problem
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 pressures.

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. This required 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 design purposes.

solution
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. To accomplish 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. This condition 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. For the above stated reasons, this project used a single-layer of triplanar geonet instead of two layers of bi-planar geonets.