| Help
for slope stabilization, road construction, drainage,
and other applications.
By
Linda Robinson
The winter
of 1995 started out as usual for the Pacific Northwest - drizzly,
wet, and rainy. But as 1995 slid into 1996, the rain
came heavily and more steadily than usual. Soils had
long passed their saturation levels, and tributaries
of Columbia River flooded small towns. As the storm
set one record after another, the uniform silt soils
around Castle Rock, WA, had trouble holding onto the
roots of vegetation and trees.
In early
February 1996, the rain-soaked soils dragged trees and
debris down a steep slope and onto both railroad tracks
of the Burlington Northern Santa Fe (BNSF) Railway in
Castle Rock. The landslide closed the double-track mainline
between Portland, OR, and Seattle, WA, for five days
while work crews moved mud that was up to 10 ft. deep
and another 5 - 10 ft. of root masses and uprooted trees
off the tracks.
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| Castle
Rock landslide area |
Stan Boyle,
Ph.D., P.E., with Shannon & Wilson Inc., a geotechnical
engineering firm in Seattle, remembers the five days
preceding the slide as producing the "highest rain
on record." The storm also set the record for the
highest rainfall in a consecutive period of four months.
"The
trees were maples and cottonwoods but primarily maples
that had about 30 to 40 years of growth and a 2- to
3-foot-deep root system that spread out and matted together
in the near-surface weathered soil," Boyle explains.
"Because very dense and hard native silt soils
underlie the site, the trees were unable to form a taproot,
which might [have helped] maintain stability."
The area
of the landslide - around 60 ft. deep and 0.25 - 0.50
mi. long - was not a natural slope but rather a constructed
slope cut in the 1930s so the railroad could be rerouted
from downtown Castle Rock. The uniform silty soils at
the site were deposited by glacial floods that eroded
the silt from Idaho and eastern Washington. The soil
particles are very fine, but because the soil does not
contain clay, the particles do not stick together. They
do, however, exhibit some apparent cohesion that might
be associated with a weak cementing agent or have resulted
from the structure of the deposit. When wet, they tend
to weather and can lose their structure and cohesion,
says Boyle.
After the
main landslide - commonly called a "skinslide"
- in 1996, the area was impacted even more by another
slide in 1997 and yet again by a skinslide in 1998.
By the spring of 1999, BNSF Railway had hired a watchman
who patrolled the tracks every hour after a storm.
Shannon &
Wilson was hired to develop methods to reduce the frequency
of landslides at the Castle Rock site. Trent Hudak,
manager of engineering with BNSF, says site constraints
eliminated traditional methods of fixing the problem.
The company was confronted with tight rights of way,
extremely steep slopes, and surface soil weathering
conditions.
"The
goal was to move loose debris down to the unweathered
materials and to make it more weather resistant and
be able to stop the skinslides," Hudak explains.
BNSF worked
with the State of Washington to secure a $1 million
Federal Railway Administration (FRA) grant for the project.
The rail corridor between Seattle and Eugene, OR, is
a federally designated high-speed rail corridor, which
is being enhanced by the Washington State Department
of Transportation to support the state's intercity passenger
rail program.
"The
funding from the FRA was established for testing new
technology that hadn't been tried before on railway
facilities," Hudak says. And that's where the unusual
solutions began. Boyle relates that his company considered
several options, but each one had restrictions. For
instance, the 45° angle and high slope didn't lend
itself well to the use of a retaining wall. The slope
could not be flattened because of limited rights of
way and residence boundaries. Debris fences weren't
an option because too much track occupancy would be
needed to keep the track clear.
Shannon &
Wilson suggested that the problem be solved by a very
nontraditional line of attack - that is, by decreasing
the rate of weathering of the surface soils. The approach
was to install a fully engineered, honeycomblike Geoweb
system from Presto Products Company to protect the surface
of the long, steep slopes along the railroad tracks
and to fill the cells with a coarse aggregate. In other
words, the idea was to protect the soils from vegetative
roots and their destabilizing effects on the soil particles.
