Mechanically Stabilized Earth - Vegetated
Steepened Slope System 96th Avenue
Roadwork’s, Surrey, British Columbia
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Dan M Donald, P.Eng.
Nilex Civil Environmental Group, Burnaby, British Columbia, Canada
German Cajigas, P.Eng., M.Eng.
Tensar ® International, Coquitlam, British Columbia, Canada.
ABSTRACT
The City of Surrey selected a Sierra® Vegetated Slope System as the structure of choice for the approach walls leading
up to this cast in place, pile supported bridge structure. This paper will discuss the use of the vegetated slope system
inclined to the horizontal at 69 degrees as the primary facing element for these approach walls. The facing of this slope
system is comprised of welded wire forms to assist in construction, along with a synthetic structural wrap comprised of
Tensar Biaxial Geogrid, Erosion Blanket, and Topsoil to form the vegetated facing. The prime structural soil
reinforcement consists of Tensar HDPE Uniaxial Geogrid.
These slopes are up to 5 meters high, and used in conjunction with rip-rap and sheet piles, effectively form the southern
riparian zone of the Serpentine River at this location. The structure crosses over the Serpentine River which is a
sensitive spawning and rearing habitat for trout and salmon, and is located immediately downstream of the Tynehead
Fish Hatchery. The paper will focus on the design methodology, ease of construction, with an additional emphasis on
the environmental benefits for this sensitive infrastructure application.
RESUMEN
La Ville de Surrey a adopté le système de talus végétal Sierra® comme structure de choix pour les murs d'approche
menant à un pont moulé en place et soutenu par des piliers. Ce document traite de l'utilisation du système Sierra® avec
talus végétal incliné à 69 degrés par rapport à l'horizontale, comme l'élément de paroi primaire pour ces murs
d'approche. La paroi de ce système de talus est constituée de coffrage en treillis d’acier à mailles soudées pour aider à
la construction, avec une enveloppe structurelle synthétique composé de la géogrille biaxiale Tensar, couverture de
l'érosion et de terre végétal pour former le talus végétal. L’élément principal de renforcement structurel des sols se
compose des géogrilles uniaxiales Tensar HDPE.
Ces murs sont jusqu’à 5 mètres de haut; ils ont été utilisés conjointement avec un enrochement et des palplanches pour
former efficacement la zone riveraine sud de la rivière Serpentine à cet endroit. Le pont permet de franchir la rivière
Serpentine qui est une des zones de frai et de l'habitat d'élevage de la truite et le saumon, et qui se trouve
immédiatement en aval de «l’écloserie de Tynehead ". Ce document se concentrera sur la méthodologie de conception,
de la facilité de construction, avec un accent supplémentaire sur les avantages pour l'environnement liées à cette
application sensible dans les infrastructures.
1
INTRODUCTION
The project was undertaken by the City of Surrey in
British Columbia. As noted in Figure 1, the site is located
immediately to the south of Highway #1 on 96 Avenue,
between 160th Street to the West and 168th Street to the
East.
This project involved an arterial road widening from
two to four lanes. This paper will focus predominantly on
the vegetated steepened slope systems which were ideal
for construction in this environmentally sensitive area.
The Sierra® Vegetated Slope System was the preferred
option where the approach fills encroached within the
gullies of the Serpentine River and its tributaries. A pile
supported bridge was also required as part of the
widening where the roadway crosses over the Serpentine
River. Other portions of this project also involved paving,
sidewalks, preloading, and sheet piling at the base of the
steepened slope along the river’s edge.
Property restoration and reconfiguration were an
additional challenge, to which a vegetated steepened
slope blended in extremely well with the lush vegetation,
and natural habitat along this river course.
The initial phase of sheet piling and steepened slope
construction commenced in 2010. The latter phases
associated with bridge construction and paving were
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substantially completed on January 3 , 2012.
The major challenges associated with this project
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occurred in the section between 160 and 164 Street,
where the roadway crosses numerous watercourses and
creeks leading into the Serpentine River.
