Geomembrane Deployment Practice for Longterm Survivability
by Mark Cadwallader

Current thinking among geosynthetic/geotechnical engineers is that survivability of geosynthetics depends on three things:

1. Good material selection
2. Good design and
3. Good construction/installation.

Installers of geosynthetics impact the third survivability requirement mentioned above. Below are some key state-of-the-practice installation techniques that affect long term survivability of polyethylene geomembranes.

If we consider the various mechanisms by which polymer materials can degrade, the survivability limit for current technology and practice with polyethylene geomembranes is the development of stress cracking. Long term tension or compression stresses focused on points of concentration are threats of the lifetime of any material. Stress cracking is accelerated by high temperatures and by contact with certain surface-active substances. Yet if stress is not present, them by definition one should not have stress cracking! For this reason, knowledgeable engineers attempt to design and construct geomembrane facilities minimizing the amount of stress carried by the liner.

Stress cracks appear at points of stress concentration in the sheet, called "stress risers." Stress risers are typically any abrupt change in material thickness, any notch or score mark of significant depth, or any material nonuniformity, etc. From the installation perspective, the avoidance of scoring of the sheet and excessive abrasion during welding is therefore important.

Because stress is meant to be kept out of the liner, installation practice in the United States has sought to incorporate sufficient slack to avoid tension stress placed in the material. (see Footnote) The thinking here has been that bent-strip stress crack testing (like that which results from folding or bending a liner and holding it, eg, a wrinkle) is more easily accommodated by the material than is "constant load" stress crack situations (like the stress of sustained tension in a liner). Of the two approaches to stress crack testing, 1) constant strain (e.g., bent strip testing, by ASTM D1693), and 2) constant load (e.g., ASTM D5397 which sustains tension along a dumb-bell strip), constant load testing is the more rigorous on polyethylene. Both tests apply stress across a razor blade slit (a "notch" acting as a stress riser), to focus the point of stress. Because Notched Constant Load is more rigorous, ASTM D5397 is becoming regarded by engineers as the proper material performance stress crack test to specify.

ASTM D1693 (the traditional stress crack standard) simulates the bending and folding stresses that can be left in a liner. In this sense, it is a very relevant performance test. If the molecular structure at the bend or fold is able to relax through "stress relaxation" (i.e. align itself to assume the "U" shape of the bend) the stress is relieved. This happens in polyethylene because polyethylene has viscous flow properties, as well as elastic properties (a "viscoelastic" material). The molecules are able to "flow" into the bent condition, relieving the stress. For this reason ESCR values cited much in excess of 1000 hours for ASTM D1693 do not generally provide additional information about stress crack resistance. By that time stress relaxation has usually set in at the elevated temperatures of the test and the specimens should continue for many thousands more hours because the stress has dissipated.

The point is that bends at wrinkles in time undergo stress relaxation. And if the wrinkle (bend) is folded flat and creased, stress is also relieved because the material will have "yielded" at the outer surface. Either way, yielded or unyielded, stress is relieved in time so the impetus to crack is dissipated.

This knowledge should also be weighed with the fact that removal of covering soil and cutting and capping a wrinkle could risk potential damage of the liner and create new seams possible perpendicular to slopes. Problems could actually be created by trying to repair wrinkles, when the supposed problem is actually not very critical. Constant load-type stresses in the liner may occur in cases of extreme and long-term contraction. Field seams can behave similarly to a lab test "notch", focusing the point of stress. Therefore, severe thermal contraction which results in tension across seams can produce cracks at seams in exposed liners. This is why the thinking is that proper installation should eliminate those stresses.

Installation of exposed liner during hot weather must mean deploying excess material to compensate for thermal contraction which will occur as temperatures decrease. Proper installation also means that seams should as much as possible run parallel to the slope direction (to prevent tension stresses from occurring across seams).In addition, slope angles must be designed to be less than the friction angle of the liner in order for loads on the liner to be transferred through to the subgrade. Compressive loads on the liner should, by design, not be allowed to become tensile stresses in the liner.

What Constitutes a Significant Defect or Score Mark in HDPE?

Modern fracture mechanics technology in polyethylene gas transmission pipe has produced an empirical method for estimating lifetimes in polyethylene for stresses sustained across a notch defect at various temperatures. M.F. Kanninen of the Southwest Research Institute, San Antonio, Texas has published "A Methodology for Forecasting the Lifetimes of Geomembranes That Failed by Slow Crack Growth" extending this technique to HDPE geomembranes.(1) The technique has certain uncertainties associated with it, but it does provide an approximation of the significance of defect depth.

According to the empirical model presented by Kanninen, the difference between a 0.08 mm (3 mil) notch depth and a 0.03 mm (12 mil) notch depth is 20% in lifetime for a "no slack" initial condition and a temperature drop of 2 degrees C. The difference in a 3 mil notch depth and a 12 mil notch depth becomes 50% for a "no slack" initial condition and a temperature drop of 15 degrees C. Two conclusions stand out:

1. It is important to add slack (compensation) and to reduce liner temperature swings so that contraction stresses are eliminated or minimized.
2. Notch depth (score or scratch depth) is significant.

(1) Kanninen, M.F., et al, "A Methodology for Forecasting the Lifetimes of Geomembranes that Fail by Slow Crack Growth", Proceedings of Geosynthetics '93, Vancouver, B.C., March 30 - April 1, 1993.

Footnote: The American approach of encouraging slack differs from the German approach where liners are installed taught, removing wrinkles and depending on stress-relaxation to get past the initial constant load stress condition. Such strategy may work in Germany where buried liners are required, and where one does not encounter the extreme temperatures that American desert climates provide.