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"Geogrids are plastics
formed into a very open gridlike configuration," writes Robert
Koerner, an authority on geosynthetics. "That is to say, they
have large apertures. Geogrids are stretched in one or two directions."
Geogrids' strength
gives them the ability to be used for base stabilization, for slope
reinforcement, to reinforce the soil mass behind retaining walls,
and for berm reinforcement. In a typical application, geogrids are
embedded horizontally behind retaining walls and between layers
of soil. The geogrids are anchored at the retaining wall and extend
deep into the soil behind the wall. "You tie the grid into
the wall to make the soil and the wall act as a monolith,"
explains Tom Collins, president of Huesker Inc., a geosynthetics
manufacturer. "The wall, the grid, and the soil mass are all
one unit."
Major Wall Construction
A recent grade separation
project in Auburn, WA, involves no fewer than 15 retaining walls–some
75,000 ft.2 of walls ranging up to 32 ft. high. The project
is located at the intersection of State Route 18 and Third Street
SW; SR 18 also passes over a railroad track at the intersection.
Several ramps are involved.
Originally the City of
Auburn called for mechanically stabilized earth (MSE) retaining
walls, with geotextile employed between 12-in. lifts of compacted
soil, to stabilize the earth. At the wall's face, the geotextile
would be wrapped up and around each lift of soil. The final facing
component would be a cast-in-place concrete fascia.
Tensar Earth Technologies
Inc. (TET), however, proposed an alternative using a combination
of geogrids to stabilize the earth and geotextile to wrap the soil
layers at the edge. In addition, TET proposed the use of full-height,
precast concrete panels. Working with a structural consultant and
the contractor, Robison Construction Inc., TET developed a scheme
by which the precast panels could be anchored into place. The city
accepted the revised wall construction method; full-height precast
panels could be used on walls up to 23 ft. high. For higher walls,
cast-in-place construction was used.
Typical lift construction
for the wall proceeded as follows, explains Gerry Kehler, senior
construction engineer with TET. First, L-shape welded-wire forms
were placed along the fill's edge, with the vertical leg sticking
up. "The forms enabled the contractor to maintain a vertical
soil face before it was supported by any concrete wall," he
describes. If only geotextile had been used to wrap the soil layers
at the edge, a "pillowing" effect would have resulted,
making the wall face irregular. To pour concrete up against such
a vertical series of "pillows" would have meant using
extra concrete at the bulges to achieve the required thickness.
In the alternative method
used, TET used geotextile for soil retention only. Geotextile was
embedded just 3 ft. into the fill, and the outside edge was flipped
over the wire basket at the edge. To reinforce the soil, the contractor
laid Tensar Structural Geogrid on top of every 18-in.-thick layer
and extended it into the fill a distance equal to 70% the height
of the wall. For example, a 30-ft.-high wall would require geogrid
reaching 21 ft. from the wall face into the fill. "Or in some
cases where there were ramps on both sides of the fill, the geogrid
extended the full width of the embankment," adds Kehler.
The next step was to
place a steel strut between the vertical and horizontal legs of
the L-shape form baskets. The fill came next. Each lift of coarse
sand was 9 in. thick, and the contractor compacted two 9-in. lifts
into place before placing more geotextile and geogrid materials.
When an 18-in. layer was completed, the geotextile was wrapped around
the sand layer's edge and laid back over the top.
After the fills were
placed, they required a period of time to settle before the concrete
walls were placed. One-piece panels, extending up to 23 ft. in height,
fitted into a cast-in-place leveling pad at the bottom. Near the
panel's top, a steel tie rod was attached at one end to the
back of the panel; the rod extended 13 ft. back into the fill and
was attached to a concrete anchor block set in the fill.
Tensar's alternative
method allowed the construction team to increase the spacing between
geogrids to 18 in. from 12 in. "That decreased the number of
geogrid layers, which meant less manual labor to install layers
of geogrid," points out Kehler. "The increased strength
of the geogrid [over the originally specified geotextile] allowed
a reduction in the required number of layers of soil used to build
the mechanically stabilized earth structure." More importantly,
the contractor eliminated a large quantity of cast-in-place fascia
and replaced it with the more economical precast concrete paver
fascia.
