30 Sep 01
consisting of bentonite clay materials supported by geotextiles or geomembranes. GCLs are used to
augment or replace compacted clay layers or geomembranes. All of the GCL products available in
North America use sodium bentonite clay powder or granules, with a mass per unit area in the range of
3.2 to 6.0 kg/m2 (0.66 to 1.2 lb/ft2). The clay thickness of the products varies between 4.0 to 8.0 mm
(160 to 320 mils). GCLs are available in widths of 2.2 to 5.2 m (7 to 17 ft) and lengths of 30 to 60 m
(100 to 200 ft). GCLs are most often considered for use where there is a limited supply of natural clay,
limited landfill space, or where differential settlements could cause unacceptable tension cracking in a
compacted clay liner. GCLs are also less susceptible to the effects of desiccation and freezethaw than
compacted clay layers. One limitation to consider is that hydrated, GCLs have extremely low in-plane
shear strength. Because of this, geotextile-backed GCLs should be reinforced with stitch bonding or
needle punching if the GCL is to be used on slopes that are steeper than 3 to 5. If a geomembrane
backed GCL is used on slopes, the bentonite component of the GCL should be fully encased with
geomembrane to prevent it from hydrating.
(2) Design Criteria. EPA/600/R-93/171, Report of Workshop on Geosynthetic Clay Liners
summarizes information on GCLs. Guide Specification CEGS-02376 should be used in contract
documents to specify material and installation requirements for geosynthetic clay liners.
k. Cover Penetrations.
(1) General. Penetrations through the landfill cover are required for gas vents, monitoring wells,
and other purposes. However, the number of penetrations should be minimized. Geomembranes
should be attached to the penetrating object in a way that ensures a watertight seal but still allows for
movement from settlement or horizontal displacement.
(2) Design Criteria. Most geomembrane manufacturers, fabricators, or installers have their own
typical penetration details. Therefore, in many cases, it is only necessary to show locations of the
penetrations on the drawings and note that penetration details must be in accordance with approved
geomembrane manufacturer, fabricator, or installer details. GCL penetration details should also be as
recommended by the GCL manufacturer. Pipes that penetrate deeply into the waste material are likely
to settle at a different rate and to a smaller magnitude than the adjoining landfill cover. The differential
settlement between the pipe and the cover system creates stress concentrations at the boot connection
that can tear the geomembrane away from the pipe. Slip couplings are typically used in this situation to
allow differential movement while maintaining a watertight seal.
l. Gas Collection and Removal System.
(1) General. Landfill gas production results from vaporization, chemical reactions, and biological
decomposition. Biological and chemical degradation of organic waste materials is the predominant
source of landfill gas. Degradation of organic materials results in the production of carbon dioxide
(CO2) and methane (CH4). Other gases may also be generated, depending on the composition of the
waste. Gases migrate by two processes: convection and diffusion. Convection is gas flow induced by