Step 1 - Select Liner System.

ERDC TN-DOER-R6

December 2004

Liner design incorporates both material selection (composition) and thickness (dimension) analyses. The

design must also take into consideration the type of contaminant present (material resistance), properties

of the environment (vicinity of groundwater table, water chemistry, location of surface water bodies,

quality and potential use of groundwater as a potable water source), climate (desiccation and freeze-thaw

effects), available materials (geotechnical qualities, relative permeabilities), equipment and placement

techniques, regulatory setting (mandated maximum level of loss), and resources available (funding).

The composition of liners for CDFs might typically consist of a compacted subsoil layer overlain by one

or more layers of low-permeability soil or clean dredged material, and in some instances, modified soils

and modified clean dredged materials, compacted clay liners, or geosynthetics such as geomembranes or

geosynthetic clay liners. Depending on the physical and chemical characteristics of the contaminant, the

permeability of the compacted subsoil layer may be sufficiently low to allow its use as the sole liner

component. It should be noted that design criteria for municipal and hazardous waste landfills provide

specific recommendations for liner material and thickness requirements; however, CDFs are not regulated

under the same authority as these systems. Section 404 of the Clean Water Act of 1972, as amended, and

the National Environmental Policy Act (NEPA) of 1969, as amended, do not specify design criteria for

CDFs and do not require specific materials or thicknesses for liners. Rather, the primary consideration is

reduction in hydraulic conductivity through the CDF liner system to meet groundwater standards at the

point of compliance.

Figure 2 illustrates a cross section of a CDF, further broken down into cross sections of example CDF

bottom liner systems. A number of different liner combinations are possible, ranging in approximate

increasing order of complexity and expense: (a) compacted subsoil; followed by any combination of one

or more layers of: (b) compacted soil lifts; (c) uncontaminated compacted fine-grained dredged material;

(d) compacted modified soils/dredged material; (e) compacted clay liner material; (f) geomembrane;

(g) geosynthetic clay liner; (h) compacted clay liner and geomembrane; (i) pelletized materials; and

(j) composite liner systems consisting of compacted clay liner and geomembrane and leachate collection

system. Each liner system offers particular advantages and disadvantages. The following sections

provide an overview of the principle of design, applicable literature for design guidance, and specific

procedures for CDFs when design varies from published guidance.

The flowchart depicted in Figure 3 illustrates the major design requirements for implementation of

containment features for liners and the sequence in which the design requirements should be considered.

As mentioned previously, there is a strong interdependence among all components of design for a CDF

project. For example, the initial consideration of the CDF location and extent of contamination within the

sediments to be dredged greatly influence all subsequent design elements. Each step in the design process

must be clearly identified and documented before a decision can be made to proceed to final design

activities. This will also assist in ensuring that unnecessary data collection and evaluations can be

avoided. If project data and sediment characteristics suggest that a CDF is necessary, and an upland site

has been selected, then the following design sequence should be followed. Detailed descriptions of liner

material selection criteria and thickness requirements are provided in subsequent sections.

Step 1 - Select Liner System. "Leachate Screening Considerations," ERDC TN-DOER-C16

(Schroeder 2000), provides a leachate screening protocol to evaluate the acceptability of confined

disposal of dredged material. If applicable guidelines or discharge standards for contaminant loss are

predicted to be exceeded by the CDF design under consideration, additional contaminant control

measures must be considered to reduce contaminant loss to acceptable levels. Possible leachate control

measures include (see Figures 1 and 2):