Geosynthetics June/July 2021 - 17

permafrost region. Wu et al. (1998) indicated
that a height of the embankment
of more than 2.6 feet (0.8 m) maintains
a steady state on the permafrost and suggested
that a 5.2-foot (1.6-m) embankment
for the gravel road is optimal.
The HBR track was not in good serviceability
condition because of permafrost
degradation causing vertical deformations
and the lateral spreading of
embankment material. The aim this time
was to bring the track to its condition just
before the flood events. The mode of failure
was washout, which had exposed the
permafrost and caused erosion at almost
all the stream crossings. The design for
rehabilitation needed to address issues
such as structural strength, erosion control,
protecting the permafrost from
further degradation and, most importantly,
utilizing the washed-out material
deposited at the washout locations. If the
material was not enough, a low-quality
3-inch (75-mm) pit run gravel would
have to be hauled from Gillam (97 miles
[156 km south]) of the first repair site.
The only access to the site was the railway
line itself. Given the time line of
five weeks, options other than geocells
seemed impossible, because there were
more than 26 damage locations along a
93-mile (150-km) stretch.
American Railway Engineering
and Maintenance-of-Way Association
(AREMA) and Canadian National (CN)
railway design principles were checked
to confirm the structural requirements
of the geocell-reinforced structure. The
mechanism of load distribution via the
slab effect to protect the weaker region
within the permafrost was anticipated
by transferring the load to a wider
and more competent area. The design
had attempted to make only 5% of the
applied stress to be transferred to the
subgrade. The geocell reinforcement was
designed to increase the modulus of the
unreinforced gravel by 3.5 to 4 times.
In the design, the thickness of geocell
plus 1 inch (25 mm) was assumed as
the reinforced thickness for stress transfer
calculations, as recommended by
Pokharel (2010).
Due to the construction constraints
and available tamping equipment at
the site, only the subballast layer was
reinforced, and the layer of reinforcement
required was determined by the fill
height. To avoid further damage to the
existing surface and to protect the permafrost,
nothing was removed, and construction
started right on top of the existing
subgrade.
At some locations, the culvert pipes
were found to have settled, rendering
them useless. The new design raised the
culvert beds to the existing streambed
level and provided a semirigid NPA
geocell-reinforced mat at the culverts to
avoid a similar situation in the future. For
erosion control at the toe of the embankment
at and near the streambank, riprap
was also provided.
Materials used
The geosynthetic materials used in this
project were woven and nonwoven geotextiles
and high-strength polymeric geocells.
Salvaged, washed-away granular material
was used wherever possible. Granular
infill material with less than 12% fines was
desired; the washed-out granular material
deposited at the downstream side was
used to save construction time and cost
in bringing similar material from Gillam.
Sand and 3-inch (75-mm) pit run gravel
were the approved materials for infill in
the geocell. Riprap used was 6-12 inches
(150-300 mm) in size.
Two types of NPA geocells were chosen
for the design, as NPA geocells have
higher tensile strength, modulus and
creep resistance than other available geocell
material. The top layer of subballast
just below the ballast was reinforced with
www.GeosyntheticsMagazine.com
17
American Railway
Engineering and
Maintenance-of-Way
Association (AREMA)
and Canadian National
(CN) railway design
principles were checked
to confirm the structural
requirements of the
geocell-reinforced
structure. The mechanism
of load distribution via
the slab effect to protect
the weaker region within
the permafrost was
anticipated by transferring
the load to a wider and
more competent area.
http://www.GeosyntheticsMagazine.com
Geosynthetics June/July 2021

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