No Waitin' for Revegetation: Landscaping Alternatives
Plantable walls, gabions, and mats help projects get back in the green.
Strong, durable, efficient - what's not to like about concrete retaining walls? Although they've made construction easier and more cost-effective in many places, for some applications, the complaint is in the concrete. Although segmented, or block, concrete walls feature texture and color, allowing them to better blend into their site, some developers feel there still are areas where concrete "just doesn't look right."
In urban areas, growing concerns about heat islands have inspired some planners to use as little heat-retaining concrete as possible. Granted, concrete isn't the only cause of urban heat retention; to minimize the heat generated by dark roofing materials, many skyscrapers in downtown Chicago now sport roof gardens. Despite such efforts, however, the United States Department of Agriculture (USDA) recently unveiled a revised Plant Hardiness Zone Map, which showed Chicago, IL, to be an island of the warmer Zone 6, while surrounding suburbs still shiver through Zone 5 winters. (Five other cities - Columbus, OH; Nashville, TN; Phoenix, AZ; St. Petersburg, FL; and Tulsa, OK - now are also rated one zone warmer than they had been on the 1990 USDA map.)
Does that extra heat matter? Undeniably as evidenced not only by higher air-conditioning costs but also by weather patterns; a recent Time magazine article noted that cities can "make their own weather." Heat-absorbing roofs, pavement, and auto exhaust trap the sun's rays and warm the air. When that heat rises later in the day, striking the cooler upper atmosphere, the clash of temperatures can produce clouds and rain. The article was accompanied by a dramatic NASA thermal photo that showed downtown Houston, TX, registering nearly 20°F warmer than outlying suburbs.
Don't Supplant It, Plant It!
Segmental retaining walls can be flexible enough to address both aesthetic and heat issues. Some blocks have openings that can allow vegetation to be planted and grown on the wall in individual blocks or pockets - in some cases practically obscuring the wall and creating a more natural-looking site. In addition, the vegetation lowers heat retention because the plants not only shade the concrete but also, through their process of transpiration, excrete moisture, cooling the concrete.
Several years ago, for example, Mahon Creek in San Rafael, CA, required a retaining wall nearly 30 ft. long and between 7 and 8 ft. high. "It's right by one of our recreation centers," explains Steve Zeiger, senior associate engineer and stormwater program manager for San Rafael Public Works. "We wanted an environment-friendly method, not like riprap. We also decided that being able to plant the wall made it more "creek-friendly.' Our Parks Department planted vegetation in the completed wall - it looks fine, and since it was put in about four or five years ago, the plants nearly cover the wall." "Living" Secura Slope blocks from Matterhorn California Inc. were used to construct the wall; their center cavities and open backs are backfilled, securing the wall and allowing planting.
"This tidal creek has 5-foot water elevation changes daily, which required very unique installation methods," says Zeiger. "There was no way to divert the water when working on the project. Matterhorn used sandbags and pumps to hold the water back while they were building the wall. They only had to key it into the base of the bank and, using some anchors, they drilled into the side of the slopes to anchor the wall off."
"We do a lot of streambank restoration," says Matterhorn California's Steven Shigematsu in Napa, CA. "Because Secura Slope blocks can be planted, the result looks much more natural. We're often called out for emergency repairs; for example, some tidal creeks will back up, especially in winter. If the problem is in a housing development, you can't dig back to make slopes. Our product allows us a build in even the steepest slopes without a lot of excavating. Also, when working in creeks, you can no longer drill into the creek sides. If we need additional wall support, we can use screw-in anchors. The Northern California Fish and Game Department often refers people to us if the homeowner wants to hold back a stream on his property.
"People have started asking for "bioengineered components,'" he continues. "Well, if you're just planting vegetation, it's not "engineered.' We're the bioengineered component for streambed restoration."
Juan Hidalgo, geotechnical engineer for R.G.H. Engineering in Santa Rosa, CA, has specified Matterhorn California's system for numerous projects in Sonoma, Napa, and Marin Counties over the last 10 years.
"We needed to protect a creek in Napa, and the wall ended up being 15 feet high," Hidalgo explains. "I find these blocks work well in retaining a cut. There was no problem with stacking these 15 feet high. For one thing, the blocks are interlocking. Because it's a gravity wall, the resistance of lateral forces is maintained by inclination. The higher the wall, the more you have to incline it."
He describes the installation: "You need a firm base for the wall and, depending on the environment, you can use one of three systems: a regular footing; a poured, reinforced concrete pier; or helicore anchors. Lately Matterhorn has used metal anchor/screws, in which a machine high-torques them into the slope, both vertically and horizontally, about 4 feet on center, but we haven't used those in our jobs. The manufacturer suggests placing a foot of drain rock behind the wall before backfilling. Then you put dirt or fill in front of the block and get vines or something to grow in the wall. A year or so later, you'll have lots of vegetation; the wall begins to disappear."
