Geosynthetics April/May 2020 - 17
Construction water
flow dynamics of the
leak detection layer
By Abigail Gilson-Beck and Larry Shilling
T
he double-lined containment facility featuring a leak detection system (LDS)
is the most accurate way of assessing leakage through a primary geomembrane
(Bonaparte and Gross 1992). This type of system also represents the gold standard
for environmental protection, since leakage through the primary liner is collected
and removed so that the hydraulic head over the secondary liner is minimized. Along
with a double-lined system comes an action leakage rate (ALR). The definition of
ALR is "the maximum design flow rate that the LDS can remove without the fluid
head on the bottom liner exceeding 1 foot" (U.S. Federal Regulation 40 CFR 264.222).
Inherent to the definition as described in the governing documents, the fluid head on
the bottom liner is presumed to come from leakage through the primary geomembrane. In practice, conformance to the regulation is determined simply by the daily
quantity of liquid that is pumped out of the LDS. An ALR value is applied to a site as
a permit condition, expressed as gallons per acre per day (gpad) or liters per hectare
per day (lphd), and if the flow from the LDS is greater than this, a site will not be
allowed to operate. The case study detailed here provided a unique opportunity to
quantify the flow dynamics of construction water through LDS. In this instance, the
site's leakage exceeded the state-mandated ALR for roughly three months. In light of
the free-flowing gravel material used for the LDS, the slow weeping observed, which
was unaffected by rainfall, defied all logic. The urgency of the construction process
in order to get a site permitted to operate before a site's airspace is exceeded is shown
in Figure 1. Placement of the LDS material, electrical leak location (ELL) testing of
the secondary geomembrane, and installation of the primary geomembrane over
previously tested areas happened all on the same day.
On liner floors with less than 10% slope, regulations for the case study site require
a 1-foot (30.5-cm) thick LDS material. This 1 foot of material provides separation
from the primary and secondary liners such that if something were to penetrate the
primary liner, it would not likely also penetrate the secondary liner. This layer, commonly referred to as the structural fill layer, has always been required. Prior to the
new regulations, there was no minimum requirement for permeability. Generally, this
Abigail Gilson-Beck, M.S., P.E.,
is director of electrical leak
location services with TRI/
Environmental Inc. and is
based in Lansing, N.Y.
Larry Shilling is market area
landfill manager at Casella
Waste Systems Inc.
in Angelica, N.Y.
All figures courtesy of the authors
www.GeosyntheticsMagazine.com
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Geosynthetics April/May 2020
Table of Contents for the Digital Edition of Geosynthetics April/May 2020
Geosynthetics April/May 2020 - Cover1
Geosynthetics April/May 2020 - Cover2
Geosynthetics April/May 2020 - 1
Geosynthetics April/May 2020 - 2
Geosynthetics April/May 2020 - 3
Geosynthetics April/May 2020 - 4
Geosynthetics April/May 2020 - 5
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Geosynthetics April/May 2020 - Cover3
Geosynthetics April/May 2020 - Cover4
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