Thickness Test Methods Used For Polyethylene Geomembranes
by George Yazdani, MSc., P.E.

The following is a brief summary of the test methods used to measure the thickness of polyethylene geomembranes.

• ASTM D 1593: Take five, 4"x 4", specimens uniformly spaced across the width of the roll. Calculate uniform thickness based on specimen weight, density, and size. Poly-Flex reports thickness based on this test method.
  
• ASTM D 374: Use a 1/4" diameter dial head micrometer. Number of measurements is not specified.
  
• ASTM D 751: Use a 3/8" diameter presser foot micrometer. Measure 5 or more places uniformly spaced across the roll width.
  NSF modified:Use a 1/4" diameter presser foot micrometer. Measure every foot across the width of the roll.
  
• ASTM D 5199: Use a 1/4" diameter presser foot micrometer. Measure 10 specimens randomly distributed across the roll width.
  
• ASTM D 5994: Use a 1/16" diameter rounded tip micrometer. Measure 10 places randomly spaced across the width of the roll. This test method replaced GRI GM8 for measuring the core thickness of textured sheet.

Typical Single Point Thickness Measurements Using Different Test Methods:

Thickness Requirements According to GRI GM13 Standard:

• Smooth Liner, ASTM D 5199:
  Minimum average value (nominal thickness)
  Lowest individual reading of 10 values (-10% of nominal)
  
• Textured Liner, ASTM D 5994:
  Minimum average value (-5% of nominal)
  Lowest individual for 8 out of 10 values (-10% of nominal)
  Lowest individual for any of the 10 values (-15% of nominal)
  

Terminology For Polyethylene Geomembrane Specifications
by George Yazdani, MSc., P.E.

The terminologies used to specify the properties of polyethylene geomembranes are defined below.

Nominal Value:
The Poly-Flex nominal value is based on the historical average lot property data taken over a period of time. Nominal values are for informational purposes and should not be used in project specifications.

Minimum Average Value:
Minimum average value designates the lowest acceptable property value for an individual roll. For example, ASTM D 638, Tensile Properties of Plastic, calls for at least 5 specimens to be tested in machine and cross machine directions. The minimum average tensile value is the lowest average value of the 5 specimens for any individual roll. Poly-Flex and the Geosynthetic Research Institute, i.e. GM13 Standard, use this terminology. This terminology should be used in project specifications.

Lowest Individual Reading:
This refers to the lowest value for each test specimen and is often used for thickness. For example, a specification may indicate a minimum average value of 60 mils and a lowest individual reading of 54 mils for liner thickness.

Minimum Value:
Minimum value is confusing since it can be interpreted as either the minimum average value or the lowest individual reading. It should not be used unless it is clearly defined. Poly-Flex interprets minimum value as minimum average value.

Minimum Average Roll Value (MARV):
MARV is commonly defined as the average test results minus two standard deviations and implies that approximately 97.5% of the test results meet or exceed this value. MARV can be misinterpreted since it is sometimes confused with the minimum average value as defined above. MARV requires a normal distribution of data, which is not the case since rolls not meeting Poly-Flex minimum average values are not included in the data. To calculate MARV, an independent laboratory must test a large population of rolls, making conformance testing time consuming and expensive. Resolution of any disputes between the manufacturer's quality control certification and the conformance testing can also be time consuming and expensive since retesting of all rolls may be necessary. For these reasons, MARV should not be used for polyethylene geomembrane properties. The minimum average value is preferred since it is much easier to qualify one or two rolls of material without having to produce a large data bank.