construction and refurbishment of earthen irrigation channel banks

22.8.2 Membrane Liner TypesGeomembranes liners are manufactured from a variety of materials in a variety ofways and this results in a wide range of properties which can be varied to suit theneeds of a particular application. An ideal geomembrane liner would have the ultravioletand chemical exposure resistance characteristics of High Density Polyethylene(HDPE), and the flexibility and mechanical properties of polyvinyl chloride (PVC),in a thermally welded package. Unfortunately this ideal geomembrane is not yetavailable and it is necessary to work between a number of areas of compromise.In discussing the different types of geomembrane materials that are available theterms and descriptions that are in common use will sometimes be recognised bypolymer chemists and others as being less than strictly correct. These common termsare used below since the aim here is to convey meaning and understanding toengineers using the materials who are perhaps more familiar with construction usingsoils or concrete.The range of geomembranes available is extremely wide with new productsappearing regularly on the market from a range of manufacturers.The following are the more common types of geomembrane and a descriptivetabulation of properties is given in Table 22-6.22.8.2.1 PolyethyleneThe most common form of polyethylene used for liners is known as high-densitypolyethylene (HDPE) and is very similar to the material used in blackpolyethylene pipes. Strictly speaking it is based on a medium density resin butthe addition of carbon black tends to increase the density.HDPE is generally cost-effective over large areas but requires well preparedsurfaces.It has a very strongly crystallised structure, which contributes to its broadchemical resistance, and excellent UV resistance for which it is well known.However it is less well known for its lack of flexibility, its high coefficient ofthermal expansion and its brittle behaviour particularly in developing brittlestress cracking at low stresses.The base polymers have been developed to improve the Environmental StressCrack performance considerably in recent years but this is still an area worthy ofgreat care in design and seaming especially in the areas around extrusion filletwelds.HDPE seaming has embraced the hot wedge automatic welder which producesdual fusion track seams with great reliability and efficiency that can be readilytested by the air pressure test.These seams still require significant attention to seaming hygiene but are lessoperator dependent and create a less influential heat affected zone. The olderextrusion fillet welding method which creates a large heat affected zone is stillused mainly for details such as connections, T-junctions and patching. It requiresskilled operators for effective repair work in service.Construction and Refurbishment of Earthen Channel Banks August 2002 - Edition 1.0 22-43