Synthetic Liners

ERDC TN-DOER-R6

December 2004

Synthetic Liners. In rare cases of highly contaminated dredged material, more rigorous methods of

containment and isolation may be necessary than those offered by compacted soil and sediment liners and

covers functioning alone. A potentially economical and volumetrically efficient method to reduce

leachate generation and transport is the use of manufactured geosynthetic materials such as

geomembranes, geonets, and geotextiles. Each of these components functions as an integral part of a

geosynthetic composite liner. Geomembranes are impermeable barrier materials designed to prevent

migration of leachate into or out of the CDF. Geonets function as lateral drainage layers to assist in the

drainage of leachate. Geotextiles are used between geonets and soil layers or between geonets and

geomembranes and function as filters to prevent clogging of geonet drainage layers.

Geosynthetic materials are rarely used in CDF liner and cover design because (1) of their high cost

relative to soil liner and cover systems, and (2) CDFs do not fall under the same regulatory standards as

municipal and hazardous waste landfills (which require the use of geosynthetic materials (Qian 1995)).

Instances when geosynthetics may be employed in CDFs include (1) a requirement by a local regulatory

authority, (2) characteristics of the contaminated sediments and potential leachate dictate the use of more

restrictive control measures, and (3) characteristics of the leachate pose significant chemical degradation

hazards to soil and dredged material liner integrity so that a more inert material (e.g., high-density

polyethylene membrane (HDPE)) must be utilized.

Geomembranes. Geomembranes offer a lightweight, low-hydraulic conductivity solution to the problem

of contaminant release, including, when used in covers, control of vapors such as volatile organic

compounds. With hydraulic conductivities that range from 0.5x10-10 to 0.5x10-13 cm/s, geomembranes

can greatly improve the effectiveness of leachate control systems. High-density polyethylene (HDPE)

and very-low-density polyethylene (VLDPE) are two types of geomembranes that are commonly used.

HDPE liners possess outstanding chemical resistance, low permeability, and exceptional durability, as

evidenced by their strong track record for employment in solid waste landfill systems (USEPA 1985).

VLDPE geomembranes possess an exceptional ability to withstand elongation due to differential

settlement; they also possess similar resistance to chemical degradation as HDPE. Each of these liners is

available in thicknesses that range from 1 mm (40 mil) to 2.5 mm (100 mil). HDPE and VLDPE liners

essentially serve as impermeable barriers to leachate transport to prevent the migration of liquids and

gases from the containment facility, if they are kept in good condition with no tears or punctures.

General design guidance for use of geomembranes in liner systems can be found in EPA 625/4-89/022

(USEPA 1989), EPA 530/SW-91/054 (USEPA 1991a), TM 5-818-8 (Headquarters, Department of the

Army (HQDA)1995), and Koerner (1994). Differences in the requirements for CDFs and municipal and

hazardous waste landfills again relate to the existence of specific requirements for materials and

thicknesses for landfill systems; specific design guidance for emplacement of geomembranes in CDFs

does not exist. Particular areas of difference include potential for the development of increased hydraulic

head on the liner system in CDFs and production of greater leachate quantity with differing leachate

quality. Because of the relative imperviousness of geomembranes, particular emphasis should be placed

on reduction of hydraulic head via employment of various dewatering systems. Adherence to similar

material testing requirements as that specified for landfill liners will, however, generally yield appropriate

material property guidance if geomembranes are selected for use in CDFs.