Geomembrane specifications from various sources are presented as they are obtained - by the regulating agency in each country, by national institutes, by trade association, and any other nationally recognized group. We also present specifications generated by industry-recognized leaders familiar with field problems and construction quality assurance practices, with brief explanations of the differences compared to the conventional specifications. It must be recognized that these latter specifications are likely more advanced and may not be accepted by regulating agencies.

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

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.

• Smooth sheet, English units
  
• Smooth sheet, SI units
  
• Textured sheet, English units
  
• Textured sheet, SI units
  

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).

• Smooth sheet
  
• Seams
  IAGI
  This specification was first published in April 2000. They use GRI GM13 specifications for the material. However, it has its own specifications for seams.

Bundesanstalt fur Materialforschung und - prufung (BAM) (1992/English - 1999/German).

• Smooth sheet
  
  1. General Physical Requirements
  2. Special Physical and Mechanical Requirements
  3. Requirements with Respect to Combined Physical and Chemical Attack
  4. Chemical and Biological Requirements for Geomembrane Seams
     
  BAM
  This specification is very detailed compared to the US specifications. The philosophical differences in approach of the two countries, not surprisingly, include different test methods, which in some cases give different results. For instance, stress crack testing in the same resins and sheet products gives higher break times by German methods than by US methods. Due care is required when comparing the different specifications.

I-CORP INTERNATIONAL, Inc. (2000) (USA)

• Smooth sheet
  
• Textured sheet
  
• Seams
  I-CORP
  This specification was developed after years of failure analysis and on-site CQA. Its major differences are in requirements for stress cracking resistance in both geomembrane and seams. Strength parameters are not required for seams since the low cross sectional area of the geomembrane tab prevents adequate challenge to seam bond strength if bond efficiency exceeds only 10 to 25% depending on geomembrane thickness. The geomembrane will always fail first.

Note that the conventionally used NSF 54 specification is no longer applicable since it was withdrawn in July 1997.

TEXTURED HDPE GEOMEMBRANE
Increased friction on HDPE geomembrane is generated in two ways - by random texturing and by specifically designed surface profiles.

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.

LLDPE

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
amorphous (noncrystalline) while LLDPE is about 5% crystalline. In comparison, HDPE geomembranes are about 50% crystalline. LLDPE's do not quite have the chemical resistance of HDPE, nor do they have the tensile strength, but they do have better elongation performance and stress cracking resistance. Hence their preferred use in landfill caps, for instance, where they may be subjected to deformation due to differential settlement of the waste - they will conform to the new profile without a significant influence as the long term performance of the geomembrane.

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
ductility LLDPE made using a manufactures metallocene catalyst technology.

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,
while lower density products have properties more approaching those of the old VLDPE. The major difference is in the multiaxial strain at burst. But also associated with
these differences are differences in the method of applying the pressure and measuring the strain in the multiaxial burst test. Whether the pressure is increased and the strain
is measured continuously or incrementally can significantly affect the results. After much initial discussion GRI GM17 was written to encompass the full range of LLDPEs.

PP

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
that you discuss actual properties with the different manufacturers.

BASELL SPECIFICATIONS (UNREINFORCED)

TYPICAL SPECIFICATIONS

(SINGLE PLY) REINFORCED PP GEOMEMBRANE 1 SPECIFICATIONS (TYPICAL)

PARAMETER	TEST MODE	VALUE
Thickness	ASTM D5199	± 10%
Density	ASTM D1505	< 0.090 g/cm3
Carbon content (1.)	ASTM D1603	2.5-3.0%
Carbon dispersion	ASTM D5596	Category 1 or 2
Tensile stress at break (2.)	ASTM D638 (IV)	> 18 MPa
Tensile elongation at break (2.)	ASTM D638 (IV)	> 600%
Puncture resistance	FTMS 101C (2065)	> 150 N/mm
Tear resistance	ASTM D1204
Dimensional stability	ASTM D1204	± 2%

1. black material only
2. gage length 50 mm, tested at 50 mm/min

PVC

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.