Geosynthetics June/July 2020 - 24

High-strength geocell and geogrid hybrid reinforcement

FIGURE 3 Partially stretched
bottom layer NPA geocell

24

At the time of design, the existing
ground was thought to be 19.7 inches
(500 mm) below the final design grade,
which would have required 9.8 inches
(250 mm) of soft soil removal. After
removal of wooden mats and debris
at the time of construction in January
2017, the subgrade was found to be 29.5
inches (750 mm) below the final grade,
so there was no need for further excavation and removal of the existing soft
soil before installing the bottom layer of
NPA geocell. It would have been ideal to
have 27.6-inch (700-mm) thick gravel
reinforced with two layers of NPA geocell and one layer of biaxial geogrid for
the design, but since the subgrade was
already at 29.5 inches (750 mm) below
the finished grade, the structure was
designed to exactly match the final grade.
The 29.5 inches (750 mm) of structure
was checked for all three design criteria.
A factor of safety of 3 chosen against
the bearing capacity of the subgrade of
16.5 psi (113.8 kPa), due to the presence of dynamic crane loads, was satisfied. A short stretch of access road for

heavy traffic load entering the pad was
designed with only two layers of NPA
geocell reinforcement.

Material used
NPA geocell, biaxial geogrid, and woven
and nonwoven geotextiles were used. The
nonwoven geotextile had a grab tensile
strength of 269.8 pounds (1,200 N) and
the woven geotextile had a grab tensile
strength of 179.9 pounds (800 N). Less
expensive 3-inch (75-mm) minus pit run
gravel was used as infill at the construction (bottom) layer and 1.6-inch (40mm) minus crushed gravel was used for
the upper structural layers.

Construction and
performance of the pad
Construction of the gravel structure
started in January 2017 and was completed in February that year. The nonwoven geotextile was first laid on top
of the subgrade; the bottom layer NPA
geocell was stretched above the geotextile
and filled with pit run gravel. A single lift
of 11.8 inches (300 mm) was allowed at
the construction layer for the compaction, so as not to damage the subgrade
and to create a safe layer for construction
equipment. The gravel brought in was in
a nonfrozen state with natural moisture
about 3% less than the optimum moisture
content. Freezing temperatures during
construction introduce complications
when the moisture within soil freezes.
However, given the time line, a control
volume/weight compaction method was
used for the granular reinforced fill to the
specified degree of compaction.
Figure 3 shows the stretched NPA
geocell and nonwoven geotextile. The
gravel was end-dumped and pushed
by dozer into the geocell pockets. Any
frozen soil in lumps and large chunks
were deemed unsuitable for construction

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Geosynthetics June/July 2020

Table of Contents for the Digital Edition of Geosynthetics June/July 2020

Geosynthetics June/July 2020 - Cover1
Geosynthetics June/July 2020 - Cover2
Geosynthetics June/July 2020 - 1
Geosynthetics June/July 2020 - 2
Geosynthetics June/July 2020 - 3
Geosynthetics June/July 2020 - 4
Geosynthetics June/July 2020 - 5
Geosynthetics June/July 2020 - 6
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Geosynthetics June/July 2020 - Cover3
Geosynthetics June/July 2020 - Cover4
https://www.nxtbook.com/ifai/geosynthetics/geosynthetics-october-november-2021
https://www.nxtbook.com/ifai/geosynthetics/geosynthetics-august-september-2021
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