Geosynthetics April/May 2020 - 43

Following the contract award, the
general contractor reached out to Hallaton Environmental Linings in 2017
about providing a geosynthetic liner
solution for the temporary stone cofferdam. The lined stone cofferdam
would be constructed in lieu of a more
traditional sheeting barrier system that
was part of the original bid documents.
The general contractor and Hallaton,
in conjunction with the supplier, proposed to install a temporary lining on
one face of a stone cofferdam that was
1,200 feet (366 m) in total length. The
stone cofferdam varied in depth based
on the existing reservoir bottom ranging from 0 to 60 feet (0 to 18 m); thus,
the depth of water varied in the area
where the temporary cofferdam would
be constructed. The general contractor
and Hallaton consulted with an engineering firm to provide an engineering
design for the proposed lined stone
cofferdam concept.
With a design in hand, Hallaton
chose an XR-3 8138 potable water-grade
geomembrane liner manufactured by
Seaman Corp. It met the requirements
for potable-grade material; therefore,
it was compatible with drinking water.
Additionally, the XR-3 is puncture-resistant to survive the installation process
with the angular riprap-type stone cofferdam. Another key factor in choosing the XR-3 geomembrane liner was
its density. Its density is higher than
water, which made it possible to sink the
liner. Hallaton also proposed utilizing a
double-sided geocomposite supplied by
SKAPS to be installed directly on the
stone cofferdam prior to installing the
XR-3 geomembrane liner.
The crew installed the geocomposite
on the top of the stone cofferdam and
the leading face down to just above the
existing waterline. The geocomposite
could not come in contact with the
water since it was not potable grade.

The crew field fabricated panels of
XR-3 in a staging area directly adjacent
to the stone cofferdam.
The installed stone cofferdam varied in width and depth over the entire
length; this presented a big challenge.
Each XR-3 field-fabricated panel varied in length and had to be measured
and cut precisely to ensure the leading
edge of the cofferdam would be covered
after the liner system was sunk below
the waterline. Each panel length had to
coincide with the exact width and depth
of the stone cofferdam spanning the
1,200-foot (366-m) length.
One of the final challenges to ensure
the successful installation of the liner
system was the sinking of the liner.
Based on the proximity of the turbidity curtains, the sinking of the installed
liner system needed to be controlled
from the stone cofferdam. A piping system fabricated and installed by Hallaton
was crucial to sinking the liner.
Once the leading-edge liner system
critical component was in place, the
temporary stone cofferdam was ready
to go into service. The general contractor installed additional stone ballast on top of the completed lined cofferdam, per design. Within a few days
of completion, the general contractor
readied the pumps to dewater the western side where the new underground
tanks are to be constructed. This was the
moment of truth. With the dewatering
process underway, the water pumped
from the western portion of the lake
behind the cofferdam began to lower in
level and the lined cofferdam held back
the exposed water storage system. The
overall Druid Hill Reservoir project is
ongoing, with a scheduled completion
date in 2021.

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Geosynthetics April/May 2020

Table of Contents for the Digital Edition of Geosynthetics April/May 2020