Why Weight and Thickness are Not the Final Specs for Geomembrane Liners
Tom PerkinsApril 10, 2020Geomembranes
The best advice we can give to anyone contemplating a geomembrane pit or pond liner is to talk to us before writing the final project specifications. This is important because the tests and standards established to measure and specify the characteristics of various geotextiles may not by themselves be adequate to meet the overall requirements of the project.
Shipping to the field and installation in the field are two huge factors in any project. Relying on specifications of permeability and puncture resistance using materials-test standards alone can result in choosing the wrong material for the project, and ultimate project failure.
While all geotextiles have a nominal thickness, for example, when created at the factory, real-world conditions such as packaging, shipping and load weight can impact the characteristics of a geotextile material. Furthermore, and most crucially, the choice of material will also dictate size of panels and the number of welds required in the field to create the final size of liner.
Our knowledge of industry practice suggests that, while a puncture from rocks and other objects penetrating the material can occur from a careless installation, by far the most likely source of failure will be a seam breaking under load from an inadequate panel weld.
What matters in most liner projects is durability; a geophysical project is exacting enough to install in the first place, no one wants to have to dig it up and replace it! One of the best measures to assure durability in real terms may be the warranty provided by the manufacturer and installer. At Western Environmental Liner, we always strive for a 20-year warranty as our benchmark when writing specifications.
While soil has accrued very dependable engineering standards through testing and calculation over the years, applying these same measures to geomembrane and geotextile materials is not quite so straightforward. Soil exists and remains in place, and its characteristics can be reliably estimated. Geosynthetic materials are the variable being brought into place, and this makes a difference.
Some Standards for Geotextile Choices
One of the common standards used to compare different materials for use in a liner project is the ASTM D4833 puncture test, which takes a small piece of material and drives a probe through it using a measurable force. This results in a PSI standard for the material. This test is used for HDPE and as ASTM cautions, this standard may not be appropriate for some woven geotextiles.
In Western Liner’s woven RPE products, we find the tightness of the weave is the greatest factor for puncture-resistance, rather than material coating, and we’ve developed the tightest weave of any comparable geotextile in the industry. We do include the D4833 measurement as one of our RPE characteristics.
The CBR test, D6241, is another standard used for puncture resistance and measured in PSI, although it’s more of a strength test than a puncture test. Various tensile strength tests are also included such as the D7004 and D7003 ASTM standards.
The tests for permeability standards all depend on nominal thickness of the material, and this is problematic because as we stated above, the resulting thickness in the field can be different after transportation stress is incurred. So the engineering calculations based on the written specifications may not hold true in the final result. The test for permittivity, ASTM D4491 is probably the most reliable benchmark for fluid flow through a geomembrane
Exceeding the Geotextile Standards
To get a sense of our material characteristics, consider our Aqua Series RPE geotextiles. Using the 12 mil thickness for comparison, this material beats all the ASTM standards for string reinforced geotextiles. It offers more than 50% greater tensile strength and more than double the puncture resistance in both the D4833 and the CBR tests.
And our Aqua Series geomembranes are almost 60% lighter! This last quality – weight – of course has a direct bearing on how it can be shipped to the field, and how easily it can be installed when welding panels together in the field. The tendency in project design is to aim for the heaviest and thickest material, but we offer a greater effectiveness with a product designed lighter and thinner, and yet stronger.
The point here is that while the Western Environmental Liner materials are created to exceed ASTM standards, we still don’t want to specify any project on engineering specs alone. It’s crucial to study the project holistically in all its aspects – including the non-measurable ones such as shipping and installation – before writing the final specifications.
What we can conclude from this brief overview of some of the testing standards at work in writing the specifications for a liner project is simple, and where we started: talk to us first, before the engineering specifications are written up. Get our suggestions, and then write the spec for a successful project result, fully warranted for durability.