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GSI News, News | August 1, 2017 | By:

Commentary on geosynthetic specifications

One can only suspect that the first specification on a given manufactured material came along with or very shortly after the first manufactured product became available. It follows then that some 200 years later, manufactured geosynthetic materials should do likewise. In fact, so-called standards organizations have evolved by publishing the following items:

  • Test methods: definitive procedures that produce a test result
  • Guides: organized collections of information or a series of options that do not recommend a specific course of action
  • Practices: a set of instructions for performing one or more specific operations that does not produce a test result
  • Specifications: an explicit set of requirements to be satisfied by a material, product, system, or service

In this column, however, we focus completely on specifications, recognizing that they necessarily include test methods, and occasionally guides and practices as well.

With the above as a very brief introduction to specifications, such as those used for geosynthetic products, they can take significantly different approaches. Following are three types of specifications: proprietary, generic, and performance.

Regarding proprietary specifications, manufacturers should have one for every product they manufacture. It is to be expected that the listed test methods and their numeric property values will be acceptable to their manufactured product(s). However, such a specification can go much further in that the specific selection of tests and corresponding values prescribed can deftly exclude competitors manufacturing similar products. As such, proprietary specifications are excellent marketing and sales vehicles for a manufacturer’s specific products but of limited value to a broader audience such as an owner, regulator, or designer.

On the other hand, generic specifications attempt to provide a collection of test methods along with minimum (or maximum) values that meet the objectives of a particular application and are acceptable to several manufacturers of competing products. That said, if manufacturers set the bar too low, all such products will be acceptable; if they set it too high, it could be excessively restrictive such that no products are acceptable. Thus, it is a difficult task for the facilitator to strike a balance in crafting a valuable and useful specification. The selection of pertinent, but not superfluous, test methods and the numeric values ascribed to the designated test methods are fraught with compromise and negotiation. Furthermore, the updating and ongoing care of such specifications is important so as to keep current with industry developments and product enhancements.

For example, generic geomembrane specifications began under U.S. EPA stimulus in 1983 by the national Sanitation Foundation. They abruptly discontinued their activity in 1993, and GSI followed with its first generic specification on HDPE geomembranes in 1997. GSI currently has 18 generic specifications (9 GMs, 5 GTs, 1 GCL, and 3 GCs); see for more information.

While the general goal of generic specifications is to set a lower limit of product acceptability, they can often have the effect of not encouraging product enhancements by the associated manufacturers. In this regard, it can be said that generic specifications have a tendency to commoditize products for the specific application under consideration.

Performance specifications avoid the aspect of commoditization just mentioned. In this case an owner, regulator, or designer states a series of objectives for a particular application and allows an engineer or consultancy to design accordingly. An example of a performance specification is the U.S. EPA regulations for solid waste landfill leachate collection and removal systems (LCRS). It requires the following:

  1. The maximum leachate head above the primary liner of a landfill must be equal or less than 300mm.
  2. The time for leachate removal from the farthest distance of the cell to the outlet sump must be equal or less than 24 hours.
  3. The minimum grade of the landfill’s liner system must be at 2% or higher.

Such criteria leaves the design engineer complete freedom as to use any type of granular soils, any type of geocomposite drains, or even a combination of drainage soil and geosynthetics. (Conversely, the Umweltbundesamt Deutschlands [German Environment Agency or UBA] is prescriptive for this same application in requiring 450mm thickness of rounded, nonlimestone gravel of size between 16mm and 32mm diameter.)

This commentary on specification types was prompted by a discussion at the GSI Annual Meeting at the Geotechnical Frontiers conference in Orlando, Fla., on March 13, 2017. Several attendees, particularly from Europe, encouraged the use of performance specifications over generic specifications. Clearly, with an available population of knowledgeable design engineers and consultancies, this is justifiable and will result in many different solutions for a given application. On the other hand, for designers not familiar with the specific application, in whole or part, it can lead to chaos or even improper practice insofar as subsequent poor performance or failures. It should be mentioned that there are tens of thousands of civil engineering design consultancies in the United States. As long as a company has a professional engineer (PE) on staff, or otherwise available, it can make such a design and have it sealed for construction. Considering that there are very few geosynthetic design courses available within universities and there has never (to our knowledge) been a single geosynthetic question on any professional engineering examination, the use of performance specifications seems risky, if not outright dangerous, in light of the guidance and directions given in performance specifications.

It is our considered opinion that generic specifications play an important and positive role in all types of geosynthetic products. As such, they should be continued and possibly expanded. Of course, an important caveat is that such generic specifications are kept current with respect to evolving new products and a watchful eye on existing issues and nonperformance via field evaluations.

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