Geosynthetics June/July 2020 - 26

High-strength geocell and geogrid hybrid reinforcement

FIGURE 5 303-ton (275-tonne) Crane lifting
compressor on May 8, 2017 (Courtesy Mark
Bonnell, Program Manager, Strike Group)

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The site condition at the compressor
station Pad B was working out great,
and the operating 303-ton (275-tonne)
crawler crane did not even make a small
rut when moving around the site, so
the hybrid geosynthetic design worked
very well (personal communication with
the contractor's project manager, Cord
Roberts, July 26, 2017). Figure 5 shows
the condition of the pad on May 8, 2017,
while lifting the compressor. After two
freeze-thaw cycles and one and one half
year of service on July 27, 2018, the pad
and access road were both performing
well without any maintenance requirements (personal communication with
the contractor's program manager, Mark
Bonnell, July 27, 2018).

Discussions
The design was successfully implemented
to withstand the anticipated loads. Initial
concerns had been brought up regarding
the weak subgrade shear strength as well
as the potential of subgrade and base
course softening during the following
26

spring after the pad was constructed. The
design had anticipated that the mechanisms of cellular confinement and gravel
interlocking would increase the stiffness
of the entire structure and thereby reduce
stress applied to the subgrade during
spring thaw. The pad held firm under
the applied loading, which validated the
designed structure and construction
methodology. The structure was basically designed for load support purposes,
hence possible subgrade settlement was
not examined; however, the designers had
anticipated no appreciable differential
settlement would occur at the pad under
the design load.
A comparison of costs and quantities was conducted between the hybrid
reinforcement design and the 51-inch
(1,300-mm) thick three layers of planar geosynthetic-reinforced conventional design for a pad area of 291,000
square feet (27,000 m 2). The conventional design would have required the
removal of 21.7 inches (550 mm) of
existing subgrade soil, which is equivalent to a total of 524,423 cubic feet
(14,850 m 3 ) of soft soil removal and
hauling out. Additionally, it would have
required 1,239,545 cubic feet (35,100
m 3 ) of crushed gravel. The hybrid
design, however, completely avoided the
excavation; the crushed gravel required
was only 429,073 cubic feet (12,150 m3)
and 286,049 cubic feet (8,100 m 3) of
less expensive pit run gravel. The average haul distance for the gravel was 37
miles (60 km). The hybrid design, therefore, reduced the total gravel quantity
by 524,423 cubic feet (14,850 m3) and
haul by 19.54 million miles-cubic feet
(891,000 km-m3).
The cost analysis showed the following results from using the hybrid
reinforcement structure: 22.8% savings
in initial project cost, 100% savings in
excavation, 42.3% savings in granular fill
volume, and 45.7% savings in granular

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Geosynthetics June/July 2020

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