Geosynthetics June/July 2020 - 23

The footprint of Pad B under consideration was about 291,000 square feet
(27,000 m 2). Geotechnical investigation supplied by the owner had recommended 2 psi (15 kPa) as the characteristic undrained shear strength (Su) of
the soft silty clay subgrade. The seasonal
water table was found to be close to the
existing subgrade in the southern side
of the pad. The project needed solutions
to support a 237-ton (215-tonne) crane
and loads from trailers carrying 154-ton
(140-tonne) loads, with individual axle
loads not exceeding local highway limits. A 51-inch (1,300-mm) thick planar
geosynthetic-reinforced granular structure was conventionally designed with
three layers of nonwoven geotextile (grab
tensile strength 160 pounds [712 N]) and
triaxial geogrid to build the pad. This
solution would have required 31.5 inches
(800 mm) of existing soft soil excavation
and 51 inches (1,300 mm) of reinforced
crushed gravel fill. Once excavated, the
material would need to be hauled off-site
and replaced with imported material. All
these activities would have contributed
to a huge increase in the project cost and
the extraction of a lot of virgin crushed
gravel aggregate that would have made it
impossible to meet the project deadline.
Thus, an innovative design with a hybrid
geosynthetics solution was presented that
not only was sustainable in terms of cost
and environmental indicators but also
would enable winter construction.
The alternative design that was proposed in lieu of the conventional design
featured a layer of biaxial geogrid perched
between two layers of high-strength NPA
geocell reinforcement and a geotextile as a
separation layer. The design was tested for
safety against bearing capacity, rut criteria
as mentioned in Pokharel (2010), and the
hoop strength of the geocell wall. The
bottom layer of NPA geocell-reinforced
gravel served a dual purpose: first as a
construction platform where construction

equipment could operate and second as
a structural base layer during operation.
A nonwoven geotextile was selected to
act as a separation layer between the granular fill and the subgrade. A layer of NPA
geocell reinforcement installed on top of
the nonwoven geotextile and was filled
with 11.8-inch (300-mm) thick, 3-inch
(75 mm) maximum size pit run gravel.
Above this layer 5.9-inch (150-mm) thick,
well-graded, 1.6-inch (40 mm) maximum
size crushed gravel was reinforced with
a layer of biaxial geogrid (Figure 1 on
pages 20-21). Finally, the top layer of NPA
geocell was installed and filled with 11.8
inches (300 mm) of the same crushed
gravel material. Figures 2a and 2b provide a sketch of the structure alongside
the conventional design for comparison.

FIGURES 2a and 2b a) Hybrid
reinforced structure compared
with b) the conventional structure

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Geosynthetics June/July 2020

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