Landfill liners are used as a low permeability barrier which is placed under landfill sites. The liner must slow the rate of flow through it to ensure migration of leachate and its hazardous byproducts into underlying aquifers or nearby rivers, preventing contamination of local water resources. Landfill cappings or “seals” are also be used to retain the gases that are generated in landfills and enable the gasses to be used for the conversion of electricity, and even injection in the local natural gas grid.
PVC and butyl rubber was used in many recreational sites, waste pits, canals, water storage, floor coverings, plating, fuel, oil storage and containment, landfills and many other applications in the past. However, PVC Liners are not perfeerred due to inferior performance in hot conditions in waste sites, and worries about the plasticisers used, and the better alternative is HDPE.
It must be remembered that even the best liner and leachate collection system will ultimately fail due to natural deterioration, and recent improvements in MSWLF containment technologies suggest that releases may be delayed by many decades at some landfills. The US EPA has voiced concerns that we have been lining landfill in this way for no more than 30 years, and thus this lining method cannot yet have ben said to have faced the test of time. A number of lining installations have needed corrective action due to leakage found. So, although corrective action may have already been triggered at many facilities, 30 years may be insufficient to detect releases at other landfills. That is not a comforting thought for anyone, let alone landfill owners who may be forced to spend money on their liners in future years. Thta is why in Europe and much of the US, composite liners are used which combined a geomembrane liner with a clay based material. By doing this, if a hole develops there will not be a leak as there will also be clay below the HDPE as a further line of defence.
The U.S. has 3,091 active landfills and over 10,000 old municipal landfills, according to the Environmental Protection Agency. However, in the “good old days,” every town (and many businesses and factories) had its own dump. According to the 1997 U.S. Census, there are 39,044 general purpose local governments in the United States – 3,043 county governments and 36,001 subcounty general purpose governments (towns & townships). One suspects that there are many more old and abandoned commercial, private, and municipal dumps than the 10,000 estimated by the EPA.
The selection and design of the geomembrane components of landfill lining and cover systems are not simple matters if optimum durability is required. A designer cannot blindly take any material of a predetermined regulated (implying satisfactory) minimum thickness “off the shelf” and apply it for all systems, on all slopes, on all subgrades, with all leachates, in all environments, and for all landfill operating procedures. If such a practice is followed, and often it is, the liner is not ‘designed’ and will probably fail before its intended service life. And there have been a- significant number of geomembrane failures in landfills and liquid impoundments, mostly the latter.
Manufactured synthetic lining products have allowed for efficient design and construction of containment structures such as ponds, lagoons, landfills, tanks and secondary spill containment facilities, including vapour management systems. Because of the wide variety of project conditions and materials being contained and with so many liner types to choose from, the design and selection of the appropriate lining materials can be a complex task.
Geosynthetics are a cost-effective method for containing solid waste. Geosynthetic liners prevent leachate from leaking into the ground and subsequently contaminating ground water. Landfills can range in size so the dimensions and site conditions will determine the capacity of the impoundment and ultimately what the liner can maintain.
GCLs can replace compacted clay in liner systems required in many regulations. They effectively minimise system leakage by providing “intimate contact” with the overlying geomembrane.
This allows a GCL based liner to outperform a compacted clay-based system. Another popular design option is to combine a geomembrane, a GCL and a partial thickness of a low permeability soil to maximise both hydraulic performance and puncture protection.
Seismic behaviour of landfills is a big concern particularly when the landfills are located in the vicinity of densely inhabited areas. Following the Northridge tremor of 1994 in LA, Madabhushi ( 1994 ), one of the key fears was the dynamic behaviour of various Waste Containment Structures storing many varieties of toxic and non-poisonous chemicals. The post tremor behaviour of these structures need to be researched to establish their effective functionality over their full intended design life that may extend to a period of seventy to eighty years. To explain, post tremor integrity of the landfill liners must be guaranteed.
The seepage thru landfill liners either simple clay liners or even more modern multi-layer systems using geomembranes, geotextiles and GCL’s has been well analyzed by Rowe ( 1998 ) and more lately by Foose et al ( 2001 ). However these studies concentrate on either the natural contaminant migration rates through intact barrier systems or accelerated rates due to defects in liner systems and imperfect contacts. But during earthquake loading we are rather more nervous about the integrity of the landfill liner rather than the defects in fabrication of the liners.
The best landfill liners today are made from a tricky plastic film called high density polyethylene ( HDPE ). HDPE has only been used in this country for this purpose since the early 1980s, so there’s not much actual long term experience to go on. Nevertheless landfill designers assure that HDPE resists attack by just about all chemicals.
Resistance to chemical attack is significant as the theory of landfill design announces the landfill liner must maintain its integrity for the length of the hazard it is meant to contain. If the rubbish in the landfill will remain noxious for centuries, the landfill liner must maintain its integrity for centuries , if the liner fails before the danger has gone away, the failed liner will allow the danger to flee, and we’re going to have simply passed today’s problem onto our youngsters and grandkids.
Desiccation of clays can cause the development of cracks and this is a very important issue for mineral barriers utilized in lining and capping of landfills. The mechanisms of post-compaction moisture loss in a clay layer are reviewed and the results from a field test cell investigating clay desiccation are presented. A giant scale instrumented test cell was created comprising a clay layer overlain by a geomembrane. Parts of the test cell were covered with a stone protection layer and parts of the test cell were sloping. Temperatures throughout the clay layer, relative humidity just above the clay, ambient air temperatures and crack patterns and dimensions wereobserved during a 36 day summer period. The amount of cracking was appraised using diverse crack indices. Cracks in the covered and flat sections were minimal whereas in the sloping revealed portion cracking was instituted rapidly and then spread till the network of cracks at the surface stabilised and further drying ended in crack depth propagation. The geomembrane, exposed to the sun, was found to possess a significant heating effect on the soil below. Heavy moisture was lost over the entire thickness of the exposed barrier leading to a modelled flow thru the barrier of nineteen times the construction value . Swift covering of the geomembrane was found to seriously cut back the temperature differentiations, moisture loss and cracking. The results are discussed in the context of current crack theories and implications for landfill design and construction.
The landfill liner system and its components, including the HDPE ( high density polyethylene ) liner are built to provide maximal protection to the environment. Installation of the liner system is supervised and checked to make certain that it meets or surpasses environmental standards.
Preparation of the current ground surface is performed prior to development of the liner system to gain the design subgrade configuration and remove rocks that could penetrate the liner. 2 feet of compacted clay or a geosyntheic clay liner ( GCL ) is then placed over the prepared subgrade surface and the 60 mil HDPE liner is installed over the clay or GCl material. Eventually, a drainage geocomposite is placed above the HDPE liner. The HDPE liner is built, installed, tested and mended as per authorized guiding principles. All manufacturer’s recommendations and installation procedures are overseen by independent Quality Guarantee personnel. Prior to implementation, Quality Assurance / QC ( QA / QC ) advisors authorized that the geosynthetic materials have been correctly installed.
A Landfill liner may be comprised of many differing kinds of material but the end objective is always the same regardless of the trail taken. The landfill liner is the first defensive position against water leaching into the ground water ( base landfill liner ) as well as rain water intruding into the landfill solid waste area. See also www.landfill-site.com/html/liner-history.html .
The landfill liner can be made from clay, synthetic flexible membranes or extra thick plastic sheeting. The membrane is there to keep the waste water from leaching into the ground water. These methods of sealing the base of the landfill are all high-priced anf take time to complete.