Paving Over Peat
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| A
Tensar welded-wire retaining wall, approximately 12 ft. high,
during construction. The precast panels were installed after
the embankment settled. |
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Sometimes it takes more
than a simple geogrid to stabilize the soil. In Kent County, MI,
officials have paved several highway projects in recent years over
unstable subgrades with the help of a geocell–the Geoweb Cellular
Confinement System from Presto Products Company. Made of high-density
polyethylene, the Geoweb product resembles a honeycomb in structure.
The three-dimensional network of interconnected, perforated cells
is filled with select infill materials such as topsoil, aggregates,
concrete, or a combination of those materials.
Sections of Geoweb come
in various sizes, cell sizes, and cell depths. Cell depths, for
example, are 3, 4, 6, and 8 in. By confining the base material within
the cell walls, the Geoweb system decreases the rate of the infill
material's lateral movement and creates passive resistance
between adjacent cells. The product helps create a stiff base and
distributes loads laterally.
In Kent County, a 1,000-ft.
section of 20-Mile Road had presented problems for years. Over time,
three culvert pipes had settled into the peat at one place, one
above the other. Construction began by widening the road on each
side with a 6-ft.-wide section of sand. "Then we spread separator
fabric full width over the original road," recalls Tom Byle,
assistant engineer for the Kent County Road Commission. For the
fabric, the county specified an 8-oz., nonwoven geotextile.
Next, 8-ft.-wide sections
of 8-in.-thick Geoweb were laid down along the sides of the road,
parallel to the centerline, with a 4-ft. width over the existing
road and 4 ft. over the widened sand section. "We filled the
geocell with blast furnace slag," continues Byle. "That
way we stiffened the edge between the old base and the synthetic
base."
The remainder of the
road base consisted of another 4 in. of slag, followed by another
layer of nonwoven geotextile, which was topped by 6 in. of blast
furnace slag. The road was paved with a 1.5-in. mat of flexible
hot-mix asphalt. "We knew 20-Mile Road would settle, but we
used the fabric and the geocell to get it to settle uniformly,"
explains Byle. It has been several years since the geocell project
was built. "We've never been back," he concludes.
"The road is in relatively good shape for its age. It was built
over 30 feet of peat. Using that technique, we were able to pave
a road that we otherwise couldn't have paved."
For the 20-Mile Road
project, Kent County also used an 8-in.-thick section of Geoweb
to stabilize the base under a culvert. The Geoweb was laid into
the culvert's trench and filled with blast furnace slag. Culvert
pipe was placed on top of the filled geocell, and the trench was
backfilled with slag. "Then we brought fabric over the whole
road grade, over the slag at the trench," states Byle. "The
purpose of the fabric was to keep the pipe from pushing down under
load. And the purpose of the geocell was to distribute the load
of the pipe so that it wouldn't push down."
Stabilizing Soft Subgrade
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| The
Geoweb system was specified to provide needed load support over
organic deposit. |
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| Geoweb
sections were placed over needle-punched, nonwoven geotextiles;
the system was then filled with an open-graded, expanded slag. |
An
unstable subgrade also was the problem facing excavating contractor
Raye Vest Corporation in building a parking lot for a Bennigan's
Restaurant in Waldorf, MD. In excavating the site, notes Vice President
Raymond Vest, "We came to a soft, silty clay. No matter what
you did to it, it wouldn't hold together. It would not compact."
Vest began construction
by using dozers and loaders to strip away 6-10 in. of topsoil from
the site. A proof-rolling revealed that the subsoil was unsuitable,
he recalls. So the contractor used an excavator to undercut about
2,000 yd.3 from the site. In terms of depth, the subgrade
excavation reached another 3-5 ft. down. Still, the bearing capacity
was inadequate. "After we undercut it, it would still pump
again under truck wheels," says Vest.
The solution was to place
some 6,000 yd.2 of Webtec TerraGrid 100 geogrid over
the unstable subgrade. TerraGrid 100 is a two-layer grid, and the
layers are held together with polypropylene stitching, describes
Marketing Manager David Snyder. When the two layers overlap, the
result is a pattern of varied aperture sizes that conform to a number
of different fill materials. The product is used for tensile reinforcement
of the soil.
