Geoysynthetics August/September 2023 - 22

Searching to maximize stress reduction ratio with biaxial geogrids and timber piling
approach embankment runs parallel to
Burlington Avenue, a municipal roadway
and the only access point to a major public
park; this roadway was also required
to stay open. A 90-meter (300-foot) long
temporary modular block retaining wall
(Figure 1) was proposed immediately
adjacent to Burlington Avenue to support
the shoo-fly track widening.
The subsurface investigation of
Initial concepts revolved
around designing a
temporary bridge
structure. However,
after discussion with
the Illinois Tollway and
BNSF, the modular wall
option was preferred and
a geogrid-reinforced,
timber pile-supported
wall foundation system
was developed as
an alternative.
the southeast approach embankment
revealed peat and soft, organic silts
with thicknesses of greater than 27 feet
(8.2 meters). Initial concepts revolved
around designing a temporary bridge
structure. However, after discussion
with the Illinois Tollway and BNSF,
the modular wall option was preferred
and a geogrid-reinforced, timber pilesupported
wall foundation system was
developed as an alternative.
Site conditions and geology
The site lies approximately 15 miles
southwest of the Chicago Loop at the
limit between the Valparaiso and Tinley
Moraines in a slight depression or basin
fill area. The youngest natural soils within
the basin fill consist of weak, compressible
clay and silt of moderate to high
organic content and muck. The compressible
soils reach 27 feet (8.2 meters)
in thickness and extend approximately
1,500 meters (5,000 feet) along the rail
alignment on each side of Flag Creek,
which represents the major drainage feature
in the area. The basin fill has been
removed in a number of select areas during
development of the I-294 corridor;
however, weak and compressible soils
remain largely in areas along the length of
the BNSF corridor beneath and adjacent
to the embankment that is close to 100
years old.
The base of the rail embankment
along Burlington Avenue sits at an elevation
of about 645 to 647 feet (196.6 to
22
Geosynthetics | August September 2023
197.3 meters) and is relatively flat. The
embankment is about 20 to 25 feet (6.1
to 7.6 meters) tall with a pre-construction
rail tie elevation of about 670 feet
(204.2 meters).
Soil and groundwater
conditions
The subsurface conditions along the
proposed temporary modular wall were
investigated by advancing seven SPT
borings to depths up to 50 feet (15.2
meters) below grade, pushing undisturbed
Shelby tubes, performing in-situ
pressuremeter testing, and installing a
groundwater monitoring piezometer.
Below the existing pavement structure
along Burlington Avenue, the investigation
encountered (A) man-made ground
(fill); (B) upper organic silt loam and
peat; (C) lower organic silt loam and
clay; (D) soft to medium stiff silty clay
loam; and (E) medium dense to dense
sand and sandy gravel.
The units showed lateral continuity
across the length of the proposed
wall. The clayey fill material, placed
to support the pavement structure of
Burlington Road, is a consistent 5 to
6 feet (1.5 to 1.8 meters) thick, while
the upper organic material and peat are
present with consistent thickness in all
but two of the borings.
The upper organic unit presents as
black and dark brown silty loam and
generally non-fibrous peat with thickness
between 5 and 10 feet (1.5 and
3.0 meters). The material is comprised
heavily of silt and sand-sized fractions
with natural water contents measuring
110% to as high as 270%, and organic
content of 22 to 30%. Liquidity indices
(LI) of the upper organic are well in
excess of 1.0.
A one-dimensional consolidation test
was performed on a Shelby tube sample
of this unit from boring SAB-18. The
Geoysynthetics August/September 2023

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