These steepened slopes were designed as wing walls
extending from the crossing structure, and generally
running adjacent and parallel to the river.
This paper will illuminate the key aspects associated
with this vegetated slope design, and its environmental
benefits.
2.2
As this project extends along the Serpentine River
riparian zone, the key design consideration was the Q 100
Flood Level which was defined for this project at elevation
9.35 metres at the bridge crossing location.
2.3
2
GEOTECHNICAL ASPECTS
The geotechnical investigation for this site was conducted
by Metro Testing Laboratories Ltd., of Burnaby, BC. A
report was prepared for the City of Surrey by its prime
consultant R.F.Binnie & Associates Ltd.
In summary, the field exploration consisted of 17 auger
holes advanced to depths of 10.7 m in the roadway area,
along with two cone penetration tests (CPT) to determine
conditions at the bridge location. These CPT tests
successfully penetrated the approach fills, the underlying
channel deposits, and the glacio-marine sediments below.
Eleven test pits were also completed predominantly along
the north side in an undisturbed area, to later be covered
with site grading fills.
It is key to note that with the environmental sensitivity
of this site, that no exploratory works were conducted
within the floodplain of the Serpentine River.
2.1
Site Topography
The topography of the site reflects the glacio-marine
origin of the area, and the post-glacial down cutting of the
Serpentine River. This region is gently rolling with
topographic relief in the range of 50 metres from the
uplands area to the base of the Serpentine River channel.
The Serpentine River steps down from the west in a
series of gently sloped benches flowing into a gully about
15m deep. The active channel is in the range of 12 m
wide, as it meanders across a floodplain of about 100 m
wide. In the vicinity of the existing river crossing the road
grade is approximately 4.5 m above the river channel.
2.4
Figure 1. General Site Location
River Levels
River Erosion and Sheet Piles
Sheet piles were required along the north side of the
bridge approaches to provide protection from river erosion
as seen in Figure 2. The piles were also needed to
satisfy global embankment stability along the north side at
the bridge crossing location. The base of the wire wall
was generally set at approximately 500 mm below the top
of the sheet piles as evident in Figure 2 below. These
piles generally extended upward about 1500 mm from the
base of the river channel bottom.
Riprap comprised of angular blast rock varying in sizes
of approximately 200 mm to 1200 mm is noted
immediately in front of the sheet pile wall. The riprap was
underlain by non-woven geotextile.
Surficial Geology
Available geological information indicates that the native
soils in this region are of glacio-marine sediments
comprised of silt till (Newton Stoney Clay), sandy glacial
till, marine silt, and lenses of fine water bearing sand.
Within the drainage courses recent sediments were
comprised of coarse gravel alluvium and organic fine
sandy silt.
Figure 2. Serpentine River, North Side, Looking East
2.5
Embankment Settlement
New fills would be placed approximately 5.3m above the
river channel on the north and south sides of the bridge
crossing. This would mean an overall rise in road grade
of 1500 mm above existing levels.
Settlement was calculated for maximum fills of 2.5 m
and 1.5 m of preload above finished road grades on both
the north and south sides. It was determined that
settlements were the highest on the north side at 125 mm,
with settlements on the south of upwards of 100 mm.
The ability of the steepened slope to accommodate
such settlement during the course of construction was a
key consideration in its selection on this project. The
system can also easily accommodate curves to suit the
topography of the river channel, and allows for easy
alignment in the field by the construction crew.
3
DESIGN OF MECHANICALLY STABLIZED
STEEPENED SLOPES
Engineering design was provided by Tensar International
in conjunction with Metro Testing Laboratories. Materials
to construct these slopes were provided by Western
Canada Distributor, Nilex, Inc. Hall Constructors was the
successful general contractor awarded this contract
valued at approximately $10M by the City of Surrey.