"Once we laid down
the geogrid, we started putting down 8- to 10-in. lifts of granular
bank-run gravel," relates Vest. "It was a sandy material
with traces of clay. We put from nine to 12 lifts of granular fill
material over the geogrid and rolled each lift with a smooth drum.
They took density tests on each lift." Vest notes that the
dozers stayed up on the fill as they pushed the first lift out over
the geogrid, to avoid tangling the grid into the dozer tracks.
Did the geogrid stabilize
the subgrade and base? "Yes, that parking lot soil was hard,"
relates Vest. "The geogrid is a very fine product. The subgrade
didn't pump at all. The grades for all the lifts held true
up to the top of subgrade."
Nonproprietary Specification
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| Some
6,000 yd.2 of Terragrid 100 geogrid was placed on the subgrade
at this Maryland parking lot to stabilize the soil. Lifts of
granular bank-run gravel, each 8-10 in. thick, were compacted
on top of the geogrid to form a stable base for paving. |
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Geogrids also were used
recently to stabilize the soil behind a block retaining wall on
State Highway 1 in Carmel, CA. But this project brought with it
an interesting lesson: a way to write nonproprietary specifications
for block-wall-and-geogrid systems.
"There haven't
been that many block walls built by Caltrans [California Department
of Transportation] because, up to now, no one has written a nonproprietary
spec that would produce a project with the kind of standards Caltrans
could be comfortable with," says Phil Gregory, principal engineer
with Cal Engineering & Geology Inc. in Walnut Creek, CA. Gregory's
firm was brought onto the Carmel project by Whitson Engineers in
Monterey, CA, the project's civil engineer.
For the project in Carmel,
Gregory demonstrated to Caltrans that a nonproprietary specification
in fact can be written for a block-wall system with geogrid reinforcement.
"We wrote a technical specification with regard to three main
elements," explains Gregory. "We specified the size and
strength of the block; we specified the strength of the geogrid;
and third, we wrote the block-grid connection characteristics. If
you put out the effort, you can write a spec that is nonproprietary
and allows multiple systems to compete."
Gregory's spec worked.
Three combinations of geogrid and block could meet the specification
he wrote for the Carmel project. The winning geogrid selected was
Huesker Fortrac 55/30-20, a coated polyester geogrid. And the winning
block was Anchor Vertica Pro. Construction started in November 2001;
by April 2002, work was complete.
The project consisted
of widening the embankment to carry the highway and building 8,000
ft.2 of retaining wall to contain the widened section.
At the start of construction, Whitaker Contractors Inc. excavated
a 6- to 8-ft.-deep trench along the toe of the embankment's
slope. The contractor next placed a leveling pad of aggregate base
into the trench. Perforated plastic pipe, 150 mm in diameter, was
installed for drainage along the slope side of the leveling pad.
Next came the segmental
retaining wall units. Each block was 18 in. wide by 8 in. high,
and extended 20 in. deep into the embankment. "The first course
of block got geogrid on top, the third course got grid, then the
fifth, and so forth," says Gregory. The geogrid was placed
to extend 11.5 ft. from the wall into the embankment. "You
cut 11.5-foot-long pieces from the roll and lay them perpendicular
to the face," he continues. "That way the strong dimension
is perpendicular to the wall." Each course of blocks was backfilled
with crushed stone. After the stone was placed against the wall,
the contractor compacted 8-in. lifts of soil into place.
"The spacing of
the geogrid is controlled by seismic design," points out Gregory.
"We placed a grid every 16 inches. If it were not for seismic
design, we could have spaced the grids every 24 inches." He
says the wall face was battered at a 1:8 slope–for each 8 in.
of vertical height the wall was set back 1 in., mainly for aesthetic
reasons.
"The geogrid creates
an internally stable block of soil," explains Gregory. "And
that block of soil acts as a gravity structure to retain the rest
of the embankment. Once you make the wall and the geogrid-reinforced
soil into one unit, that unit acts as if it were a gravity retaining
structure."
Frequent contributor
Dan Brown is the owner of TechniComm, a communications business
based in Des Plaines, IL.
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