Designers discuss drainage, subsoils, settlement, cost, and appearance.
For some people, appearance means everything.
While a retaining wall has always served the purpose its name implies, these days, those who build them are concentrating on looks as well as substance.
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Photo: Rinker |
| The industry trend is moving toward more natural-looking retaining walls. |
“Appearance is everything,” Glen Reddemann says of retaining walls. He’s vice president in charge of operations for TPC Landscape in Burnsville, MN. “We’re a higher-end company, so functionality is one thing, but appearance is everything,” he adds. “We like to structure a product that is maybe a little bit more unique than just a gigantic modular wall.”
For that and other reasons, his company has turned projects down. “They may spec a product we don’t really want to use or put our name behind,” he says. TPC Landscape favors Interlock Concrete Products or Anchor Retaining Wall Systems for its retaining walls.
Reddemann says the trend for retaining walls for the past two decades has been modular block walls that look more like a concrete block. “They have turned the corner and are trying to look more natural, which has been exciting,” he says. “People don’t necessarily want to use natural stone, or there may not be a place where you can apply natural stone. You can use a lot of other products that have multiple sizes and pieces and roughed base that look more natural. It beautifies the area more than just a concrete wall that has no aesthetics in appearance whatsoever.”
As for the engineering aspects, Reddemann says structural considerations often determine the options his company has available when building retaining walls. The subsoils—heavy clay or sand—in combination with such factors as nearby parking lots or residential buildings determine the products his company will use. Other factors include the load rates and whether the wall will be structural or ornamental.
Regarding heights, Reddemann believes “you can just about engineer any product to go any height you want. As long as the engineering specs come out, we’ll build it. I don’t believe that there’s a maximum height to really any product.”
He notes soil type is another engineering consideration that affects the strength and height of a wall. “The soil conditions under the wall and behind the wall may call for amending prior to doing anything,” he says. “But whatever that may be, it’s done pre-construction. They can take any project that’s engineered and put it up, based upon what you have to do in amending soil. Maybe they want to lay the wall on a concrete pad. We have a lot of frost issues we deal with. The heavier soils, like the clays, move more frequently than sand. We’ve got just about every kind of material you could ask for, from rock to sand to heavy clay to loam.”
Whether soil is used from onsite or imported depends on the development, Reddemann says.
“If they’ve got good material or the engineering specs say to use the material that is there, we don’t import it if we don’t have to because the additional costs incurred would be great, especially with the current costs of trucking,” Reddemann says. “For the most part, we like to use the product that is there.”
Reddemann says his company likes to use clear crushed limestone for compaction behind the wall.
“The majority of that is to disperse that hydrant pressure, the water limitations from the soil behind you,” he says.
Reddemann believes that for addressing drainage issues, drainage swales are always the best approach. “If the grade is done correctly, the majority of the water will either run off the wall or come over the top of it; some will leach through or run through the drain tile system you place in the back,” he says.
“The best idea anybody ever came up with is keeping water away from areas you don’t want water. Drainage swales being cut in front of the wall so the water isn’t coming to it is your best bet.”
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Photo: Rinker |
| Retaining walls may be aesthetically pleasing as well as functional. |
Slowing the Waves
Wall appearance was the primary reason Mark Thomas’s company was chosen for a recent concrete retaining wall project in the Flathead Lake near Glacier National Park in Montana. “Because it is concrete, it can be stained, and the number of stains available are phenomenal—they’re water-based, oil-based, acid-based—and it depends on what you want to do with it and what color you want. That’s very popular in the upper end of the client market,” says Thomas, owner of Diversified Materials & Construction of Missoula and Polson, MT.
But this particular project had many other factors involved as well. The Confederated Salish and Kootenai Tribes, who oversee construction below the high-water mark of Flathead Lake, maintain strict regulations for shoreline protection retaining walls. Thomas did not have many options, as he had to abide by the tribes’ restrictions for permitting. The Tribes’ 64A Shoreline Protection Ordinances recommend wood or riprap walls for the natural look they provide, with concrete walls being used as a last resort.
Additionally, tribal regulations call for wave speed, upon hitting a wall, to be slowed in an effort to prevent erosion and sand transport.
But in Polson, a Flathead Lake resident’s rotting wooden retaining wall on the shoreline of his residence led to the tribes’ considering other alternatives to traditional shoreline protection walls: a Redi-Rock concrete retaining wall.
Thomas explains that Flathead Lake is a large lake, measuring 28 miles long north to south and 5 miles wide, and can generate some rough action as a result of storms. “It’s not uncommon to see 10-foot rollers on this lake,” he says. “The wall we were replacing was 9 feet tall—8 feet above lake bed and another foot buried below lake bed—and the house sits approximately diagonal to shoreline but as close as 30 feet.” Losing the wall during a storm would “definitely threaten the home very quickly,” he adds.