This steepened slope consists of a facing unit made of
welded wire mesh forms with a wrap of Tensar Biaxial
Geogrid (secondary reinforcement) that provides surficial
stability to the reinforced fill. A pocket of plantable topsoil
fill and seed ultimately provides the vegetated facing,
which in turn is protected with a North American Green
Erosion Control Mat (ECM). The primary reinforcement
consists of the family of Tensar Uniaxial geogrid. A
typical layout of the slope is illustrated in Figure 3.
The SierraSlope constructed at this site was upwards
of 5.5 m high in the vicinity of the bridge crossing
structure. The inclination of the facing elements is just
less than 70 degrees, and was designed officially as a
slope, as opposed to a wall.
3.1
Design Methodology
The design of the reinforced slope was conducted
following the guidelines and methodology proposed by the
Federal Highway Administration in its document FHWANHI-00-043 (2001) and under the project specifications.
Consequently, the design was carried out using limit
equilibrium slope stability methods, and in this particular
case the Bishop Method was modified to account for the
stabilizing effect of the geogrid reinforcements. The
GSlope version 4.15 computer program (Mitre Software,
2006) was used for this purpose.
3.2
Soil Design Parameters
The cohesionless reinforced fill within the Slope consisted
of a granular soil with a maximum particle size of 25 mm,
with the portion passing the 0.425 mm sieve being nonplastic, as per the specification.
The design parameters utilized for this slope design, and
for the varying foundation conditions are noted in Table 1.
Table 1. Design Parameters
Soil Type
Reinforced
Silt
Moist Unit
Weight
(kN/m3)
20
20
Effective
Friction
Angle(Φ)
38
24
Effective
Cohesion
Cu(kPa)
0
5
Blue Clay
20
20
5
Till
20
37
0
Clayey Silt
20
10
24
3.3
Geogrid Design Strengths
The primary soil reinforcement consisted of Tensar
Uniaxial HDPE geogrid, arranged in vertical spacing(Sv)
as required by design. In general primary grids were
more concentrated in the lower wall portion.
Secondary soil reinforcement comprised of biaxial
geogrid was utilized in upper grid layers where possible.
The geogrid strength properties are noted in Table 2.
Figure 3. Typical SierraSlope Layout
Table 2. Geogrid Parameters
Geogrid Type
UX 1100
UX 1400
Material
Type
HDPE
HDPE
Ultimate Tensile
Strength kN/m
58.0
70.0
BX 112060
PP
20.5
BX110075
PP
20.5
North American Green was used to retain the plantable fill
or topsoil pocket, to support the future vegetated growth
on the steepened facing system. Prime reinforcement is
noted at the mid-height of facing cage as illustrated in
Figure 4.
The following reduction factors (RF) were used in the
design when considering the overall structure life of 100
years:
Creep reduction factor (FRcr) = 2.6
Install reduction factor (FRid) = 1.1
Durability reduction factor (RFd) =1.1
Internal design included the analysis of potential
circular failure surfaces passing through and behind the
reinforced fill (compound stability). Minimum factors of
safety under static conditions were set at 1.5.
For conditions of Seismic the factors were set to 1.1
for the 975 year event, and at 1.0 for the 2475 year
event. Other conditions analyzed considered the effect
of the temporary preload surcharge(50kPa), and traffic
surcharge(16 kPa).
A maximum differential hydraulic head of 1m was
designed for, along with the Q100 flood elevation defined
at elevation 9.35 m for this site.
3.3
External Design
Figure 4. Facing Element Details
5.
EXISTING CROSSING FOR REMOVAL
The existing crossing consisted of a two lane arterial
roadway crossing over two culverts as indicated in Figure
5. The headwalls around these culverts was comprised of
cement filled bags not uncommon during the early years
of construction. The Serpentine River is noted to flow in a
southerly direction and into the culverts into the distance.
External design involved the analysis of potential deep
seated circular failure surfaces and seismic loading
analysis. Due to the geotechnical conditions of the
foundation soil and including the groundwater level, the
design was governed by the potential occurrence of deep
seated circular failure surfaces affecting the underlying
silt, gravely sand and clay layers under the seismic
condition for an earthquake with a probability of
exceedance of 2% in 50 years (2,475-yr return period).