This solution
made sense to everyone involved. It was affordable,
discouraged plant growth, decreased infiltration, and
slowed the flow of rain down the slope. With the perforated
geocells, the rate at which the water entered the drainage
system would be slower than with other slope cover methods
and would not overwhelm the retention system that was
about a half mile away, says Boyle.
The project
contractor, Wilder Construction from Everett, WA, rose
to the task of developing some alternative construction
plans. Although clearing surface vegetation and debris
sounds like a fairly typical task for a construction
crew, the steepness and height of the slope had the
workers head over heels.
Mark Hillyard
is the Wilder Construction project engineer who worked
on the Castle Rock site. "The BNSF flagmen did
an awesome job with scheduling while we were working
there," he says. "We had high lifts down by
the railroad lifting crews up the slope, in addition
to crews tethered from up above being lowered down the
slope. And that's with about 80 trains in any 24-hour
time frame. They were so very well coordinated."
The considerable
challenges the construction crews faced included not
only the height and degree of steepness of the slopes
but also access restrictions to the top and bottom of
the slopes and the train schedules. In addition, the
soft silt soils prevented the use of heavy excavators.
Instead of
larger excavators, crews used a spider excavator. Hillyard
describes the machine as having six individual hydraulic
pumps so the motor won't lose oil on steep slopes. Cabled
to a bulldozer at the top of the slope, a hydraulic
winch was used to move the excavator across the face
of the slope. "The operator of the hoe has total
control over the winch to control movements on the slope
and up and down the hill," Hillyard explains.
Working the
individual legs, boom, and bucket of the spider excavator,
the operator had to keep one step ahead of the geotextile
crew blanketing the slope. After covering the slope
with geotextile, it was time to start installing the
Geoweb sections and tendons. Panels were measured, stapled
together, and placed strategically at the top of the
slope. After the tendons were threaded, they were attached
to a deadman pipe.
"The
deadman pipe system is pretty basic," says Boyle.
"We dug a hole, tied the tendons to a pipe, and
lowered the pipe into a 2-foot-wide trench at the top
of the slope. Then we compacted the soil over it."
This got
the Wilder Construction crew swinging. Tethered to safety
harnesses, laborers moved the Geoweb structure slowly
down the slope, expanding it as they went. They then
fastened the tendons at the bottom of the slope. Working
from hoists stationed at the base of the slope, the
crew worked to drive the stake anchors, secure each
Geoweb to the slope, and finally install the restraint
clips in each tendon.
Expanding
the Geoweb system to accommodate the No. 4 and No. 5
aggregate was a feat in itself. The crew used a concrete
conveyor truck equipped with 100 ft. of a 30-m conveyor
boom. Hudak describes it as a telescoping conveyor system
based on a truck with a 100-ft. reach. "We didn't
want it to free-fall, so they dropped it down a tube
on the end of the conveyor. A man in a safety harness
placed the rocks by positioning the tube. All of this
[happened] on a very busy rail corridor."
The Geoweb
cells that were used measured 3 in. deep, and as Boyle
recollects, that caused a few challenges. "The
slope is at a very steep angle, with some areas at 45°,
and it was locally steeper. In these steeper areas,
they couldn't fill the cells all the way. The next time,
I think I would use a tackifier to hold the stones in
place better."
As part of
the FRA grant, a 75-ft. test strip of grass was planted
on the southern end of the project for future comparative
monitoring purposes. Approximately a year and a half
after the project was completed, the test strip was
weathered to 24 in. Boyle reports that when he checked
again in August 2003, the depth of weathering was pushing
2.5 ft. in the hydroseeded test strip. The soil protected
with the Geoweb systems was moist and loose to a depth
of about 1 in., and the next 2-3 in. of soil were moderately
stiff. Still deeper, the soils were dry, firm, and unweathered.
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| Installing
the geocellular confinement system in southern New
York |
Restructuring
Ramps and Overpasses
The New York Department of Transportation (NYDOT) is
converting some of its older roads to interstate standards.
