Geosynthetics August/September 2019 - 38

An innovative cover system to close an ash/sludge impoundment

erosion repairs and vegetative maintenance (i.e., mowing).
The project team estimated an 80%-
90% cost reduction of post-closure
maintenance activities offered by the
TG-FCS in comparison with the traditional method.

Project construction

With the TG-FCS in
place, precipitation
is prevented from
contributing to the
water content in the
waste, and existing
water can be removed
through a network
of drainage pipes
and pumped from
the perimeter of
the installation.

38

Geosynthetics | August September 2019

Construction of phase I took nine months.
Phase II also took nine months. The time
it took to install the TG-FCS was less than
the time estimated to dewater the sites for
the traditional cover system.
The upper 10 feet (3 m) of the waste
profile had a CBR of less than 1%, which
complicated the installation process
even for the TG-FCS. First, to get maximum capture of gas coming from the
waste, a geocomposite drainage layer
was installed with 100% coverage. The
geocomposite drainage layer was tied
into an active gas collection system for
proper gas disposal.
The installation contractor utilized
a slip sheet of sacrificial geomembrane
for the field technicians to stand and
operate on in deploying and seaming
the geocomposite layer. This simplified the installation while providing an
increase in worker safety and quality of
the work. Installing the geocomposite
was the most challenging part of the
installation. Once the geocomposite was
in place, it provided a more stable platform for the technicians to deploy and
install the geomembrane layer. Standard
geomembrane installation practices
were utilized for this step of the process, including overlap and seaming, and
construction quality assurance (CQA)
procedures. Since odor elimination was
the first priority, the team installed 100%
of the geomembrane before proceeding
with the engineered synthetic turf layer.
Once the geomembrane was in place, the
gas was captured, and the corresponding

odor was eliminated. The installation
process for the turf involved unrolling
the turf and deploying the panels into
place, securing adjacent panels together
using a prayer seam.
The final component of the TG-FCS
was the specified infill. Low ground pressure vehicles were utilized in deploying
and placing the infill.
The profile of the site is fairly flat.
This is due in part to the high water
content and low strength characteristics
of the waste. Precipitation drainage is
managed through a network of bench
drains that empty into a perimeter channel. Runoff from the TG-FCS has low
turbidity due to the lack of soil in the
drainage cross section as well as the filtering capabilities of the C33 sand infill.
A common scale for measuring turbidity
of water is the nephelometric turbidity
unit (NTU) scale. Typical NTU ratings
for runoff from the TG-FCS system are
11 NTUs. As a reference point, drinking
water standards are usually 1-5 NTUs.

Post-installation activities
The decision to install the TG-FCS
prior to dewatering accelerated the
project schedules and helped to mitigate the odor problem very quickly.
However, the water still needed to be
removed from the waste. Fortunately,
the TG-FCS allows for dewatering activities after installation. With the TG-FCS
in place, precipitation is prevented from
contributing to the water content in
the waste, and existing water can be
removed through a network of drainage
pipes and pumped from the perimeter
of the installation.
As the dewatering process is continued, settlement of the final cover surface
can be observed. Due to the very high
water content, the settlement expected
and ultimately realized exceeded the
category of differential settlement. The
Geosynthetics August/September 2019

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