It was necessary to supplement the design with sheet
piles installed at the toe of the steepened reinforced
slopes. The seismic analysis was carried out using a
pseudostatic approach employing the Bishop’s Modified
Method of limit equilibrium and the GSlope computer
program. The seismic acceleration coefficient used in the
analysis was kh = 0.25, corresponding to half of the Peak
Ground Acceleration for the site per the 2006 BC Building
code.
Figure 5. Existing Culvert Crossing To Be Replaced
4
FACING DETAILS
As noted in Figure 4 the facing system is comprised of a
black wire facing form.
This wire form enables
confinement at the facing fills during the compaction
phase. Tensar UV stabilized biaxial geogrid (BX112060)
was utilized as a wrap facing immediately behind the wire
facing element. An erosion blanket manufactured by
As evidenced in the early construction photo above,
the vegetated walls and associated preload are noted on
the left hand side just above existing road grade. The
reader will also note the early establishment of sheet piles
and rip-rap along the river’s edge leading right into the
culvert inlet to protect the sensitive habitat.
Figure 7. New Proposed Bridge Crossing
Figure 6. Looking South From the East Approach Fill
5.1
Figure 6 is looking in a Southwest direction as viewed
from atop the east approach fills. Sheet piles are also
noted in the distance at higher elevation and in front of the
east abutment wall. The Serpentine River flows to the
south towards the left hand side of this photo.
Staged construction and preloading was initially
implemented on the north site perimeter where preload
fills were the highest.
This approach enabled the
substantial construction of approach fills, sheet piles, and
later the bridge infrastructure. Traffic was moved over to
the north side upon substantial bridge completion. This
allowed the final removal of the culverts on the south side
of the river crossing.
One key advantage of this method of slope construction is
that it does not require any unique footing design. An
environmental construction advantage is that placement
of a concrete footing is not required. The slope is simply
founded upon undisturbed native or granular soils. Prior
to construction of the slope the contractor cleared and
grubbed the reinforced soil slope footprint in accordance
with the specification requirements to good bearing.
5.
NEW CONSTRUCTION
This project was comprised of seven steepened
vegetated slopes, with five of the slopes located along the
north perimeter. The overall vertical facing area was
comprised of 996 M2 of vegetated facing.
The project also incorporated a pile supported bridge
structure with a span of 21.2 m as designed by
Associated Engineering for the City of Surrey. A typical
elevation of the bridge structure is noted in Figure 7 to
illustrate the general concept.
Piles for this structure were 508 mm diameter, and
filled with concrete. The bridge structure was comprised
of a cast-in-place abutment, with precast box girder
arrangement, and a cast-in-place deck.
5.1
Foundation Preparation
Challenges in Construction
The primary construction challenge associated with this
project was the strict requirements for the contractor to
not encroach within the riparian zone during the course of
construction.
A key benefit of the Sierra facing system is that the
wall components are lightweight and easy to erect
adjacent to the river’s edge without the need for heavy
equipment.
Once the facing units are installed complete with the
biaxial wrap facing grid and erosion blanket, the backfill
behind the wall proceeds with conventional construction
and compaction equipment.
This method affords
protection by installing the wire facing initially, with the
backfill operation occurring thereafter.
5.3
Slope Installation Sequence
Immediately upon bearing surface approval, the
contractor installed the black welded-wire facing elements
(3000 mm long × 508 mm high), complete with the
diagonal support struts.
It is important to note that the black wire cages are not
relied on for long term design strength, and the design
relies solely on the strength of the biaxial wrap. The black
wire form simply serves as a temporary formwork to
confine the outer face of the fill slope during compaction.
The facing cages are then lined with a UV stabilized
biaxial geogrid directly behind the wire cage, along with
the erosion control blanket. The biaxial geogrid is pulled
tight returning horizontally for a distance of 2000 mm at
the top and bottom return of each facing cage (also
known as the wrap).
