problem
In a traditional sawtooth landfill design, leachate collection pipes
are installed at the bottom of each lateral, and surrounded by gravel
with geotextile wrap. The hydraulic performance of traditional landfill
bottom liner systems significantly limits leachate recirculation
and fullscale bioreactor operations. In addition, traditional designs
require extensive coordination between specialized subcontractors,
resulting in long construction schedules and high costs.
solution
Recognizing this, Jones, Edmunds & Associates developed
a non-traditional bottom liner system with increased hydraulic performance
to meet the operational challenges of bioreactors and to allow for
a streamlined construction process. The innovative design features
a high flow triplanar geonet in a configuration that eliminates
the leachate collection lateral pipes, and a precast drop inlet
that collects leachate from both the leachate collection and leak
detection systems.
The drop inlet design eliminates the need for
pipe penetrations and their accompanying boots associated with more
traditional liner penetrations and seaming techniques. The overall
performance benefits of the new bottom liner design over traditional
designs include the significant increase in the hydraulic performance
of the liner system.
The form and properties of the triplanar geonet used in the leachate
collection system greatly improves the hydraulic characteristics
of the liner system. The robust collection system can more easily
accommodate increased hydraulic loading rates resulting from leachate
recirculation associated with bioreactor operations.
Another system
benefit includes the associated decrease in threat to the environment,
due to very low expected maximum head on the liner. Modeling results
show that less than 6 mm of head are expected on the liner. The
potential for leakage has been reduced by having only one liner
penetration and by improving the weld quality with the drop inlet
design.
The orientation and access of the drop inlet results in
high quality welds, which greatly minimizes the potential for leachate
leakage. Welding the penetrated geomembrane to the drop inlet permits
the weld to be tested, which is not possible with traditional pipe.
conclusions
Replacement of traditional collection laterals with a triplanar
geonet liner system has led to reduced construction complexity,
duration, and costs. The elimination of collection laterals greatly
simplifies the grading plan, and therefore enabled grading to be
accomplished by landfill personnel and reduces construction time.
In addition, the simplicity of the bottom liner system allowed owners
to perform major elements of the construction without risk of impacting
their contractors.
Another significant innovation incorporated into
the new bottom liner system, which allows a modular construction
process, is the development of a composite HDPE/concrete precast
drop inlet for leachate conveyance from within the lined cell.
The HDPE-lined top surface of the drop inlet easily facilitates
the equipment that is necessary to weld the landfill liner directly
to the top surfaces of the drop inlet. Significant reductions in
project duration (over 60%) and cost (over 25%) over traditional
bottom liner designs were achieved for these two projects.
The engineers of Jones Edmonds & Associates, in an effort to
develop a flat bottom design for a landfill at New River Regional
Landfill, were in search of a high flow geocomposite to incorporate
into the design.
TENAX TENDRAIN was determined to be the best solution
for use in the design due to TENAX TENDRAIN's high flow capacity
along long and flat slopes. TENAX TENDRAIN is also capable of maintaining
high flow under heavy loads.
The combined effect of simplifying the bottom liner grading plan
and eliminating the leachate collection laterals results in significant
cost savings to the facility owner. It provides an estimated 31%
reduction in construction costs, and an approximate 42% reduction
in construction duration.
The other benefits of this "low-head
high flow" design include: reduced potential for leakage into
the leak detection system and environment due to minimal leachate
head in the leachate collection system; reduced potential for construction
claims and owner liability due to simplified construction requirements
as well as separation of subcontractor work; and reduced potential
for leachate system failure and associated regulatory exceedances,
due to the entire triplanar geonet layer overlying the leachate
collection geomembrane functioning as the "conveyance"
channel, rather than relying on a small number of collection lateral
pipes.
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