Page 559 - Materials Chemistry, Second Edition

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530 Waste Management Practices: Municipal, Hazardous, and Industrial
In designing a SWCR system, three highly practical issues must be considered: (1) cover stabil-
ity, (2) puncture resistance, and (3) the ability of the system to withstand considerable stresses due
to the impact of settlement. The stability of the geomembrane cap supporting the SWCR system can
be affected by the materials used to construct the drainage layer and by the slope of the unit layers.
Geomembrane caps must resist penetration by construction equipment, rocks, roots, and other natu-
ral phenomena. Traffic by operational equipment can cause tearing. In a Subtitle C landfill, a
geomembrane is always underlain by a clay liner, which will protect against puncture. Additionally,
a geotextile placed on top or beneath a membrane increases its puncture resistance by three or four
times. The impact of settlement is a major concern in the design of the SWCR system. A number of
facilities have settled 6 ft in a single year and 40 ft or more over a period of years (U.S. EPA, 1989).

17.3.11 GAS CONTROL LAYER
Gas collection systems are typically not used in Subtitle C landfills. Landfills dedicated to receiving
only hazardous wastes are relatively new and gas is rarely detected in these systems. This is attrib-
uted to the fact that most wastes are received in stabilized or solid form, and there are no putrescible
materials present as would be found in a conventional MSW landfill. Because the long-term effects
of gas generation are not known and costs are minimal, the U.S. EPA recommends the use of gas col-
lector systems. Gas collection systems are installed directly beneath the low permeability clay cap in
a Subtitle C landfill.

17.3.12 BIOTIC BARRIER
A biotic barrier is a gravel and rock layer designed to prevent the intrusion of burrowing animals
into the landfill. Animals cannot generally penetrate a geomembrane cap but they can widen an
existing hole or tear the material where it has wrinkled (U.S. EPA, 1989).

Cover soil
SWCR
Compacted clay

Operational cover

Waste
Compacted clay
Primary anchor trench
Secondary anchor trench
Cap anchor trench

Geomembranes

FIGURE 17.8 Profile of a Subtitle C hazardous waste landfill (U.S. EPA, 1989).