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Photo: Rinker |
| A site's subsoils are among the factors that help determine what products to use. |
The situation was complicated by the fact the space above the wall had previously been built up with walkways, requiring the removal of large amounts of concrete before the wall could be replaced.
Preliminary testing demonstrated that the Redi-Rock stone texturing caused waves to accelerate significantly less than a poured-in-place wall. But further measures were needed to slow the waves to meet the tribes’ standards, so Thomas proposed using Redi-Rock protruding planter blocks to slow wave action and provide a safe place for swimmers to enter and exit the water.
The protruding planters—usually used for ornamental plants in aboveground walls—were manufactured with the planting area filled with concrete. The planters were staggered across the walls every 16 to 20 feet, slowing wave speeds to well below the tribes’ standards. Thomas calls the protruding planter—which exceed the height requirement to be considered a step—modified ladders.
The large block size enabled the 9-foot-high wall to be installed without using geogrid or tiebacks. By the end of the project, Diversified Materials & Construction had built a 660-foot-long wall with 11 modified ladders. The project began on January 15, 2006, and was completed on February 26, 2006.
Meanwhile, in an effort to evaluate the effectiveness of the Redi-Rock modified ladder design, the tribes have installed sensors at various points on the wall to determine how the wall affects wave speed in open areas, around curves, and near the modified ladders. Preliminary testing results have been positive.
Thomas says when his company designs a wall near water, it starts by designing one that has a variety of wave patterns “to try to get an optimal position based on the seasonal storms. Which kind of wave pattern is going to throw more of the waves back off of the wall rather than allowing them to run down the wall?”
In this case, the wall was more than 600 feet long, the bulk of it being 9 feet high, but some 200 feet of it shorter where it wraps around a point built into a boat ramp and a private marina.
As for drainage issues, Thomas says they are addressed by elevations. The house sits on a low point coming off of a steep hill. “In this instance, we lowered the new wall by 6.25 inches lower than the previously existing wall,” Thomas says. He cites a few reasons for the decision. The wall transcends two separate pieces of property owned by the same family. The smaller of the two homes is situated higher than the other, and by lowering the wall by 6 inches, Thomas’s company was required to alter the shape of the ground coming into the wall area.
Thomas says drainage wasn’t hard to control “because we were removing the entire wall and chewing up anywhere from 12 to 15 feet above the wall by running heavy equipment to build the wall. This allowed us to reshape pretty easily to compensate for any hillside slopage.”
After Ivan
Hurricane Ivan passed through the Soddy-Daisy, TN (Chattanooga), area in 2004, filling up a wide creek with water. Runoff from a mountain had swelled up, eroding land behind houses located near the creek.
In one case, the land—part of the North Chickamauga Creek Watershed and owned by the US Department of Agriculture Natural Resources Conservation Service—had eroded so much that a house was close to hanging off of the edge of a cliff area. After investigating the situation, the US Army Corps of Engineers chose to use gabion retaining walls to restore some of the land in the neighborhood, located near the intersection of Dayton Pike Road and Montlake Road.
Erosion and Retaining Wall Structures Inc. (ERWS) of Lewisville, TX, installed gabion retaining walls on either side of a bridge; on the top of the walls sloping up to the surrounding homeowners’ property lines, the company installed riprap to provide creek bank erosion protection. In executing the project, ERWS used 4,500 cubic yards of Maccaferri gabions and 6,000 tons of riprap. The project lasted about four and a half months in the latter part of 2005.
Joe Schweighofer, ERWS’s operation manager, says access was an issue for his crews because the work was being done in a creek, so maneuvering the machines was going to be a challenge. “If it rained, you had to pull your equipment out,” he says. “And people live there, so you couldn’t go into their yards.” In fact, Schweighofer says the job’s biggest challenge was dealing with the homeowners. “They were happy we were there to do it, but we were working right up behind their houses, so we had to be mindful of that,” he says. “Plus, we were working in a creek. The benefit was that the whole time we were there, it didn’t rain enough to where the creek swelled up.”
When the project was bid in the summer of 2005, one of the stipulations was that it had to be done within a particular time period to avoid rainstorms—or snow runoff in the winter—that would curtail work time by causing more erosion problems.
Because the creek was a couple of hundred feet wide, workers were able to channel the water through the center as the gabions were being constructed and as they worked perpendicular to a bridge.
Another challenge was working up the height of the walls, Schweighofer adds—18 feet tall, with 12 feet of that being under the ground.
The gabion’s appearance was quite important in this project, Schweighofer says. “This was in people’s backyards—you had to go with something that was going to look good, not just put up any old wall,” he says, adding that the gabion also was easily visible from the road.
Building Vertical
A value-engineered solution saved $1.5 million in a retaining wall project in Santa Clarita, CA, executed by Key West Retaining Systems in Wilsonville, OR. The Golden Valley Roadway Project entailed construction of a retaining wall utilizing materials from Lock+Load of Vancouver, British Columbia—of which Key West Retaining Systems is an affiliate construction company—along Golden Valley Road where it meets with Soledad Road.
