Understanding Geomembrane Asperity

Q: Twice in the past few weeks colleagues asked me the same question about geomembrane asperity, so I come to you to understand it. 

Manufacturers tell us not to have a requirement regarding the “minimum of 10 readings must average specified height. Eight of the readings must be >= …”, or 

Delete the acceptance criteria for asperity that says: “The minimum average of each sheet must be greater than the minimum average as established by the interface shear results.” 

One manufacturer mentioned this criterion caused problems in a project in Ohio. Did GRI-GM13 have a requirement related to this in the past? If so, why has that changed? 

A: I think there is some confusion between core thickness (ASTM D5994) Standard Test Method for Measuring Core Thickness of Textured Geomembranes and asperity height (ASTM D7466) Standard Test Method for Measuring the Asperity Height of Textured Geomembrane. Asperity height has always been 16 mils (0.4 mm) minimum average in GRI-GM13. On February 20, 2006, Revision 7 added a footnote on asperity height clarification with respect to shear strength to the tables. On November 4, 2015, Revision 13 removed the footnote due to a protest by the manufacturers that GRI-GM13 is a Manufacturing Quality Control (MQC) specification and we should not reference a performance test in it. In short, clients were asking them to run direct shear tests on their geomembrane prior to shipment. This was felt to be way beyond the intent of the specification. 

In contrast, core thickness does have three criteria in the specification (minimum average thickness, lowest individual for eight out of 10 values, and lowest individual for any of the 10 values). This was a compromise, knowing the variability of many textured geomembranes. 

What really matters is the performance of the measured shear strength of the interface in question. All of this asperity conversation has a rough correlation with shear strength. This work needs to be done with site-specific parameters in mind by a third-party laboratory directed by the engineer of record.