Advances in Environmental Geotechnics

Advances in Environmental Geotechnics

Despite reductions in waste generation, landfills will
continue to be required for the safe disposal of municipal
solid waste (MSW) for the foreseeable future. These
landfills will generate both leachate and gas whose
escape from the facility must be controlled to
environmentally acceptable levels. The leachate is
predominantly water but typically contains dissolved
organic and inorganic chemicals and suspended solids
(e.g. microbes, particulate matter etc.) whose escape
from the landfill must be controlled to negligible levels.
Landfill gas is predominantly comprised of methane and
carbon dioxide which are of concern as greenhouse gases
(especially methane) but it also contains trace amounts
of volatile organic compounds. From an engineering
perspective, the long-term performance of the modern
MSW landfill will be governed by the performance of a
system comprised of three primary subsystems: the
barrier system below the waste, the landfill operations,
and the landfill cover and gas collection system. To
provide long-term environmental protection, this system
must contain contaminants for what is called the
contaminating lifespan of the landfill (i.e. the period of
time during which the landfill will produce contaminants
at levels that could have unacceptable impacts if they
were discharged into the surrounding environment). For
large modern landfills this could be hundreds of years
(Rowe et al. 2005).

The release of contaminants contained in landfill
leachate can be reduced to environmentally acceptable
levels with a suitable barrier system below the waste that
includes a leachate collection system and a liner system.
The leachate collection system minimizes the driving
force for leachate escape (i.e. the leachate head acting on
the underlying liner). The liner system provides
resistance to the migration of contaminants both by the
pressure driven movement of leachate containing
contaminants (often referred to as leakage or advection)
and the concentration driven movement of contaminants
by a process of diffusion (those not familiar with the
terminology and contaminant transport processes should
refer to Rowe et al. 2004 for details). The leachate
collection system typically involves a series of
perforated pipes in a granular drainage layer together
with a means of removing the leachate that is collected.
The barrier system may involve a single liner or a double
liner with a secondary leachate collection system (also
called a leak detection system) between the two liners.

Referring to the present state of the art, the
theoretical modeling approaches applied to
Environmental Geotechnics can be split in two main
domains: flow-transport phenomena in porous media and
mechanical behavior of particulate media. Only in few
cases the two domains are fully coupled in order to set
up more sophisticated models that theoretically should
be able to result in a better representation of some actual
Practical application aspects within the
Environmental Geotechnics field require to model mass
and energy flows in order to be able to cope with
situations such as: 1) subsoil pollutant migration and
control, 2) assessment of short and long term
performances of mineral and polymeric barriers and 3)
polluted sites contaminant treatment and extraction.
On the other hand, the theoretical prediction of
particulate material mechanical behavior is often applied
to Environmental Geotechnics problems involving: 1)
waste landfill stability, management, extension and
reclamation, 2) re-use of waste and by-products and 3)
assessment of pollutant barriers performances subjected
to large stress/strain variations.
Just a very basic treatment of the main general
aspects related with mass and energy transport and
mechanical behavior of porous and particulate media
would require more than a textbook. Therefore, in the
following, after a very short review of the basic
framework and equations concerning the two aforementioned
main domains, a number of practical applications will be
considered as illustrative examples including: (1)
prediction of short and long term behavior of
geosynthetic clay liners (GCLs) for pollutants control;
(2) modeling of multiphase mass and energy flows for
speeding up contaminant extraction by electrical currents
and; (3) high vacuum; and (4) assessment of mechanical
behavior of lining systems on old landfill for allowing
side and vertical extensions.

Geosynthetic clay liners (GCLs) are factory
manufactured liners that consist of a thin layer of
bentonite (∼5-10 mm thick) that is either sandwiched
between two geotextiles or attached to a polymer
membrane and held together by needle-punching,
stitching or gluing with an adhesive. GCLs are
increasingly used in covers and bottom liners for
landfills because of their low hydraulic conductivity to
water (i.e. k typically ≤ 3.0⋅10-11m/s). The main
advantages of GCLs are the limited thickness, the
material quality assurance, the ease of installation, the
good compliance with differential settlements of
underlying soil or waste and the low cost (Bouazza,2002).
On the other hand, the limited thickness of this
barrier can produce vulnerability to mechanical accidents,
limited sorption capacity, and an increase of pollutant
diffusive transport if an underlying attenuation mineral
layer is not provided. Moreover, when exposed to water
solutions containing high ion concentrations or di-valent
cations, bentonite can undergo a significant increase of
the hydraulic conductivity.
The evaluation of GCL performances as pollutant
barriers needs an adequate theoretical approach for
modeling the simultaneous migration of water and
solutes through bentonite. Bentonite is a clay soil
containing typically at least 70% of the three layered
(2:1) clay mineral montmorillonite, which is
characterized by a very high total specific surface (∼760m2/g)
 and a negative electric charge. The ultra-fine pore
size (< 100 Å) of this clay soil and the electric
interaction between montmorillonite particles and ions in
pore solution determine macroscopic phenomena that

cannot be modeled on the basis of the advective-diffusive


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