Presented at: ANTEC 2001, Dallas, Texas, USA, 6-10 May 2001
Annually, the United Kingdom deposits around 20 million tonnes  and the United States around 2 billion tonnes  of waste into landfill. To protect the environment from the harmful effects of leachate from the waste, landfill sites are protected using a system of thermoplastic liners, typically made from polyethylene. Due to manufacturing limitations on the size of the lining sheets, welding is employed to join adjacent sheets at the landfill site. This paper reviews current welding practices, the industry approach to quality, and discusses the moves towards certification of welding personnel in order to raise standards across the industry.
Although recycling of household waste is on the increase in modern society, many countries still deposit large volumes of waste into landfill. The United Kingdom alone deposits around one third of a tonne of waste per head of population per year. In environmentally conscious societies, it is important that products, in the form of leachate, do not re-enter the environment through the landfill subsoil. Therefore, landfill sites, typically disused quarries, are lined with plastic sheet to prevent subsoil contamination and to collect the leachate. Collected leachate is usually decontaminated and returned to the water supply system.
The plastic lining systems used in the protection of landfill sites are usually made from HDPE (High Density Polyethylene)  , typically around 2mm in thickness. The sheets are generally manufactured in maximum widths of six metres and therefore need to be joined at the site during installation to cover the area being protected. Joining of the sheet involves welding, using either the electric heated wedge welding process or extrusion welding. Both these processes need a high degree of operator skill and monitoring to ensure the quality of the welds that are produced. Since there is no commercially available non-destructive technique to quantify the integrity of the extrusion  , dependence is placed on the operator to have a good understanding of the process and ensure the quality of the weld. It is, therefore, important to ensure that the personnel engaged in the welding of environmental lining systems can demonstrate their competence, preferably through an accredited welder approval scheme.
Welder approval schemes for the welding of environmental lining systems exist in Europe and United States. There is currently, at an advanced stage of development, a standard (CEN (a) Draft Standard prEN 13067  ) within the European member states to harmonise the approval testing of plastic welding personnel across a wide range of industries. Although, many industry sectors have been slow to embrace the spirit of the scheme, the environmental lining installation sector has been quick to recognise the benefits of introducing welder approval among its welding personnel. The benefits of welder certification include raising the level of competence, raising the standard of liner installation, individual job enrichment and reduction in the number of 'cost cutting' companies carrying out poor quality work at a low price.
(a) 'European Committee for Standardisation'
Welding and testing practices
There are two main techniques used in the welding of lining systems, wedge welding [3,6]
and extrusion welding. For the welding of long, continuous seams, the electric wedge welding process is usually used. Electric heated wedge welding is a semi-automatic welding process that uses a heated wedge to heat and melt the two surfaces of the lining system being welded whilst it moves along the liner seam. The operator is required to ensure that the equipment is operational, clean and at the correct welding temperature prior to commencing the welding operation. During welding it is important that the operator checks that the welding machine stays on the seam line. With electric wedge welding it is possible to produce either a single weld or a double parallel weld. The double parallel weld is achieved by the inclusion of a metal tongue on the welding machine to prevent welding at the centre of the seam. Figure 1
shows heated wedge welding of HDPE sheet.
Extrusion welding of environmental liners is generally carried out where a short length of weld is required, for example where a patch is placed over an area where a test sample has been taken. The extrusion welding process is a manual welding technique where the quality of the weld is dependent upon the skill of the operator. In this process a thermoplastic rod or granule feedstock is introduced into the barrel of a hand held extrusion-welding gun. As the material feeds through the barrel it becomes molten and is extruded from the front of the welding gun. A PTFE (polytetrafluoroethylene) nozzle, attached to the front of the gun, is used to profile the molten weld bead. A hot-air gun, attached to the side of the welding gun, preheats the weld. Figure 2 shows the extrusion welding process for a HDPE sheet.
In both welding processes, the operator sets up the equipment by performing test welds. These are typically a 1 to 2 metre strip from which 10 test specimens are taken and tested on site using either a tensile testing machine or manually operated pliers to test the specimen in shear or in peel.
During the installation of the landfill liner, routine testing of the welded seam is carried out to ensure quality. This takes two forms, a non-quantifiable non-destructive test 
or a destructive test [8,9]
The non-quantifiable non-destructive test is generally one of the following: a pressure decay test, a vacuum test or spark test. The pressure test can only be performed on wedge welded seams that have two parallel welded sections. Both ends of the double seam, possibly up to 100 metres in length, will be sealed using the hot gas welding technique and a small needle valve, with a pump attached, pushed into the centre un-welded channel. The channel is then inflated and the pressure monitored by the operator. A decay in the pressure will be evidence of a through weld width defect in the weld.
The vacuum test can be performed on both parallel and single seam welds. In this test, a box is placed over the welded seam and a vacuum applied. If there are any through weld width defects, or through thickness defects in the sheet, it will be impossible to maintain the vacuum and the equipment will detect a leak.
A third method, used only for extrusion welded seams, is the spark test. During the fabrication of the seam, a copper wire is embedded into the weld. To test the weld, a high voltage spark gun is moved along the welded seam. Where there is a through weld width defect, a spark will jump from the gun to the embedded copper wire.
Due to the lack of commercially available quantifiable non-destructive testing techniques, destructive tests are required. In a typical landfill lining construction, a site installation specification will detail the number of destructive tests to be taken from each welded seam for both peel tests and shear tests [8,9] . An on-site, independent, Quality Assurance engineer is responsible for ensuring that the company installing the lining system carries out the relevant tests. The engineer will generally be a geologist and is also responsible for ensuring that the subsoil meets the site specification. The destructive testing regime has the disadvantage of leaving holes in the liner where the test samples are taken. The holes then need to be repaired by extrusion welding a patch into position.