A steep slope in a residential neighborhood in Maryland sent Timothy Caslow, a Baltimore-area independent contractor, on a quest for workable options. Between the homeowner's preferences and the technical challenges of the site, he ruled out a number of possibilities.
|Top: Empty Strata Cubes are placed in the level sub-base. Middle, bottom: The tubes inside the cubes are filled with soil.
"A homeowner had a one-to-one slope in his yard that was hazardous to maintain with power lawn equipment," Caslow explains. "The area, which was about 18 feet long, had a 3-foot drop-off, causing erosion. We considered several options. Revegetating with grass was never really considered. The homeowner didn't want a "stark' block retaining wall. A landscape tie tiered garden would have encompassed too much room. Geocellular confinement systems would have been filled either with stone, which would have been unsightly, or with soil and vegetated. However, due to the rise and the run of the slope, a vegetated cellular confinement system would have been difficult to maintain.
"Traditional gabions were eliminated as an option," he continues. "Though the thought of a vegetated wall was appealing, the neighborhood would be staring at a basket filled with stone until the vegetation grew in." However, Strata Cubes - a plantable gabion created from extruded plastic products - provided a solution. Consisting of 260 individual rigid plastic tubular nets that are bundled and fused together to form a rectangular cube measuring 2 ft. high x 2 ft. wide x 3 ft. long, Strata Cubes from NSW LLC of Roanoke, VA, are made from a biologically and chemically resistant, ultraviolet-stabilized polypropylene. The tubes' open-mesh design provides ample drainage to prevent hydrostatic buildup behind the completed wall.
After preparing a level sub-base for the cubes, Caslow placed the first cube and made it level. The remaining cubes were placed along the foot of the slope adjoining the others. Each soil-filled tube in each cube was tamped for compaction. Backfill was added behind the cube wall to level the slope, and vegetation (ivy on the side and front, herbs on the top) was planted on the wall.
"I didn't attach the cubes to each other, although you can do that with nylon ties. The cubes are black; when plants grow over them, you don't see them at all," Caslow says. "Strata Cube provided an economical, easily installed vegetated wall that's easily maintained, which completely eliminated the homeowner's problem. The vegetation's root structure provides additional reinforcement for the soil, and the cubes' nets provide additional reinforcement for the vegetation's root structure. The garden looks great; the cubes haven't moved in a year - I checked 'em!"
Strata Cubes also can be filled with aggregate. "I could have used pea gravel, but this was in a yard, and we wanted it to look nice," remarks Caslow. "Each empty cube weighs about 18 pounds - handling it was a dream. If I can do it, anyone can do it - it's that easy! One person with a backhoe could do it - put in aggregate, get a vibrator to tamp the soil down, and then you're done. The compaction was actually the longest part of the job. Strata Cubes can be made into a small standalone garden in your yard, which would be pretty cool. I'm thinking of doing one with a sundial in the middle.
"In areas where appearance is as important as durability, such as residential landscaping, traditional measures can appear stark until vegetation is established. "Building a typical gabion - just putting the rocks in the box - takes awhile, and for a long period of time, that's all you have - a box of rocks," says Caslow. "It's cool to see the area planted instead of retaining walls. As I haven't built very high walls, I don't know the cubes' height restriction, although they can go to 6 feet. I'd imagine you'd need to place a rod inside each cube to go higher than that, and we're playing with the idea of using geogrid in the backfill to keep it from moving forward. You could possibly also put a French drain underneath it. With lower walls, though, there's no hydrostatic buildup because the cubes drain, and you can backfill with aggregate to get lots of drainage.
"I think the cubes would work well in high-water-table areas too," he concludes. "They have no problems with freeze-thaw cycles, and NSW is thinking of suggesting using Strata Cubes around shorelines."
Take It to the Mats
When designing the Bell Road improvements for the City of Scottsdale, AZ, Jonathan Bastianelli of Phoenix-based Gannett Fleming Engineers and Planners decided a soft-armor technique might solve the project's difficult problems.
"Since the highway department put in the aboveground [Highway] 101 loop, all of the water from the surrounding hills gets dammed up behind that big berm of freeway, which retains water, and eventually crosses Bell Road and goes into a box culvert with sideslopes of four-to-one," Bastianelli explains. "When investigating solutions, I did studies. The water velocities in the channel were too high - 9 to 11 feet per second. They should have been no more than 5. This caused the culvert to fill with dirt, which of course rendered the culvert useless - no water would flow through it.