In the first project in the state to use geocells, a
slope along Route 17 in the Southern Tier area of the
state needed some creative reinforcement. Matthew Barendse,
a civil engineer with the Geotechnical Engineering Bureau
of NYDOT, says the department is removing many of the
substandard constructed roads and putting in ramps and
overpasses that will handle the additional traffic loads
when the system is extended to Interstate 86.
Barendse
describes the challenges of one particular area of the
Route 50 West off-ramp: "We had to install a wall
that would be steep - a vegetated wall. Most of it is
[sloped] 1.5 to 1, and one area is 240 meters long with
a maximum height of 9.5 meters. And it is all crossed
by wetlands."
Considering
the need for low costs, an easy installation, and the
requirement to use native soils for infill materials,
NYDOT chose the EnviroGrid geocellular confinement system.
Six-inch x 8-ft. x 4-ft. perforated sections were installed
horizontally and stacked. Additionally, 6-in. x 8-ft.
x 8-ft. sections were used. The plan was to create a
terraced wall system with green fascia strips to blend
in with the vegetation and surrounding environment.
"We
stacked geocells with every fourth layer being laid
with geogrid," says Barendse. "There was a
little learning curve with the geogrid installation,
but we had the manufacturers helping us a lot."
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| Building
the EnviroGrid wall |
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| The
finished product |
At the beginning,
Barendse says, the four-person crews were installing
90-100 units per eight-hour shift. But by the end they
were doing about 50 2 a day or 140 units
per eight-hour shift. All in all, he adds, it took about
a month to install the entire 15,000-ft.2 wall, using
a total of 121,920 ft.2 of geocell.
B. Anthony
Construction Company was contracted to install the EnviroGrid
wall. Guy Rucki, the project supervisor, says the erosion
and sediment control practices on the project could
have been a full-time job by themselves. "The
problem was that it was entirely encompassed by wetlands,"
Rucki explains. "Staying on existing rights of
way, you are still bordered by wetlands, and that poses
a challenge. The state was very critical on erosion
control, even more so than on a normal project."
NYDOT requires
temporary seed and mulch cover to prevent erosion every
time crews leave an area, even for a short time. Rucki
says it makes the job very labor-intensive. In addition,
crews were required to install silt fences at the top
and bottom of the site to prevent sediment from entering
or leaving the property.
Outer cells
of the wall were planted with native wildflowers; vines
and other ground cover were used to present a natural-looking
landscape. As the project was the first in New York
to use a geocellular confinement wall system, Barendse
says the department is excited to see how the wall looks
in the spring when it is overgrown with vegetation and
flowers.
North
Saskatchewan River and Yellowhead Highway
Old roads and new highway construction both are subject
to severe erosion when the right climatic events occur.
When the Yellowhead Highway in Alberta, Canada, was
constructed, the contractors became concerned about
the potential heavy rains and spring runoff eroding
the newly created highway ditches. Because runoff from
the ditch flows directly into North Saskatchewan River,
sediment control was a very high priority.
After considering the options, the contractor decided
to install Nilex GeoRidge berms and erosion blankets.
Prior to laying the erosion blankets, grass seed was
broadcast over the area after soil grading. After the
blankets were stapled down using specs issued by North
American Green's DOT System, the GeoRidge berms were
installed perpendicular to the flow in the ditch.
The permeable
plastic berms controlled the velocity of the stormwater
in the channel and prevented erosion of the soil and
seed under the blankets. GeoRidge berms reduce the velocity
and energy of the water and slow the water movement
over the erosion control blanket, allowing sediment
to build up over the blankets. This aids in reducing
the velocities further and allowing vegetation to establish.
Eventually the vegetation in the Yellowhead Highway
ditches became established, creating a more permanent
system.
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| The
1.5:1 slope of the mini-landfill (above)
and geosynthetic placement (below) at the
Northeast Philadelphia airport site |
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Geosynthetics
Arrive at Philadelphia Airport
Road construction is only one of many uses for geosynthetics.
