must also possess

sufficient strength after compaction to support themselves and the overlying materials without failure.

Other design parameters influencing the selection of liner materials include: (a) potential for desiccation

(e.g. Hawkins and Horton 1965, Boynton and Daniel 1985, Kleppe and Olson 1985, Montgomery and

Parsons 1989, Daniel and Wu 1993); (b) resistance to freeze-thaw action (e.g., Benson and Othman

1993); (c) resistance to chemical degradation (USEPA 1985, 1986a, 1986b, 1986c, 1991a, 1991b); and

(d) ability to perform without cracking during settlement (Qian 1995). CDFs should also utilize liner

materials that provide the minimal cost liner system that functions to meet all design requirements during

operation and post-closure. Consideration should also be given to the time required for specific liner

system design and installation.

Step 6 - Evaluate Site Geometry, Stability, Erosion, and Consolidation. Consolidation of liner

materials following placement of contaminated material needs to be examined for its effect on

permeability and flux of contaminants through the liner system. Typically, consolidation of liner systems

results in reduced permeability and consequent reduction in contaminant flux. Consolidation of

contaminated material also needs to be examined for its effect on the volume occupied within CDF sites

and potential for impact on cover integrity and stability. In general, consolidation of the contaminated

dredged material will result in more stable conditions.

If the potential for consolidation of the liner, contaminated material, or cover is unacceptable, an

alternative site, alternative liner, or alternative placement techniques can be considered.

Step 7 - Evaluate Long-Term Performance of Liner System. The long-term performance of a

CDF liner system includes its inherent ability to maintain its design effectiveness throughout the design

life of the facility. Emphasis must be placed during the design process on the selection of materials and

methods that will tend to maintain design characteristics for extended periods of time. Specific emphasis

must be placed on compatibility of liner and cover systems with the contaminated sediment. Case studies

of CDF projects requiring control measures, including conclusions drawn from field monitoring efforts,

are described in "Confined Disposal Facility (CDF) Containment Features: A Summary of Field

Experience," ERDC TN-DOER-C18 (Palermo and Averett 2000).

Step 8 - Develop Design Specifications. Following evaluation of the long-term performance of the

proposed containment system, detailed design specifications should be written to ensure that construction

of the CDF containment system meets specific performance criteria at minimum cost. Table 1

summarizes the types of information that may be used to demonstrate that the performance standard for a

CDF liner system is met.

LINER MATERIALS AND COMPONENTS: Since contaminants normally associated with dredged

material tend to remain bound to sediment particles, confined disposal of dredged material usually does

not require the employment of highly engineered liner system components. The level of contamination of

the dredged material, local availability of low-permeability construction materials, and the potential

chemical flux through these materials dictate whether additional containment measures are necessary.

Potential liner system components, in order of increasing complexity, include compacted subsoil, clean

dredged material, modified soil/dredged material, compacted clay liner, synthetic liners, and composite

liners and leachate collection layers. Construction and installation considerations specific to CDFs are

provided in the "Construction Considerations" section.