Engineering Geology

Chapter 8
recommended that at dry sites, tipping could take place on coarse-grained material that has
a thickness of 15 m or more, while any water wells should be located at least 0.8 km away.
Carbon dioxide and methane are generated by the decomposition of organic materials within
sanitary landfills. However, the amount of gas produced by domestic waste varies appreciably,
and a site investigation is required to determine the amount if such information is
required. Nonetheless, it has been suggested that between 2.2 and 250 l kg -1 dry weight
(0.0022 to 0.25 m 3 kg -1 ) may be produced (Oweis and Khera, 1990). Both the gases mentioned
are toxic, and methane forms a highly explosive mixture with air. Unfortunately, there
are cases on record of explosions occurring in buildings due to the ignition of accumulated
methane derived from the landfills on which they were built. Accordingly, planners of residential
developments should avoid such sites. Proper closure of a landfill site can require gas
management to control methane gas by passive venting, power-operated venting or the use
of an impermeable barrier.
Protection of groundwater from the disposal of toxic waste can be brought about by containment.
A number of containment systems have been developed that isolate wastes and
include compacted clay barriers, slurry trench cut-off walls, geomembrane walls, diaphragm
walls, sheet piling, grout curtains and hydraulic barriers (Mitchell, 1986). A compacted clay
barrier consists of a trench that has been backfilled with clay compacted to give a low
hydraulic conductivity. Slurry trench cut-off walls are narrow trenches filled with soil–bentonite
mixtures that extend downwards into an impermeable layer (Bell et al., 1996). Again, they
have a low hydraulic conductivity. In a geomembrane wall, a U-shaped geomembrane is
placed in a trench and filled with sand so that its shape conforms to that of the trench.
Monitoring wells are placed in the trench so that they can detect and abstract any leakage
from the fill. Diaphragm walls are constructed in a similar manner to slurry trenches but are
an expensive form of containment. Their use is therefore restricted to situations in which high
structural stability is required. If leakage occurs via interlocks in steel sheet pile cut-offs, then
their watertightness is impaired. In such instances, wells may be needed to ensure effectiveness.
Grout curtains may be used in certain situations and are likely to consist of three rows
of holes, the outer rows being grouted first, followed by the inner to seal any voids. Extraction
wells are used to form hydraulic barriers and are located so that the contaminant plume flows
towards them.
When a new site has been chosen, Mitchell (1986) maintained that a properly designed and
constructed liner and cover offer long-term protection for ground and surface water. Clay
liners are suitable for the containment of many wastes because of the low hydraulic conductivity
of clay and its ability to absorb some wastes. They are constructed by compaction in lifts
of about 150 mm thickness. Care must be taken during construction to ensure that the clay
is placed at the specified moisture content and density, and to avoid cracking due to drying
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