The compacted granular fills were then placed in
approximate 250mm lifts. At each lift increment of
250mm the appropriate primary or secondary geogrid was
installed in accordance with the stamped engineering
drawings. This method of progressive slope construction
is illustrated best in Figure 8 below.
Figure 9. Well Established Vegetation in August 2012
Figure 8. Looking Toward The East Approach Fills
As each wire lift progressed upward each subsequent
facing cage is offset horizontally by 190mm, to achieve
the final design batter of 8 Vertical to 3 Horizontal, or
alternatively at 69.3 ° from the horizontal. This process is
repeated continuously until the top of final grade is
achieved based on construction survey.
At steps in the top of wall profile, the wire baskets were
returned 600mm into the backfill to effectively form a
corner, by cold-bending the wire cage.
6.
As noted in the Figure 10, the man standing atop the
bridge deck looking down into the north Side of
Serpentine River. This photo is taken from the west side
of the crossing looking upgrade to the east.
It will be obvious to the reader that during this short
time frame of vegetation grow-in, that the steepened
slope is totally obscured by an abundance of vegetation
established along this river course.
The reader will also note the 750mm diameter water
main on the north side of the bridge deck.
ENVIRONMENTAL ADVANTAGES
This site is located immediately south of Tynehead
Regional Park which also home to a fish hatchery facility.
Salmon fry are routinely released from this hatchery
facility as they make their way into the Serpentine River.
The photo shown in Figure 9 was taken approximately
one year after final completion. The reader will observe
the well established vegetation along the North side of the
Serpentine River when looking eastward.
The selection of this vegetated slope system illustrates
the success that is possible where new development,
environmental sensitivity, and engineering all meet.
Figure 10. Looking East across The Bridge Deck
This underside of the completed bridge structure is
viewed in Figure 11, where sheet piles and rip-rap are
noted.
This photo is viewed in a downstream direction as the
Serpentine River flows under the new bridge deck, and
into one of the many deep pools just observed in the
distance.
Although not immediately apparent to the reader, the
author has observed many salmon that frequent this
resting pool, during their downstream travels.
7.
CONCLUSION
The use of green vegetated facing is an attractive
engineering alternative to the more traditional facing
methods involving concrete or block wall solutions.
The slopes utilized on this project will further enhance
the local habitat, and restorative efforts which are
mandated by the City of Surrey on its new construction.
An often overlooked technical advantage is that the
use of HDPE geogrid materials are cited within AASHTO
and FHWA references as not aggressively affected by
corrosion due to the presence of de-icing salts. This is
another long-term feature that should be assessed when
making comparative assessments for clients and owners
where new infrastructure is required.
ACKNOWLEDGEMENTS
The writers would like to thank the City of Surrey and Hall
Constructors for their input in preparation of this paper.
We would also like to thank the engineering professionals
listed in the reference section below, for their overall
contribution in making this project an overall success.
REFERENCES
Figure 11. Serpentine Flow South Beneath Crossing
As noted in Figure 12 deeper portions of the river channel
and associated small pools(lower right of photo) are noted
beneath the crossing, and provide excellent habitat for the
salmon and aquatic creatures frequently seen all along
this river course.
Eivemark, M.M.(P.Eng.) and Hii, P.(M.Eng).
Metro
Testing Laboratories Ltd. Geotechnical Investigation
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Report, Widening of 96 Avenue(160 to 168 Street),
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dated January 29 , 2010.
Eivemark, M.M.(P.Eng.) and Hii, P.(M.Eng).
Metro
Testing Laboratories Ltd. Geotechncial Engineering
Analyses and Recommendations, Widening of 96th
st
Avenue(160th to 168th Street), dated May 21 , 2010.
Associated Engineering, AE Project No. 20092519,
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Issued for Tender, 96 Avenue Bridge at Serpentine
River, Revision 0, dated 2010/06/11.
Figure 12. Looking East Along Serpentine River Crossing