Large-scale retaining walls take center stage. By Tara Beecham
The capabilities of retaining walls have grown in recent years, both through the versatility of the materials they are composed of as well as in their actual largesse. Whether they are segmental, reinforced earth, poured concrete, or made from a completely different material, retaining walls—long associated with small commercial properties—have blossomed from just supporting into starring roles.
While they continue to be used along highway embankments, beside shopping centers at risk for erosion, and anywhere difficult terrain can be located, retaining walls can be found literally holding their own in large-scale erosion control projects across the United States.
Runway Resilience in Seattle
The Seattle-Tacoma International Airport (Sea-Tac), which serviced nearly 30 million travelers in 2006, needed a third runway to reduce flight delays as well as improve operating efficiency in all weather types. Located in a cloudy climate, the airport often has traffic that on a clear day could utilize its two runways for landings, restricted to one runway. Runways must be separated by a minimum of 2,500 feet for them to be available for use during bad weather, according to the Federal Aviation Administration, and the Seattle area has low-cloud conditions approximately 40% of the time. But limiting plane use to one runway during inclement weather causes delays and costs the airport money, particularly in terms of personnel and fuel.
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Photo: Sea-Tac Airport |
| Construction of a third runway at Sea-Tac involved bringing 16
million cubic yards of fill onsite to create an embankment. |
Because the third runway, which is expected to cost between $1.1 billion and $1.2 billion, needed to be raised to an elevation comparable with Sea-Tac’s other two runways, the Port of Seattle brought more than 16 million cubic yards of fill into the site to create an embankment.
Three mechanically stabilized earth (MSE) walls—walls that use layers of fill material and galvanized steel straps placed behind concrete wall panels—were planned for the site to protect both the runway embankment and the surrounding wetlands and creeks.
The largest of the three walls, all designed by Vienna, VA–based Reinforced Earth Co. (REC), is the west wall, at 1,460 feet long and about 130 feet tall at its highest point. The wall has an average height of 74 feet, with four tiers. This wall is considered the tallest MSE wall in all of North America.
The face area covered by the west wall is approximately 12,100 square meters. A length along the top tier is approximately 450 meters, and a length along the bottom tier is about 190 meters, as cited in a statement by project engineers. Slip joints were added to this wall to better anticipate the differential settlements at critical elevation changes within the wall, according to a paper provided to the airport by project engineers. According to the paper, “The additional design measures resulted in a reduction in total settlement to approximately 150 mm, with a maximum differential settlement of about 1/100 to 1/200.”
The two-tiered, 1,220-foot-long north wall reaches 86 feet high in places and has an average height of 54 feet. Like the north wall, the 1,035-foot-long south wall also protects the runway embankment. The one-tiered south wall, which is 55 feet at its tallest point and has an average height of 27 feet, both preserves and protects the neighboring wetlands and Walker Creek.
“They went through a very rigorous evaluation of methods, at least five or six years of evaluation and design,” says John Shall of the Reinforced Earth Co., explaining that the Port of Seattle is the overseeing engineer of the project. “It’s almost complete.”
The project began in 1997, but the last of the permits weren’t in place until 2002, explains John Rothnie, airfield program manager for Sea-Tac Airport. The runway itself will be 8,500 feet long, 150 feet wide, and 17 inches thick, and it is designed to last 40 years.
REC was chosen because the company has done the most types of these walls in the United States, according to Rothnie. Used in many civil engineering and transportation projects, reinforced earth walls have high load-carrying capacity and are useful when a project site requires tall or heavy-loaded retaining walls. Because the composite material is flexible, workers can use these types of walls in areas where there are slopes that are unstable or where there are compressible foundation soils.
“For a fill project like this, it’s the most efficient wall from the cost and the time to do it. You reduce the amount of fill required with a reinforced earth wall and the straps that tie in to the fill that you place,” says Rothnie. “Essentially the fill behind it becomes part of the structure.”
Tests were performed on the select granular backfill that was used.
Shall explains that proper drainage considerations are integral to the project’s design. “We have free-draining material behind the wall. It either gets conveyed out or behind the wall. Very careful consideration was made on the design. It was monitored throughout the construction.”
Though necessary to meet the airport’s challenges, the third runway project faced several difficulties. The site itself posed environmental challenges, the most prominent being the adjacent creeks and wetlands.
“Miller Creek and the adjacent wetlands probably had the greatest challenge. There was unstable soil underneath the west wall,” says Rothnie. “Our excavation at the wall was 50 feet away from the creek but 20 feet lower than the creek. That had to be sheet-piled, the unsuitable soils removed, and [new] soil added before we could build the wall. The north-end wall is 10 feet away from wetlands, with no impact to the wetlands.”
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Photo: Key West Retaining Systems |
| Drainage proved the biggest challenge during construction of a 40,000-square-foot
retaining wall at Roseville, CA’s Fairway Creek Business Center. |
The project’s north wall, like the west wall, helped limit the amount of wetland filling. It also reduced the embankment footprint to the extent that it was not necessary to impact nearby Miller Creek.