Welder Approval Testing
Approval Testing Scheme
Once a lining system is installed and covered with waste, there is little chance of repairing a leaking weld in the liner. Routine testing, as described in the previous section, ensures high standards of liner installation. It is also important to test the personnel who are engaged in the welding of the lining systems to ensure that they have a good understanding of the fundamentals of the welding processes. This is particularly important for those using the extrusion welding process that relies more on manual skill than the semi-automatic wedge welding process.
The CEN standard (prEN13067) being developed for the approval testing of plastic welding personnel defines the requirements for the qualification of plastic welders. The scheme aims to standardise, across Europe, the way in which plastic welders are tested for the fabrication of pipe work, vessels, structures and environmental lining systems. For the welding of lining systems, the materials and processes covered are listed below
- PVC-P (plasticized polyvinylchloride)
- PE (polyethylene)
- ECB (ethylene block copolymer)
- Manual hot gas welding
- Hot gas welding by machine
- Heated wedge welding (wedge heated by hot gas)
- Electric heated wedge welding
- Manual extrusion welding
Although prEN13067 has yet to be accepted by the European Member states, the United Kingdom has already in place the infrastructure for delivering the examinations according to the standard to the environmental lining industry. Additional to the three materials listed in the CEN scheme, the United Kingdom scheme includes the welding of PP liners. The United Kingdom scheme is run as part of an internationally recognised welder approval scheme; CSWIP (Certification Scheme for Welding and Inspection Personnel)  that is accredited to the United Kingdom Accreditation Service.
For the examination, the candidates are first required to demonstrate that they have had a minimum of two years experience in the installation and welding of environmental lining systems. The candidate must then undergo a theoretical training course and sit a multiple-choice examination. The examination is designed to test the candidate's knowledge in areas that cannot be tested in a practical examination, for example, how the liners are produced and what procedures to follow when working in wet conditions.
For the practical examination, the candidates weld test seams of approximately 2 metres in length using the chosen processes and materials. Typically, a candidate may choose to produce two test pieces, a manual extrusion weld and an electric heated wedge weld. At the start of the examination, the equipment would be switched off, allowing the examiner to assess the candidate's ability to set up the equipment from cold. Equipment set up consists of checking the condition of the machine and producing a pre-examination test weld to ensure that the machine parameters are correctly set. The candidate then produces the examination test weld for the examiner who will monitor the candidate's technique.
Welded samples produced during the test are sent to an independent test house for testing. Ten test specimens are taken from each welded sample with five being tested in shear and five in peel. It is required that the test samples fail in the parent material. The geometry used for the test sample in the CSWIP examination is shown in Fig.3 (according to ASTM D6392). The peel test geometry defined in draft CEN standard prEN12814-4 is shown in Fig.4. The results of the mechanical tests and of the multiple-choice examination are then reported to the candidate as a pass or fail.
Candidates are allowed a retest within 16 weeks of the original test date if the test is failed. Both sections of the test must be retaken and passed to gain the welder approval certificate. The certificate is valid for two years with a further two-year prolongation provided that the candidate can demonstrate continuous employment in landfill liner welding applications. At the end of the four-year period, the complete examination must be retaken.
In the United Kingdom the Environment Agency has overall responsibility for the safe function of landfill sites. Control of site procedures and specification of the landfill installation lies individually with the regional arms of the Agency. There are currently moves from within the United Kingdom lining industry to seek Environmental Agency approval for the welder certification scheme, demonstrating that the industry is taking the initiative in driving up welding and operator standards.
In the United States the International Association of Geosynthetics Installers has developed a similar scheme  , although there is no United States wide requirement to use approved welding personnel on the installation of landfill liners. The United States Environmental Protection Agency dictates minimum requirements for the design and installation of landfill lining systems across the country, with individual States adding additional requirements if necessary. The additional requirements may or may not include the need to use approved welders on the landfill liner installation.
The introduction of welder approval into the landfill liner installation industry can only serve to maintain or improve the quality of welded liner systems. The additional benefits will include greater operator awareness and job enrichment.
- BBC Panorama Programme - 26 June 2000.
- Peggs I D, Private communication - Nov. 2000.
- Kolbasuk G M, 'Hot Wedge Welding of HDPE Geomembranes', in book 'The Seaming of Geosynthetics', edited R.M.Koerner 1990, pp25-37
- Koener R M and Lord A E, 'Non-destructive Evaluation of Geomembrane Seams; Methods in development', in book 'The Seaming of Geosynthetics', edited R.M.Koerner 1990, pp151-163.
- PrEN13067:1999 Plastic welding personnel - Approval testing of welders - Thermoplastics welded asemblies.
- Ojeshina A O, 'Extrusion flat welding', in book 'The Seaming of Geosynthetics', edited R.M.Koerner 1990, pp15-24.
- Overmann L K, 'Geomembrane Seam Non-destructive tests: Construction Quality Control (CQC) Perspective', in book 'The Seaming of Geosynthetics', edited R.M.Koerner 1990, pp135-149.
- Peggs I D, 'Destructive Testing of Polyethylene Geomembrane Seams: Various Methods to Evaluate Seam Strength', in book 'The Seaming of Geosynthetics', edited R.M.Koerner 1990, pp125-134.
- prEN12814-4:1999 Testing of welded joints of thermoplastic semi-finished products - part 4: peel test.
- CSWIP-PW-6-96 - Requirements for the Certification of Plastic Welders - January 2000.