In developing 100 ac. at the privately owned Northeast
Philadelphia Airport, the Philadelphia Industrial Development
Commission ran into problems when an old dump area,
used for sewage dumping some 35 years previously, was
discovered. Options were considered, but hauling all
of the debris off-site was totally cost-prohibitive,
says Archie Filshill, manager of InterGeo Services.
"So
they had to excavate it and basically build their own
landfill and contain it in order to develop the site,"
he says. "The area had two sides with a three-to-one
slope. But the other side was near vertical at about
a one-and-a-half-to-one slope."
Filshill
explains that high-strength fabric, SI Geosolutions
Geotex 4X4 and Tensar BX1100 Geogrid, was used as a
face wrap. InterGeo Services is primarily a geosynthetics
installer and in certain areas of high erosion opted
for ACF Environmental Landlok 450 turf reinforcement
mats as a long-term solution. When finished, the mini-landfill
site measured 20 ft. high and 600 ft. long and was fully
vegetated.
"On
the three-to-one slope, it would have reduced their
area. But by pushing it up to the property line, there
was a savings in area to develop," explains Filshill.
"That gave them about 4 acres that could be developed
for a parking lot for employees."
A 1 million-ft.2
retail storage warehouse now stands on what was once
only a weed-infested lot.
Stabilizing
a Vertical Slope
High-strength geogrid and geocellular confinement systems
have made it possible to develop in high-priced areas
as never before. These systems are exactly what the
Irvine Company discovered would help develop a pricey
southern California property known historically for
landslides, steep slopes, and poor soils. Geogrids made
it possible for the land development without compromising
views of the renowned Pelican Hills Golf Course or the
Pacific Ocean.
A more-than-200-ft.-high
mechanically stabilized earth slope required in excess
of 400,000 yd.2 of Mirafi Miragrid to withstand a large
displacement of adjacent landslide soils sprawling along
the property lines. The majestic-looking reinforced
slope measures more than 500 ft. wide. After receiving
the master rolls of geogrid, the contractor put the
rolls on a large spool and unrolled the 3,000 lb. of
material over the face of the slope. After stabilizing
the area, he moved the spool to the next area and unrolled
for the next cut.
"Surprising"
was how the contractor with Sukut Construction described
the process and the ease of installation. Crews installed
more than 5,000 yd.2 a day to complete the project.
Lots once described as "unbuildable" are
now vegetated and look natural enough to blend in with
the rolling hills of the Newport Beach coast.
How
Old Is a Road Before It’s Not a Legal Road?
As communities spread out - whether to expand to
the suburbs or in search of open space for recreational
use - development of the transportation infrastructure
seems imminent. Old roads are being scrutinized by transportation
departments and the courts and are being distinguished
based on how they were created. Some historic roads
are even being questioned in court regarding issues
of expansion, intensification of use, or abandonment;
debates take place over whether these should be treated
as new construction or reconstruction projects.
The State
of Utah's Department of Natural Resources found the
issue of roads and public rights of way important enough
to publish a Web site that categorizes roads according
to their creation and their use (www.nplnews.com/library/rs2477/roads.htm).
When a 1995
flood washed out a public road in the small town of
Jarbidge, NV, it sparked a court battle that has lasted
for more than eight years. After the washout, Elko County
attempted to repair the road because historically it
was used by locals and tourists to reach public forestlands
and some private land in Elko County. In 1999, the United
States Forest Service placed a moratorium on road construction,
allowing 18 months to draft a new road-management policy
- and time to let the dust settle on South Canyon Road.
With 380,000
mi. of road to manage, the US Forest Service currently
has eight times more miles of roads in the national
forest system than in the whole interstate highway system.
Public lands need public roads, and in April 2003, Humboldt
National Forest Supervisor Robert Vaught announced that
the Jarbidge Canyon draft environmental impact statement
was available for public comment.
The draft
includes alternatives from Elko County to reestablish
South Canyon Road to preflood configuration. Other alternatives
include building a trail instead of opening the road
and a plan that would relocate the road to a place outside
the canyon bottom. As of September 2003, Doug Clark,
who is the Humboldt National Forest Service planner
for the Jarbidge area, says no decision has yet been
made, but he says any roadwork that does take place
will have to involve the use of geosynthetics to stabilize
the historic road. South Canyon Road dates back to the
1800s when sheepherders moved their stock from Elko
to Jarbidge and over the passes toward Boise, ID.