Potential earthquakes were also a consideration during the project’s design.
“It was certainly a consideration. It was modeled for seismic events,” says Rothnie, explaining there is a several-hundred-year interval on seismic events. “This is the recurrence factor of an earthquake like this. The model is created to withstand that.” In general, MSE walls are considered largely earthquake resistant.
A peer review with national experts examining the wall design was performed before the wall was built.
“We couldn’t have a 2-to-1 slope. We had to protect the creek and adjacent wetlands in a couple of places, and we relocated a road. For 130 feet high, the slope would extend and we would have had to fill wetlands. I think it was an environmental mitigation measure.”
The third runway’s retaining walls have to be strong, but their prominence in the surrounding area demands they be beautiful as well.
Because of the high number of people who travel through the airport and the sheer size of the outdoor project, aesthetics was a concern. A portion of the project’s budget was allocated to create artwork that would be integrated into prominent exteriors of the north and west walls. The north wall is visible to travelers along Highway 518 and Des Moines Memorial Drive.
Rothnie says project workers are using embedded art—concrete panels within which the art is formed—a technique he commonly sees as a trend in the western United States. Artist Carolyn Braaksma and the Surface Strategy Studio based in Denver, CO, created the design.
“It’s pretty intricate,” Rothnie says. “It’s a nice effect.”
A ship is depicted on the north wall, with mastheads located at each end. There are also layered images of the Puget Sound, including plants and native birds. The top tier of the west wall features companion elements. As a whole, the art helps the wall mirror the commercial—as in the ship—and natural elements present in the greater surrounding area.
Interchange Support in Tennessee
Murfreesboro, TN, experienced a population boom of about 53% between 1990 and 2000. It was only natural that highway projects would be built to support the growth of the “most livable city in Tennessee,” as it has come to be known, with its combination of good schools, an expanded housing market, and a variety of employment opportunities.
Along Interstate 24 at Manson Pike in Murfreesboro, a Tensar International Mesa Retaining Wall System was used as part of a new interchange project for the Tennessee Department of Transportation that was completed in 2004.
“The interchange required a new bridge over I-24. The bridge abutments and the grading for the off-/on-ramps required retaining walls,” explains Joseph Harris, a project engineer and president of Cumming, GA–based Pinnacle Design/Build Group. The company, a geotechical subcontractor that designs and builds retaining wall and erosion control systems, built the retaining walls at the site.
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Photo: Redi-Rock |
| Easy to install and inexpensive, wet-cast blocks provide a
natural-looking façade and minimize deterioration. |
The wall is an MSE structure, and bridge abutments are supported by steel piling driven through the geogrid-reinforced zone.
Geogrids became a critical stabilization element as part of the wall at the interchange. How can geogrids work to stabilize retaining walls?
Made with high-molecular-weight polyester yarns that are cosseted by a PVC coating, geogrids emerge when the yarns are woven into a grid that resists degradation, according to SRW Products, a geogrid supplier based in Princeton, MN. Soils located above and below the geogrid are permitted to connect together and develop a form of resistance to movement.
When geogrid is installed, soil is compacted before the geogrid is positioned in front of a block and over a connector. Geogrid should not overlap in layers but instead touch at the product’s edges. Stapling the grid into the site’s soil should be the next step. After the next block is added, crushed rock should be added to the core of the block, according to SRW.
Backfill soil then should be placed allowing a void for crushed rock to be left just behind the block, with a cushion of soil remaining between the geogrid and equipment. SRW recommends crushed rock be added into the 6- to 12-inch void present between the block and the compacted backfill soil.
Constructed at both ends of the bridge, according to Harris, the wall is 3,352 feet long and has a maximum height of 28 feet. The total wall area is 56,840 square feet. The polymeric geogrids, from Atlanta-based Tensar, have open aperture designs that allow them to interlock with natural fill materials. As in this example, they are often used to reinforce slopes and also when building low-height segmental block retaining walls. “The retaining wall backfill was imported crushed rock from a local quarry,” says Harris. “The free-draining imported select crushed rock backfill provided a retaining wall that did not allow hydrostatic pressure.”
Finally, the retaining wall style that was chosen helped allay budgetary concerns. “There was a cost savings over conventional construction,” says Harris, “while still providing the structural integrity.”
Retaining Wall Choice Marked Drainage Solution
Drainage was the biggest challenge during the construction of a retaining wall at the Fairway Creek Business Center in Roseville, CA.
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Photo: Patrick Flanagan, Piedmont Precast MSE Site Solutions |
| Access is proving difficult on the Claire Rose Lane project in Sandy Springs, GA;
all but one of the development’s high-end homes are already in place. |
Key West Retaining Systems based in Oregon’s Portland region began work on the project, which included a concrete gray-colored wall that was about 40,000 square feet. Marvin Wyatt, president of Key West, says onsite backfill was added for use with geogrids. The sandy gravel, native to the site, was then screened to 4-inch minus gravel at 1 foot of road base, and 0.75-inch minus gravel at the base.
