Faced with significant environmental
protection, cost, and
operational constraints, the St.
Lawrence and Hudson Railway
(now Canadian Pacific Railway) chose
to reinforce a deteriorating railway
trestle bridge with a geogrid reinforced
mechanically stabilized embankment.
In June 2000, the St. Lawrence and
Hudson Railway commissioned the reconstruction
of a 15m high x 60m long
railway trestle bridge across a river valley
in St. Thomas, Ontario. The overall
design was developed by
McCormick Rankin Consultants, with
Thurber Engineering Ltd. carrying out
geotechnical design, and construction
was by J-AAR Contracting of London,
Ontario. Layfield Geosynthetics Ltd.
supplied the geosynthetic material and
on-site installation supervision.
The trestle was replaced with a
geosynthetic reinforced granular
embankment constructed from the valley
floor up to the base of the railway
tracks at a 1:1 slope. A granular
embankment was chosen over other
structural techniques based on constraints
such as reduction of environmental
impact, land ownership cost,
railway operations, and timing.
Several unique construction techniques
were utilized to ensure a fast
and efficient project completion within
a tight timeline.
Design constraints
The design engineer faced several
design constraints including:
1. Operational - A key design consideration
was to minimize the down time
of the railway line. Re-constructing a
new trestle bridge would have forced
the closure of the railway line for an
extended period of time. The mechanically
stabilized embankment (MSE)
design allowed the railway to remain
open during most of the construction
except for the short period of time
when the tracks were removed to facilitate
construction of the top portion of
the embankment.
2. Environmental - Minimizing the
impact on the environment is a challenge typically faced when crossing a
river valley. Steepening the embankment
slopes achieved three important
objectives:
Minimizing the footprint of the
embankment on the valley floor.
Reducing the number of truck trips
in and out of the site, since all the
materials for the project were imported.
Utilizing a wrap wall design allowed
the designer to eliminate the need for a
bench as would normally be required
on a steepened slope. A bench would
further increase the footprint of the
embankment on the valley floor.
3. Hydraulic - Steepening the slope of
the embankment also achieved two
important hydraulic objectives:
Less fill volume minimized the
impact on the storage volume available
in the river valley.
Typically, the length of culvert
affects its hydraulic capacity. The
longer the culvert, the larger the diameter
that is required due to losses in
conveyance efficiency, or alignment
with natural channel becomes undesirable.
4. Land ownership - Typically, if possible,
it is best to keep construction
works within the bounds of the railroad
right-of-way. Going outside this
boundary involves the purchase of
more land at a high cost. Utilizing a
steepened embankment design allowed
the minimization of land area purchased.
Geosynthetic technology
employed
There has been a dramatic expansion
of reinforced soil structures in the
United States and Canada over the last
decade. The projection in the US is for
over 70,000 square metres of new wall
face per year. Geogrid reinforcement
in slopes is typically used to achieve
two objectives: increase slope stability,
and to improve compaction at the
edges of the slope to reduce sloughing.
As in this case, a typical MSE
design employs two types of geogrid
reinforcement, primary and secondary.
The primary geogrid acts like reinforcing
steel in concrete, adding strength
in tension to the soil mass. Utilizing a
high strength polyester yarn geogrid allowed
the construction of a 45 degree
embankment, utilizing OPSS 1010
Granular B Type 1 Soil with an angle
of internal friction of 32 degrees.
Table 1. Geogrid lift spacing
1. Primary geogrid reinforcement -
The primary geogrid reinforcement
utilized in this design has a minimum
long-term design strength (LTDS) of
60kN/m based on GRI-GG4(b) in
Sand and Gravel. The geogrid is constructed
of a high strength, low creep
polyester yarn coated with a polymer
for protection against UV degradation
and mechanical damage. From the bottom
to the top of the embankment, the
spacing and strength of the geogrid is
detailed in Table 1.
The lower strength geogrid and
wider spacing could be utilized at the
top of the embankment due to the
reduced design loads that, in turn, provided
a cost savings.
The primary geogrid was laid out in
continuous lengths perpendicular to
the slope face, through the trestle, to
reinforce the granular soils in the
embankment. A total of 11,200 square
metres of primary geogrid was used in
this project.
