Civil and Environmental Applications of Geosynthetics - Reinforced Soil Engineering: Advances in Research and Practice - page 10

pressure, and the displacement at the center of inflated geomembrane specimen is
measured. The stress-strain behavior is deduced from the measured results.
The seams of geomembrane rolls and panels are provided in the field. The
watertightness of the seam constitutes a successful function of a containment
system. Four major seaming methods are available: extrusion welding, thermal
fusion/melt bonding, chemical fusion, and adhesive seaming. High-density
polyethylene (HDPE) and polyvinyl chloride (PVC) geomembranes are typically
welded using extrusion fillet and solvent, respectively. The strength of the seam
can be weaker than the geomembrane itself and is determined by the shear tests
and peel tests. Several nondestructive testing methods are available to assess the
seams in the field, where the vacuum box method is widely used. However, the
ultrasonic shadow method seems promising considering its use in difficult
locations, such as slopes and corners, where the use of vacuum box method would
not be possible.
The shear strength between the geomembrane and soils, and between the
geomembrane and other geosynthetic materials used in the liner, is one of the
most important design issues, especially when the liner is constructed along
the slopes. Liner failures attributed to inadequate interface shear strength were
reported. A shear box of dimensions 10 cm by 10 cm would be adequate for
conducting direct shear tests between the geomembrane and site-specific soils
under simulated field conditions.
Although HDPE is chemically resistant, it may suffer from environmental
stress cracking. The notched constant-load tension test appeared to be a more
relevant test for determining the potential of stress cracking. In the test, a
dumbbell-shaped test specimen is sustained under a constant tensile load and
immersed in a surface-wetting agent at an elevated temperature. The transition
time for the behavior to change from ductile to brittle is recorded. A transition
time of 100 hr or greater is considered acceptable.
4.2 Application of Geosynthetics in a Waste Containment
System
Geosynthetics are used in solid waste containment systems to create a watertight
environment. Geomembranes are used with clay to form a single or double
composite liner along the side and bottom of the landfill to prevent migration of
leachate into the underlying groundwater (Fig. 6) . They are also used as a cover
liner, acting as a barrier against precipitation, after a landfill has been closed. In
the watertight environment, other geosynthetics, such as geotextiles, geocompo-
site, and pipes, are used to facilitate drainage, collection, and removal of leachate
generated by waste decomposition and/or precipitation. In the United States, the
use of geomembranes in a waste containment system is regulated by federal laws
(Subtitle D).