Slack calculations

Geosynthetics | April 2008

GMA techline answers from Bob Koerner

Subject: Slack calculations — Do you have a reference that discusses the impact of environmental conditions and the procedures for determining “slack” into an HDPE liner at the time of installation? (Rod, California)

Reply: The subject of calculating slack in geomembranes for the purpose of backfilling, and then having the lower backfilled temperature bring the GM back into a nonwrinkled condition, is something I am not proud of at this time. Many years ago, I had written on this possible scenario. While the calculation is simple (it is the coefficient of linear contraction/ expansion of the resin, times the length involved, times the temperature difference), it really doesn’t matter, since no amount of entombed wave will disappear upon backfilling. In other words, one cannot backfill entombed waves or wrinkles of geomembranes. This has subsequently been shown in the thesis of Te-Yang Soong, and he and I have published widely on the subject. Even waves as small as 14mm in height do not get straightened upon backfilling.

Thus, backfilling must be on completely flat geomembranes. No exceptions. In turn, this often requires backfilling during early morning hours or even at night. There are other schemes, but all have extra costs involved.

Robert M. Koerner, Ph.D., P.E., NAE, robert.koerner@coe.drexel.edu.

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