Geosynthetics October/November 2019 - 14

Alternative coal ash pond closure design and construction

FIGURE 2 Structured geomembrane component

monetary savings over the life cycle of the
project. A structured geomembrane/turf
component system was used in design
and construction quality assurance for
ash pond closures and will be the turf
product discussed herein.
The structured geomembrane/turf
component system involves a smooth
subgrade overlain by a linear low-density polyethylene (LLDPE) or HDPE
geomembrane (typically 40-to-50-mil
[1.0-to-1.3-mm] thick), a geotextile with
turf material (Figure 1 on pages 12-13),
and a 0.5-inch (1.27-cm) sand or sand/
concrete mixture. The structured geomembrane can have either large or small
knobs depending on the surface water
flow that needs to be controlled.
The subgrade material is typically a
clay for solid waste facilities to protect the
liner from the municipal waste material.
For CCR ponds and landfill caps, fly ash
is a suitable subgrade material that has
good physical and chemical properties
for placing the structured geomembrane
directly on top of the fly ash. Bottom ash

and clinker cannot be in direct contact
with the structured geomembrane, both
for abrasion issues and for reducing the
usefulness of the spikes on the underside
of the structured geomembrane.
The knobs on top of the structured
geomembrane (the knobs come in various sizes and structures) control the water
flow down the slopes (Figure 2). The geotextile with turf material (turf component)
protects the geomembrane from impact
and solar radiation that damages exposed
geomembrane material, while breaking
up wind uplift pressures. The 0.5 inch
(1.27 cm) of sand infill provides protection to the geotextile and allows for light
vehicles to drive on top of the system. The
sand material can be replaced with sand/
concrete or sand/epoxy mixes in channels
where the maximum stormwater velocity
exceeds 4 feet per second (1.22 m/s).
The structured geomembrane/turf
component system eliminates issues such
as soil erosion, high TSS in stormwater
runoff, difficulties establishing and maintaining vegetation and long-term maintenance. In some areas, in-place structured
geomembrane/turf component systems
have had solar systems installed on top
for long-term beneficial reuse. However,
if borrow soils are readily available, the
up-front cost of installing the structured
geomembrane/turf component system
can be prohibitive.

Power plant in Illinois

FIGURE 3 Saturated CCR at one edge of an ash pond
14

Geosynthetics | October November 2019

During the design of the CCR pond closures, a variety of alternatives including traditional RCRA caps and alternative caps were analyzed. During this
process, consolidating ash within two
basins and a structured geomembrane/
turf component system cap was chosen.
This preserved existing roadways and
pipelines constructed on a bottom ash
pond berm while minimizing the area
of in-place closure. The chosen method

Geosynthetics October/November 2019

Table of Contents for the Digital Edition of Geosynthetics October/November 2019