Sarasota County, Florida is located on the west coast of peninsular
Florida. This area is very flat, with natural slopes less than 1:2.000.
As expected, drainage is poor. Consequently the water table is very
near the surface of the ground during much of the year.
The project site elevation is about 6.1 m above mean sea level.
The Florida Department of Environmental Protection (FDEP) has established
extensive rules regarding landfill construction.
The Florida Administrative
Code (FAC) contains specific design criteria for composite and double
synthetic liner systems. Also included are requirements for maintaining
liner systems above seasonal high groundwater to protect the liner
components from the potentially harmful effects of fluctuating hydrostatic
The physical characteristics of the project site, coupled with regulatory
requirements, mandate that the entire liner area for the new landfill
be constructed above the natural ground elevation.
the excavation of large onsite pits to provide over 3 million cubic
meters of soil fill to construct access roads and elevate the landfill
area above the natural ground. To reduce the amount of fill required
and thus the construction costs for the facility, the amount of
slope used for the drainage system needed to be minimized.
The minimum design slope allowed for a primary drainage system by
Federal Subtitle D landfill regulations is 2 percent. The size of
each landfill cell also needed to be minimized to reduce construction
and operating costs.
Early in the design process, the decision was
made to eliminate penetrations of the liner system by using leachate
sump pumps in each landfill cell.
The sump pumps would remove leachate from the low point in each
cell continuously. Each cell would, therefore, have a dedicated
sump pump and related appurtenances. For the 24 hectares first phase
it was desired to use five cells to limit the costs for construction
and future operations.
To meet this requirement, each cell needed a maximum cell width
of 120 m. This results in a drainage layer slope length of 60 m
from each side of the cell to a central collection lateral. A 60
m long drainage layer slope is considered the maximum for practical
To achieve the most cost-effective landfill design from initial
construction through long-term operation, the primary design criteria
became maximum slope length and minimum degree of slope.
maximum slope length and minimum degree of slope, a highly efficient
drainage layer was required so that FDEP design criteria for the
allowable head on the liner could be met.
The original design calls
for a double layer biplanar geonet system. An evaluation has been
conducted to demonstrate that the tri-planar drainage net could
exceed the design requirement by over a factor two even when long-term
variables which may restrict flow were considered.
There is a considerable concern regarding the actual performance
of double geonet systems due to the quality of the installation.
Koerner and Hwu (1989) discuss the problem of reorientation of the
two layers such that they may fold into one another.
may occur if the panels are not aligned properly during construction.
The result of this condition could be a greatly reduced flow capacity
for the system. Another important consideration in using double
geonets is the low interface friction between the two layers.
the above stated reasons, this project used a single-layer of triplanar
geonet instead of two layers of bi-planar geonets.
The original design of this landfill in Florida called for two layers
of conventional drainage geonet in the Leachate Collection and Removal
System in order to meet the flow requirements of this landfill.
After extensive evaluation, the engineer discovered that two layers
of traditional geonet could be replaced with one layer of TENAX
TENDRAIN. TENAX TENDRAIN is a triplanar high flow geocomposite capable
of replacing two layers of geonet.
Use of the triplanar drainage
geonet core simplified field construction, minimized the potential
for construction problems caused by using two layers of drainage
geonets, and produced an efficient leachate drainage layer and critical
component of the total liner system.
The triplanar geocomposites have a structure with thick vertical
ribs, these ribs significantly increase the compressive resistance
and the tensile strength of the geonet. The vertical ribs are also
supported by inclined planar ribs that reduce geotextile intrusions
into the flow channel.
The triplanar geonet has both mechanical
and hydraulic advantages over typical bi-planar geonets, such as
high insoil flow capacity under high normal load, great compressive
strength, and high tensile strength. High tensile strength of a
geonet is an added advantage especially when the geonet is placed
on steep slopes.