2. Secondary geogrid - The secondary geogrid reinforcement utilized in this
design has a minimum LTDS of
6kN/m based on GRI-GG4(b) in Sand
and Gravel. The secondary geogrid
was constructed of polypropylene
yarns with carbon black additive for
protection against UV degradation.
The yarns are also coated with a polymer
for protection against mechanical
damage and increased friction with the
soil. The geogrid was utilized as a
compaction aid at the extreme edges of
each granular soil lift as well as to
encapsulate the topsoil, seed, and erosion
control blanket wrapped behind
the geogrid. The secondary geogrid
wrapped configuration was also used
as an aid to minimize surface erosion
and sloughing, which is often found in
steepened slopes. A total of 13,500
square metres was used in this project.
Construction techniques
The primary geogrid was placed
over each lift of granular. Of greatest
importance was that the entire lift surface
was covered with the geogrid with
the strength direction oriented perpendicular
to the slope face. To achieve
this, and to reduce waste, incremental
lengths of geogrid were manufactured,
specific to the surface area of each lift.
This also eliminated the need for splicing
in the strength direction of the grid.
The challenge of placing the geogrid
around the vertical columns that
remained in the trestle was overcome
by simply slitting the geogrid in the
direction perpendicular to the slope
face (non-strength direction). This
technique allowed for the maximum
number of reinforcing bars around
each column. The geogrid was then
staked in place and the granular materials
were pushed from the centre of
the embankment toward the edges to
ensure that the geogrid was tensioned
in the process.
The secondary geogrid and erosion
control blankets that comprised the
wrap face were laid out in continuous
lengths parallel to the slope face. The
lower leg of the secondary grid was
embedded a minimum 1.1m. The erosion
control blanket was then placed in
intimate contact with the secondary
geogrid, then wrapped over the leading
edge of embankment fill, topsoil, and
seed. Finally, the upper leg of the
geogrid was placed a minimum of 2m
over the exposed surface of the top lift
of granular material. Temporary forms
and a skilled excavator operator maintained
the alignment of the 1:1 slope as
they progressed upward.
Post-construction follow-up
A site visit was performed in May
of 2002, almost two years post-construction.
The exposed slope faces
were completely covered by dense
native vegetation. At the inspection,
the dense vegetation was pulled back
to expose the secondary geogrid which
was visually unaffected by the sun’s
UV rays. The general contractor was
pleased with the project, noting that it
was completed on time and within
budget. The contractor also noted that
it was a unique and technically challenging
construction project, which
made it interesting to participate in.
Typical costs
Mechanically stabilized embankments
consist of three primary components:
the reinforcement, backfill, and
face treatment. Typically, these costs,
as a percentage of the total cost, break
out as follows:
Reinforcement 45 - 65%,
Backfill 30 - 45%,
Face Treatment 5 - 10%.
In this case, constructing a reinforced
steepened slope proved to be
more cost-effective than an unreinforced,
2:1 slope alternative. In a simplified
analysis, the reinforced
embankment proved to be 15% less
expensive to construct than a 2:1 unreinforced
slope. This is a very conservative
analysis since it does not
account for the additional material
required to construct the mandatory
benched slope, the cost of a lengthened
culvert, or the cost of additional land.
Further, steep, unprotected slopes tend
to require additional ongoing maintenance,
particularly on the slope face
due to sloughing and surface erosion.
In this case the wrap wall construction
protected against these maintenance
problems.
The project was successfully completed
on time and within budget.
Utilizing geosynthetic technology, the
designer was able to overcome many
of the constraints involved in building
a steep slope in a narrow parcel of
land. Mechanically stabilized embankments
have become a much more common
design practice with an evolving
body of knowledge on the subject.
For more information, contact e-mail:
msimpson@layfieldgroup.com. The
author would like to thank Paulo
Branco, Thurber Engineering, Barry
Palynchuk, Canadian Pacific Railway,
Kirk Smith, J-AAR Excavating and
Pierre Boislard, Layfield Group Ltd.
for their support with this article.
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