Geosynthetics April/May 2020 - 18

Construction water flow dynamics of the leak detection layer

The first suspect for the
observed leakage was
naturally leaks in the
primary geomembrane.
ELL testing of the
primary geomembrane
was performed as part
of cell construction,
per ASTM D8265. The
method was performed
during active rainfall
with extreme sensitivity,
and a very small (~1/8
inch [3.2 mm]) knife
slice was found.

layer was an on-site soil material placed at
95% compaction. The soil layer, once covered with the primary liner, would only
weep water into the secondary collection
system when weight was added and the
water was squeezed out of it. The newer
regulations require a minimum permeability of 1 × 10-2 cm/sec. Generally, this
requires a large sand or small pea gravel
type of material, which is intended to
release water more quickly.
The first suspect for the observed leakage was naturally leaks in the primary
geomembrane. ELL testing of the primary
geomembrane was performed as part of
cell construction, per ASTM D8265. The
method was performed during active rainfall with extreme sensitivity, and a very
small (~1/8 inch [3.2 mm]) knife slice
was found. Regardless, the method was
performed again when the persistent leakage continued for more than a month after
the end of cell construction. No further
leaks in the primary were found. In addition, dye testing was performed, confirming no leakage through the primary. This
comprehensive testing, along with the lack
of leakage response to rainfall, shifted the
authors' focus to other potential culprits.
Construction water has been pointed
out as a potential cause for exceeding a
site's ALR before (Gilson-Beck 2019). The
flow attributed to construction water can
be quantified when the soil properties
of the leak detection layer are known,
along with the moisture content of the
LDS material once encapsulated by the
overlying geosynthetics. If the material
is placed wet (or experiences rainfall),
and the moisture content is greater than
the material's specific retention (Sr), then
there will be drainage of construction
water from the LDS material. At this site,
the ELL testing of the secondary geomembrane was also performed during active
rainfall, with additional rainfall in the days
following the testing, immediately before
the LDS was covered by the overlying

geosynthetics. Specific retention quantifies the portion of water that will remain
attached to the material particles and will
not be released through drainage by gravity. The counterpart of Sr is specific yield
(Sy); this is the portion of water that will
drain by gravity. If the porosity (ø) of the
material is known, either can be calculated
if the value of one of them is known using
the equation: ø - Sy = Sr.
Construction water flowing to the
LDS has been the bane of engineers of
double-lined systems since their inception. A set of equations authored by J.
P. Giroud was presented 30 years ago to
estimate both the quantity to expect from
the LDS and the flow rate (Gross et. al
1990). With these equations, one should
be able to predict how long to expect
the drainage to last if one obtains values
for the volumetric moisture content at
the time of material placement and the
specific retention of the material along
with the hydraulic conductivity and the
design-specific drainage geometry.
In order to compare the results of the
equations to what was being collected
from the LDS at the case study site,
soil testing was performed on the LDS
material to augment the typical testing
already performed on the material as part
of cell construction. The equations predicted that a total volume of 33,986 gallons
(128,651 l) would be released from the
3.5-acre (1.4-ha), 1-foot (30.5-cm) thick
layer of gravel over a period of 19 hours.
This was due to the fact that the material
was placed at field capacity (measured
to be 0.0378) and the Sr was measured
to be 0.008. However, 150 days after the
leak detection layer was covered by the
primary geomembrane, the drainage was
at approximately 10 gpad (94 lphd) and
had been slowly decreasing from nearly
80 gpad (748 lphd). The only other trend
in the data that could shed light on what
might be happening was that the flow rate
increased with increasing temperatures

Geosynthetics | April May 2020

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3/23/20 4:32 PM

Geosynthetics April/May 2020

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