Geosynthetics August/September 2020 - 8

UPDATE

Methods to estimate pressures and
strains in pond geomembrane bubbles
By Richard Thiel, Hesham Eldesouky and Richard Brachman

T

FIGURE 1 Bubbles in pond with 60-mil
(1.5-mm) HDPE geomembrane on verge
of bursting

Richard Thiel is the president of Thiel
Engineering in Oregon House, Calif.
Hesham Eldesouky is a post-doctoral fellow
in the Geo-Engineering Center at Queen's
University in Kingston, Ont., Canada.
Richard Brachman is a professor in the GeoEngineering Center at Queen's University in
Kingston, Ont., Canada.
All figures courtesy of the authors.

8

his article provides an update, validation and improved approach for predicting the shape, pressure and strains that can occur in an exposed geomembrane
bubble (aka "whale") created by gas trapped below
the geomembrane in a pond (Figure 1). The October/
November 2017 issue of Geosynthetics magazine presented an article by Thiel (2017) that provided analytical methods to predict the shape, pressure and strain
of gas bubbles that often occur in ponds with exposed
geomembrane liners. Two follow-up articles were
presented in the February/March 2018 and April/May
2018 issues of Geosynthetics magazine (Thiel 2018a
and 2018b) that discussed the design and operational
approaches for exposed geomembrane pond liners that
should be considered in light of the understandings
gained related to the mechanics of these bubbles. The
method was based on the assumption that the portion
of the bubble above the point of inflection was circular, while the portion below the point of inflection
was a clothoidal spiral. The stresses and strains in the
geomembrane bubble were estimated by summing the
forces in the vertical direction based on the derived geometry using an iterative
calculation approach to reach a solution. The method, which can be referred to as
an "approximate static-equilibrium model," was not able to account for the influence of the interface friction between the geomembrane and the subgrade, nor for
a realistic continuity of strain variations throughout the bubble and into its merge
with the subgrade. The details of the calculations were provided by Thiel (2016) in
a manner that could be replicated in a spreadsheet.
Recent finite element analysis (FEA) modeling of this problem was performed
by Queen's University in Kingston, Ont., Canada. The results of the FEA modeling
were found to be relatively close to the approximate static-equilibrium model and
provided a continuity in the strain along the bubble surface that continued into
the surrounding free-field geomembrane on the pond floor. The FEA results were
able to show the changes in bubble pressure and strain distribution depending on
the assumed interface friction between the geomembrane and the pond subgrade
material below the geomembrane.

Geosynthetics | August September 2020

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7/22/20 11:21 PM
Geosynthetics August/September 2020

Table of Contents for the Digital Edition of Geosynthetics August/September 2020