Geotextiles
and Other Geosynthetics Applications
According to the Geosynthetic Research Institute (GRI),
geotextiles now form one of the largest groups of geosynthetics.
Unlike traditional textiles, geotextiles are made of
synthetic fibers instead of natural ones, and this lessens
the importance of biodegradation. Geotextiles are permeable
to water flowing across their plane and within it, and
they always perform at least one of five functions - separation,
reinforcement, filtration, drainage (when impregnated),
and barricading moisture - according to GRI.
Although
geotextile uses are constantly growing, listed below
is a selection from the GRI Web site, www.drexel.edu/gri/geomat.html.
Separation
of Dissimilar Materials
- Between
subgrade and stone base in unpaved roads and airfields
- Between
subgrade and stone base in paved roads and airfields
- Between
subgrade and ballast in railroads
- Between
landfills and stone base courses
- Between
foundation and embankment soils for surcharge loads
- Between
foundation and embankment soils for roadway fills
- Between
foundation and soils and flexible retaining walls
- Between
slopes and downstream stability berms
- Beneath
precast block and panels for aesthetics paving
- Between
drainage layers in poorly graded filter blankets
Reinforcement
of Weak Soils and Other Materials
- Over
soft soils for unpaved roads
- Over
soft soils for airfields
- Over
soft soils for railroads
- Over
soft soils for landfills
- Over
unstable landfills as closure systems
- For lateral
containment for railroad ballast
- To wrap
soils in encapsulated fabric systems
- To aid
in construction of steep slopes
- To stabilize
slopes temporarily
- To halt
or diminish creep in soil slopes
- To anchor
facing panels in mechanically stabilized earth walls
- To create
a more stable sideslope due to high frictional resistance
- For use
in insitu compassion and consolidation of marginal
soils
Filtration
(Cross-Plane Flow)
- In place
of granular soil filters
- Beneath
stone base for unpaved roads and airfields
- Beneath
stone base for paved road and airfields
- Around
crushed stone surrounding underdrains
- Around
crushed stone without underdrains (i.e., without French
drains)
- Beneath
landfills that generate leachate
- As a
silt fence
- As a
silt curtain
- As a
flexible form of containing sand, grout, or concrete
in erosion control systems
- As a
flexible form of restoring scoured bridge-pier bearing
capacity
- Between
backfill soil and void in retaining walls
- Between
backfill and gabions
- Against
geonets to prevent soil intrusion
- Against
geocomposites to prevent soil intrusion
- As a
filter beneath stone riprap
Drainage
(In-Plane Flow)
- As a
drainage interceptor for horizontal flow
- As a
drainage blanket beneath a surcharge fill
- As a
drain behind a retaining wall
- As a
drain beneath railroad ballast
- As water
drain beneath geomembranes
- As an
air drain beneath geomembranes
- As a
capillary break in frost-sensitive areas
- To dissipate
seepage water from exposed soil or rock surfaces
Geogrids
Function as Reinforcement Materials
They are plastic and formed with an open gridlike configuration
and have rather large openings that are frequently filled
with aggregate. They are high in strength and resist
creeping. GRI reports that geogrids can be used as follows:
- Beneath
aggregate in unpaved roads
- Beneath
surcharge fills or temporary construction sites
- To reinforce
embankment fills or earth dams
- To repair
slope failures and landslides
- To construct
of mattresses for fill over soft soils
- As sheet
anchors and facing panels to form entire retaining
walls
- To reinforce
disjointed rock sections
- As inserts
between geotextiles
- To reinforce
landfills to allow for vertical expansion
- To reinforce
landfills to allow for lateral expansion
- To stabilize
landfill cover soil as veneer reinforcement
- As three-dimensional
mattresses for embankments over soft soils
Linda
Robinson is a journalist specializing in agriculture
and land-use planning.
EC
- January/February 2004 |
