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 TENAX DRAINAGE NETS |
SCHEMATIC EXAMPLE OF LANDFILL APPLICATION
Each day, million tons of municipal and hazardous wastes are disposed of in landfills or other land disposal sites. A landfill is a large area of land or an excavated site that is specifically designed and built to receive wastes.
A typical modern landfill is lined with a layer of compacted clay and synthetic geomembrane to prevent the waste and leachate from leaking into the ground or ground water. Geocomposite drains and aggregate material are used at the sides and bottom of the landfill to collect the leachate that flows through the decomposing waste. The leachate is then collected sent to a leachate recovery facility to be treated.
Municipal Solid Waste System
Minimum liner system
Conventional design |
Municipal Solid Waste System
Minimum liner system
Geosynthetics design solution |
While landfills that accept municipal solid waste have typically a layer of clay and synthetic liners and a leachate system to prevent leakage, for hazardous waste landfills extra precautions must be taken. For example, a hazardous waste landfill must have two impermeable liners layers, one consisting of a geomembrane (Primary liner), and the other composed of both a geomembrane and a thick layer of compacted clay material (Composite Secondary liner).
In addition, a landfill accepting hazardous waste must have two leachate detection systems: the upper layer for Leachate Detection and Removal System (LCRS) and lower layer for Leak Detection and removal System (LDS). Leaks of leachate from the primary waterproofing liner can occur fundamentally for two reasons: defects in seams of the geomembrane or damages to the liner during placement and compaction of waste materials.
Hazardous waste landfill
Minimum liner system
Conventional design |
Hazardous waste landfill
Minimum liner system
Geosynthetics design solution |
When a landfill reaches capacity, it is usually capped to ensure that rainfall will not infiltrate the surface. The leaching of the rainwater through the landfill could cause contamination of the groundwater, and nearby surface waters. Thus covers are always required on completed landfills. A composite cover, in which use has been made of soils and synthetic materials, can effectively eliminate infiltration but must be carefully designed to take care of considerations such as slope stability, long-term degradation and erosion.
In addition significant amounts of gas can be produced from decomposition of waste and an appropriate Landfill Gas Venting system (LFG) shall be designed. Methane extraction system wells are used to collect landfill gas. The gas may be used to generate electricity on site, burned at flare locations on the landfill, or used in the sludge combustion process. Once a landfill is capped (closed), the site shall be monitored for gas and leachate for a minimum of 30 years after the closing date. After a landfill is capped, the land may be used for recreation sites such as parks, golf courses.
Typical landfill cover system
Conventional design |
Typical landfill cover system
Geosynthetics design solution |
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Landfill: Leachate Collection and Removal System (LCRS)
The leachate collection and removal systems (LCRS) is commonly the only lateral drainage system provided in a MSW landfill and serves as the primary means for leachate collection and removal. The LCRS systems are installed on top of the primary liner and therefore within the landfill core.
The leachate collection system must be capable of providing adequate flow capacity for the life of the landfill. Traditionally, natural materials, mainly gravel, have been used, but these materials take up a great deal of valuable airspace and need a heavy cushion layer to protect the underlying liner, in addition it can be very costly if quality gravel is not readily available and needs to be transported long distances.
Sarasota County Landfill, Florida (USA): TENAX GNT Geonet for Leachate Collection and Removal System.
TENAX TENDRAIN and TENAX GNT are high flow triplanar drainage geocomposites and geonets specifically engineered for leachate collection and removal system (LCRS). TENAX TENDRAIN and TENAX GNT have three structural ribs, the top and bottom ribs keep the filter geotextile layer out of the central flow channel while the center structural ribs provide separation and create flow channels. Moreover, the significantly larger central ribs run parallel to the flow direction, resulting in tremendous hydraulic performance, especially for long term flow requirements under heavy compressive loads.
TENAX TENDRAIN assures a leachate collection and removal system which can withstand heavy compressive loads as well as maintain the flow capacity necessary to limit the head within the drainage layer. In fact conventional geocomposites may experience a reduction in its drainage capacity due to compressive creep, caused by sustained normal load, and biological and chemical clogging caused by the leachate. Bi-planar geocomposites, fall short of this goal due to lack of resistance to high compressive forces and excessive geotextile intrusion. In addition, bi-planar geonets direct leachate flow at an angle, which requires the leachate to weave back and forth in order to follow the flow direction. This dissipates flow energy and may enhance clogging.