For drainage, a burrito drain was used, explains Wyatt, adding that 2 feet of open graded rock was added and then covered with a nonwoven filter fabric. Woven fabric, he explains, would hold up the water.
Lock+Load panels were used for the wall. “At the back cut of a wall,” he says, “we have to cut out [about] 18 feet. You have to have a 70% to 80% cut ratio for the height of the wall.” Geogrid embedment extends about 80% of the wall’s height. “It’s like having a big soil nail holding your panels on,” says Wyatt. “The geogrids work with all the systems.”
Access was less of an issue at this site, located about 16 miles north of Sacramento, than for some other projects. “We had pretty good roads cut down,” he says.
“Lock+Load is an incredible product. That was a definite advantage. You can pack everything at 95% of its density. This wall system, because it has a counterfort, allows you to compact with heavy equipment at the wall face. It takes uniform compaction from the base all the way back to the end of your cut,” says Wyatt. “It was a tight site, and we were able to go near vertical with the batter, saving them real estate. You save a person 2 or 3 feet for a quarter-mile—that’s a lot of real estate.”
Aesthetics Concerns Abated in Georgia Neighborhood
Access is proving challenging during the ongoing Claire Rose Lane project in Sandy Springs, GA, an Atlanta suburb where posh residences are situated just above a very high retaining wall.
“We’ve got a very tight site,” says Patrick Flanagan of Piedmont Precast and a director at MSE Site Solutions, an associated company, located, like Piedmont Precast, in Atlanta. He says the wall tops out at 37 feet. “It’s about 250 feet long, with an average 24-foot height.”
Construction on the site began midsummer 2007, in the hot, arid climate. Although the area is under water restrictions because of serious drought conditions, the lack of rain hasn’t dampened the site workers’ spirits.
“Rain is not our friend while we’re building,” says Flanagan. “Water is the enemy of a wall.”
Piedmont Precast manufactures Charlevoix, MI–based Redi-Rock’s precast concrete blocks used in the company’s retaining wall systems. Piedmont chose to use the cobblestone face blocks. The average weight of each block is 2,000 pounds; the blocks are made with 3,000- and 4,000-psi wet-cast concrete. They are formed to create the appearance of natural stone, though they are easier to install and less expensive. Each block has 5.75 feet of textured face material. In addition to allowing a more natural rock-styled façade, deterioration as a result of freezing and thawing is minimized through the use of the wet-cast block process.
Aesthetics was a huge concern for the neighborhood, where home prices begin in the $4.5 million range. The wall is structurally “a cast-in-place wall with a veneer,” explains Flanagan, adding that the company saved the customer money by choosing this wall option.
Although these types of walls can be stacked as high as 13.5 feet without geogrid, geogrid can be used for walls that extend higher. The soil—a silty sand—had to be imported, as did the stone, both applied as backfill in some areas as the site area was raised. Stone was placed 12 inches behind the block for proper drainage.
“It’s there for the settlement of the geogrid behind the wall. The failpoint on geogrid connection is at the block. That’s the most critical connection. If dirt settles behind and grid goes down behind the block, we’ll put stone [in],” says Flanagan. “That will minimize settlement behind the wall. We do use drainage below the footing, perforated 4-inch pipe behind the wall. If we see water coming out of that pipe, we’ve got problems. We rarely ever see water coming out of those things.
“There are vegetated things on top of the wall. If you’ve got hydrostatic pressure on the wall, you have much bigger problems than you could ever design for.”
Access to the site was challenging, according to Flanagan, who explains that with the exception of a single house adjacent to the site, all the development’s homes were in place.
“That obviously slows our production rate down,” he says, explaining that crews had to use a large excavator to add dirt to the wall. “We’re at a point where we can’t pull dirt over the wall. We’ve had to build a temporary access road to get to the site.”
Often, the easiest and the fastest retaining walls to install are those with straight lines. The Claire Rose Lane project’s design, however, also threw contractors some curves: those related to the actual design necessary for the wall’s success and others that had to do with the simple natural features located at the site.
“Most retaining wall contractors like to build a straight wall with no curves,” says Flanagan. “This one has concave and convex curves in it.”
The wall had to be installed around some large, natural boulders, and waterfalls are also planned for the site, which contains two additional walls. A natural spring has been incorporated into the project’s water feature.
Natural or threatened areas located near business hubs or homes, as in this example, remain common locations for retaining walls.
Just as challenges in the form of shoddy soil, steep slopes, access, and climate will present themselves time and again on unstable project sites, evolving engineering technology will allow retaining walls to continue their reach toward the sky.
Based in Morgantown, PA, Tara Beecham writes frequently for Erosion Control.
EC - January/February 2008
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