Geosynthetics August/September 2019 - 28

Stabilization of subgrade soils during Interstate 95 lane widening

The team evaluated
the site conditions and
developed an option
that used a mechanically
stabilized aggregate
layer (MSL) to create a
stable surface above
the unsuitable material
while requiring greatly
reduced soil excavation
and disposal.

The logistics of addressing the problem
were equally challenging. Because the construction was occurring between the existing northbound GPLs and northbound
ETL exit ramps of a major interstate, any
material that would be required to enter
or leave the site would do so through limited access points. Depth of excavation
was also an issue, as the northbound exit
ramps, to the west of the northbound
ETLs, were supported on mechanically
stabilized earth (MSE) walls about 25-feet
(7.6-m) high. There were active GPLs
immediately to the east of the northbound
ETL. Even though the active traffic lanes
were separated from the work zone by a
concrete barrier, if the depth of undercutting was too deep, it could undermine the
MSE and the active travel lanes, therefore,
requiring a support of excavation (SOE).
Design and construction of this would
have delayed the opening of the highway
and increased the cost. Further, space to
operate equipment was limited, and any
significant excavation would have had to
consider the impact on the existing roadway just a few feet away. The soft subgrade
was encountered only a few months prior
to the scheduled opening of the toll lanes
and a delay would have been costly.

Solution development
The contractor approached MDTA to
determine a solution. The initial proposed option was to remove and dispose
of 6 to 8 feet (1.8 to 2.4 m) of unsuitable
material and replace it with suitable fill.
While this approach would have resulted
in an acceptable surface for construction, its impact on the project would
have been extremely problematic due to
the logistical and scheduling challenges
noted previously. After evaluating this
option, the contractor and MDTA concluded that the impact on the project
schedule would be so severe that a different approach was needed.
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Geosynthetics | August September 2019

The contractor then approached
RK&K, the designer of record, and Tensar
International Corp. to see if a solution
could be developed that would keep
the project on track. Together, the team
evaluated the site conditions and developed an option that used a mechanically
stabilized aggregate layer (MSL) to create a stable surface above the unsuitable
material while requiring greatly reduced
soil excavation and disposal.
The design was developed based on
the Giroud-Han method for unpaved
roads. This was suitable for the contractor's haul road. It should be noted that
while Giroud-Han served as a basis for
the design, the final application was not
an unpaved road subject to channelized
traffic, for which the method is intended.
However, Giroud-Han is the most rigorous and widely accepted method for
designing gravel-surfaced roads over soft
subgrades, so there is a large body of
experience with conditions that are like
those encountered in this case. Therefore,
Giroud-Han can be effectively used to
estimate thickness requirements as an
initial step in the design.
Extensive field experience has shown
that a Giroud-Han design based on a typical scenario (20-kip [89-kN] axle load,
100-psi [689-kPa] tire pressure, 1,200
passes, 1.5 inches [38 mm] max rut depth)
consistently results in an improved subgrade surface with a CBR of 6% or greater.
Based on this experience, a preliminary
cross section can be developed. Since field
conditions and constructibility issues not
considered in the Giroud-Han method can
affect performance, field testing is often
used for design verification, depending
on how critical subgrade strength is for
the final design.
The highway pavement design used
SpectraPave software. This application uses the empirically based 1993
American Association of State Highway
and Transportation Officials (AASHTO)

Geosynthetics August/September 2019

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