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Landfill: Leak Detection System (LDS)
The purpose of the leak detection layer is to determine if (and to what extent) leakage is occurring through the upper or primary liner, and to provide a mechanism for removing liquids that enter this layer. The LDS systems are installed between the primary and the secondary liner.
A leak detection system must:
- provide rapid detection of a major breach in the primary liner system
- limit the head acting on the secondary liner to less than the thickness of the LDS.
New Hanover Landfill, North Carolina (USA)
Achieving this efficiently is the challenge. TENAX GNT high flow triplanar geonets provide the most efficient results for rapid detection of leaks in the overlying liner systems. Geonets have very limited storage capacity and a high transmissivity. The combination of these characteristics means that the geonet will not store fluids, but move them quickly through the net to detection locations, therefore minimizing the time between leak occurrence and detection. To ensure unconfined flow and increase the Action Leakage Rate (ALR) High Flow Triplanar geonet systems are traditionally used in an attempt to maximize flow and reduce head. TENAX GNT and TENDRAIN provide an ideal geosynthetic solution. TENAX TENDRAIN is a triplanar geocomposite with great flow capacity. Triplanar geonets are also extremely resistant to compressive forces, even under the heaviest loads. With TENAX TENDRAIN as the LDS, only one layer of geonet is required to limit head and provide rapid fluid removal, which reduces the costs associated with the installation of other drainage system.
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Landfill: Surface Water Collection and Removal System (SWCRS)
Landfill capping are typically waterproofed using either a geomembrane or a geosynthetic clay liner (GCL). Infiltrating rainwater flowing over the liner drastically reduces the coefficient of friction at the soil/membrane interface producing, in addition, overpressures that can certainly promote slippage of the soil layer over the covering system. TENAX TENDRAIN geocomposites, properly installed on top of the waterproofing layer, shall be selected to reduce the amount of rainwater that might reach the liner interface.
Cerro Maggiore (Italy):
Aerial view of the final closure of the municipal
solid waste landfil |
Cerro Maggiore (Italy): Landfill capping
1: TENAX geocomposites for rainfall drainage
2: TENAX geocomposites for biogas drainage |
Surface water infiltrating through the cover soil will accumulate above the barrier layer and generate detrimental pore water pressure if it is not properly drained. Excessive head buildup due to inadequate drainage can cause catastrophic failure of the cover, numerous seepage-induced landfill slope failures have been recorded and analyzed to confirm this.
TENAX TENDRAIN geocomposites and TENAX GNT provide the highest flow capacity among any geonet. The long-term in-soil transmissivity of TENAX TENDRAIN is in the order of 6 x 10-3 m2/s at a gradient of 0.1, and 4 x 10-3 m2/s at a gradient of 0.33. Long-term (10,000 hour) testing indicates that TENAX TENDRAIN maintains over 90% of its thickness when exposed to long-term compressive loads. This retained thickness maintains an open flow channel into which geotextiles are kept from intruding and rain water is free to flow.
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Landfill Gas Venting (LFG)
Landfill gas (LFG) pressure underneath a lined cover system can significantly reduce the effective normal stress on the liner, which can affect cover soil stability. Large-scale landfill cover slope failures have been recorded over the past decade to be directly attributed by an inadequate LFG venting layer. According to recent research, based on intrinsic permeability theory, into gas transmission rates, the rate of LFG transmissivity is ten times lower than the hydraulic transmissivity in any porous media. In the past, however, this relationship was believed to be inverse, i.e. the air transmissivity was believed to be 100 times greater than the hydraulic transmissivity. The resulting miscalculations significantly under design the required transmissivity of the LFG venting layer, which in turn may cause landfill cover slope failures. This reveals the great need for a high flow geocomposite layer for gas venting design.
TENAX TENDRAIN and TENAX TDP geocomposite have at least 2-3 times the transmissivity of conventional bi-planar geocomposites, whether fluid or gas transmissivity. This high performance geocomposites allow for greater compatibility when variances occur that generate higher amounts of gas, necessary to prevent pore pressure build-up, and ensure cover soil stability. This allows for wide levels of gas venting to occur preventing pore pressure build-up and ensuring cover soil stability.
TENAX TN and TENAX TNT geocomposites are conventional drainage geosynthetics and are suggested for low critical design situations. |
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