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Photo: Lock + Load |
| Constructing a vertical structure presents particular challenges. |
The wall is 1,100 feet long and 86 feet wide from side to side and ranges in height from 11 feet to 40 feet. It supports six lanes of traffic. Because of right-of-way considerations, it needed to be built vertical, explains Ben Stores of Lock+Load. Construction of the wall took about 50 days.
Stores says the retaining wall needed to have the ability to take differential settlement. “When you stack 40 feet of dirt vertically, you exert such a great amount of pressure on the ground that sometimes your ground may not be able to support 5,000 pounds per square foot without getting a little bit of settlement. The subgrade gives a little bit. It had to be able to take a little bit of differential settlement without affecting the aesthetics or performance of the wall.”
Another consideration was the desire not to have to use the imported structural fill—crushed gravel—specified in the original design. “If we didn’t use crushed gavel for the fill, we could use onsite soils available across the street at a significant savings over the crushed gravel,” Stores says.
Stores says the original design also called for using steel reinforcement. “By going to onsite soils, geogrid instead of steel was used for the reinforcing zone.”
He says the biggest challenge on this project was constructing a vertical structure of 40 feet. The contractor, Security Paving of Sun Valley, CA, had never built this type of wall system before, “and anytime you are building vertical, there’s always a little bit of consternation, because what happens if it goes over vertical?” says Stores. To help address that issue, Lock+Load provided onsite construction assistance.
Stores says most retaining walls typically have a lightly compacted zone behind the wall face. “One of the major structural considerations was that this particular system can accept full compaction to the back of the wall panels.
“If this starts settling and twisting, we would have major problems with the structure we were putting on top,” he says. “One of the reasons for choosing this system was it could take full compaction to the back of the reinforcing wall panels.”
Again, appearance was important for the retaining wall. “Santa Clarita wanted something that was fairly attractive from an aesthetic consideration,” Stores says. That led to a consideration in the project that the wall system had to be able to take a fairly large surcharge because the traffic barrier was cantilevered 14 inches over the wall face, Stores says.
After the main wall was completed, pedestrian lights resembling old-style gas lamps were incorporated from an architectural standpoint into the top of the structure and were 18 inches hanging over “so the entire lighting structure was hanging cantilever over the top of the retaining wall … there was nothing underneath it,” Stores says.
Matching Quarry Rock in Vancouver
The Lasst Construction Co. in Vancouver, WA, constructed over the last nine months of 2005 what is believed to be the largest sculpted nail wall by square footage in the state of Washington along 192nd Avenue in Vancouver.
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Photo: TenCate Mirafi Geosynthetics |
| Constructing a soil nail wall in Vancouver |
Drilled into a freshly excavated cliff, the wall was sculpted, textured, and stained by artisans at Boulderscape Inc. and given a fractured-basalt finish to match its environment, which includes an old quarry on the west side of the street. Rinker Materials supplied the shotcrete for the project.
The soil nail wall was installed on the east side of the street in lieu of a large soldier pile wall, because that alternative would have been more costly, says Gary Lasst, owner of Lasst Construction Co. He says the soil nail wall was done at one-third of the cost; the project involved both state and federal funding.
The wall face is 40,000 square feet, with the wall being 72 feet high at its highest point. The road was realigned because the soil nail wall fits flush with the slope; the original soldier pile wall protrudes out from the wall.
So impressed is the concrete industry with the wall that it received first place in ACI Oregon’s 2006 Excellence in Concrete Awards and second place in Washington’s Excellence in Concrete Awards.
Lasst says one of the factors on the job that required a different approach was that the anchors had to be longer than they typically would be on this type of wall to compensate for the absence of rock. “There turned out to be more soil than rock, so the engineer had to increase the length of the anchors to give stability to the wall,” he notes.
Another tricky aspect to the project was that 240 linear feet of the upper part of the wall retains wetlands, he adds. In constructing the wall, Lasst Construction used drainage swales and horizontal drains to control the water.
Continuously controlling the water coming from behind the wall as his company was working so productivity could keep moving forward was another challenge, Lasst says. Some of the main parts of the job were executed during the wettest part of the season.
“We had to put in some natural drains and pipe the water out and down away from the work area to control and contain it,” he says. “Guys were working in the mud up to their knees on some days.
“The weather was very wet and actual springs were shooting up out of the work bench we were working off of. At one point, you could put your finger on one hole where the water was squirting out and it would just move over and squirt out of another hole.”
Such challenges affected the schedule and the ability to execute the work, Lasst notes. In the end, however, the result was an impressive-looking wall. Lasst says aesthetics weighed heavily in the project, because the wall serves as an entrance to the city.
“The height goes from zero to 72 feet and then wraps around and ties into the quarry wall—that’s unique,” he says.