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Title: Geomembrane
Material Specifications
Written by: I-CORP |
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We acknowledge the fact that all materials are not yet represented, but we are working to make the list complete. (Items underlined are linked.)
HDPE resins are made by many manufacturers using different manufacturing processes. Each adds different comonomers to the HDPE to modify its molecular structure, and different additive packages to protect against oxidation and UV radiation, thus there are many HDPEs. However, all their conventional mechanical and physical properties are very similar. They differ only in one respect - their resistance to stress cracking. And this difference can be a factor of 500 to 1000. Therefore, this is the single-most important parameter that must be specified. The AO and UV additives are consumed during service, but they are present in sufficient quantities so that, without abuse, the geomembranes will last far longer than is practically required. Properly designed, manufactured, specified, installed, CQA'd, tested, and operated, an HDPE geomembrane should have a lifetime of a few hundred years. Four sets of HDPE specifications are presented: Geosynthetic Research Institute, GRI GM13, Index Revision 2, generated in 1997.
Note that this standard was developed strictly for quality control purposes, and was not intended to be a material specification, although it undoubtedly serves as one. Note especially the frequency of QC testing - it depends on the parameter being measured - there is no requirement to test all properties at the same frequency. The same consideration should be extended to CQA conformance testing specifications. There are no seam specifications in this standard. International
Association of Geosynthetics Installers (2000) (USA). Bundesanstalt
fur Materialforschung und - prufung (BAM) (1992/English - 1999/German). I-CORP
INTERNATIONAL, Inc. (2000) (USA) Note that the conventionally used NSF 54 specification is no longer applicable since it was withdrawn in July 1997.
TEXTURED
HDPE GEOMEMBRANE The random texturing is created in one of two ways - integrally with the geomembrane manufacturing or as a secondary process. In the former, nitrogen gas is pumped into the PE in the outside layer extruders of a three layer coextrusion system. As the geomembrane exits the extruder and the external pressure is reduced the gas explodes to the surface thereby roughening it like a turbulent ocean. The minimum thickness of the resulting sheet is essentially controlled by the thickness of the core extruded layer. In the secondary process, pulverized PE (usually LDPE) is dropped onto the hot surface of just-extruded or reheated geomembrane. The lower melting point of the LDPE results in it melting and welding to the surface of the HDPE as a small blob. Alternatively, small droplets of molten PE can be "sprayed" on to the surface of the HDPE. The minimum thickness of these geomembranes remains the same as if the material were smooth sheet. With secondary process texturing, a balance between sufficient bonding to minimize the ability to scrape off the texturing and over-welding that might induce residual stresses and susceptibility to stress cracking adjacent to the weld is required. In primary process texturing, the potential for stress cracking is reduced. The texture cannot be removed, but minimum thickness is much less predictable. In structured surfaces the hot extruded geomembrane is squeezed between calendering rolls, one or both of which are profiled to produce the surface geometry as the geomembrane thickness is also established. Such profiles cannot be scraped off, there is no welding associated with them, residual stresses can be eliminated by careful design, texturing is consistent over the surface of the geomembrane, and structures can be designed for optimum interaction with different adjacent materials. Thus, all textures/structures are not equal. Velcro-type hairs will interact better with a nonwoven geotextile than will a smoother structure, for instance. Some will stick better to clay, others to geotextile. Some are designed as a compromise for every possible mating surface - geotextile, clay, sand. Some are textured on one side, some on both sides. However, in the finished lining system they should be "stickier" on the bottom than on the top in order to keep a permanent stress out of the geomembrane. If sliding on a slope is going to occur, it is generally better that sliding occur on top of the liner so the liner remains intact. The required veneer stability in the soil above a smooth liner can be obtained with a woven geotextile or a geogrid.
Linear
low density polyethylene (LLDPE) is a linear microstructure version
of the LDPE commonly used for packaging and in construction as a
vapor barrier. LDPE is Two
standards for LLDPE are the new (2000) GRI
GM17 specification, and
a manufacturer's typical specification. Also included is specification
for mPE, a new high strength, high Note
that LLDPE geomembranes are made with resins with a fairly wide range
of density - higher density products have properties approaching
those of MDPE/HDPE,
At present PP geomembrane resin is made by only one manufacturer, Montell, although others are now looking at it. There are only two or three slightly different modifications to the basic resin that produce geomembranes with slightly different properties, predominantly in the melting point. PP geomembranes contain similar AO and UV additives to HDPE. When
PP is reinforced, the properties become a function of the type of
reinforcement. Typical specifications for a reinforced material are
presented. However, we recommend BASELL SPECIFICATIONS (UNREINFORCED)
TYPICAL SPECIFICATIONS (SINGLE PLY) REINFORCED PP GEOMEMBRANE 1 SPECIFICATIONS (TYPICAL)
PVC resins are made by several manufacturers, but the performance of a geomembrane is controlled more by the plasticizer (approx 30%) that is added than the resin itself. The plasticizer gives the geomembrane its flexibility. Liquid plasticizers can be volatilized or extracted out of the geomembrane, hence the requirement that they be covered. However, the PVC can be plasticized or alloyed with another polymer (KEE, ketone ethylene ester) to form ethylene interpolymer alloy, known by several different trade names. Such geomembranes have good weathering performance. See PVC Geomembrane Institute's (PGI) 1104 specification for conventional PVC geomembrane.
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