"We didn't want to go with a concrete-lined channel," he continues. "Riprap was a little costly and hard to install, and its maintenance isn't great. Since I do a lot of drainage work, I knew that Contech has a lot of good products, and I read about Pyramat." A high-performance permanent erosion and reinforcement matrix, Pyramat, from SI Geosolutions in Chattanooga, TN, has tensile strength exceeding that of an AASHTO M288 stabilization geotextile.
"The information said Pyramat could handle up to 15 feet per second velocity without vegetation, and with vegetation it would work even better. Plus the estimated cost was reasonable - about $1.25 per square foot to install," notes Bastianelli. Despite the fact that little application data existed on how Pyramat worked, Bastianelli chose the product, which was installed by Achen-Gardner of Chandler, AZ.
"We came up with a seed-mix design of natural desert seeds and sagebrush and hydroseeded the area," Bastianelli recalls. "We installed the Pyramat, and a couple of months later the ground cover grew in. Then we got a big rainstorm. We had taken our channel down around Westworld. Someone else had worked a channel, but they didn't have any Pyramat around it. That channel south of us had lots of erosion, but ours held its ground. We still had ground cover - we had installed it with 4 inches of dirt over the mats - but downstream you can see a definite line where our channel ends and theirs begins."
Although Bastianelli hasn't measured water velocities in the channel lately, he explains his calculating formula: "At the Phoenix Coyotes' training facility, AllTel Ice Den, they keep rain gauges and equipment because, as a hockey team, they need to know how to keep the ice in good playing condition. I would check their rainfall information; for example, let's say we got 1 inch there, which would be my intensity factor. I'd plug that into my equations, which would allow me to come up with a flow rate. The ground will soak up so much, to a certain point - you have to know soil types - and the water velocity goes up with slope and width of a channel. When they're built, they're sized to handle a certain capacity. Then of course the stormwater goes into retention ponds."
Bastianelli was able to offer one other benefit to his client in choosing mats for the Bell Road improvements. "Pyramat is available in black and a sand color - did they do that because of me? I remember that in Scottsdale they did little flips when they saw the sand color - it blended right in!"
Eureka - Excelsior!
|The wall of cubes is backfilled and planted.
Eco-Systems, a Bloomington, IN, company, performs a great deal of erosion control work in southern Indiana on channels, ditches, waterways, hills, slopes, and other projects, such as the Monroe County Landfill.
"We're distributors, and we have a crew that does installations. Eco-Systems offers "interrelated systems' - natural restoration systems, interrelated with economic analysis," says Senior Environmental Scientist/Vice President of Operations Steve Chafin. "We work to find the most cost-effective-per-unit solution; for example, the lowest cost per ton to control sediment. Our cost-benefit analyses look for the best plan of attack. Not simply the cheapest initial plan but the most cost-effective for the long term."
A combination of products and techniques was used for the Monroe County Landfill. "The landfill has slopes as much as two-to-one, with lengths of more than 200 feet," Chafin explains. "We used Curlex blankets on this particular site, where we installed an intermediate [10- to 15-year] landfill cap. Before applying any product, we tested the soil, which was actually just subsoil mined from elsewhere on-site, and it had a pH of 5. As we hydroseeded, we added powdered lime and mycorrhizal fungi to the slurry. We also used guar gum tackifier, gypsum, and American Excelsior Excel wood fiber mulch and paper mulches to ensure seed germination, and we used a combination of seeds suitable for the site conditions. The landfill staff installed all the RECPs [rolled erosion control products] after we performed the hydroseeding. Curlex II and III blankets, which are contained within UV-resistant polypropylene nets, were placed on the area."
For long, steeper slopes, Chafin also used American Excelsior Curlex Sediment logs. "The 10-foot logs - which "break up' the slope length, function as terraces, and direct flows of water - are made from curly aspen wood shavings that have been packed into a sock net. The logs are permeable and don't tend to overly concentrate water, unlike straw wattles, which don't let water trickle through - water stops then over-tops straw wattles. Eventually sediment builds up behind the Curlex logs, creating terraces on the slopes, which direct water flows to waterways lined with Recyclex TRMs [turf reinforcement mats]."
Seed selection for such projects is entirely site specific. "We sometimes use wildflowers, cool- or warm-season grasses," he continues. "If the site has wildlife habitat issues, we'll tailor the seed mix for that. For instance, deer will come out and graze on this particular landfill cap, so we used endophyte-free fescues, which won't harm the wildlife, and stay away from KY 31. We also used mammoth red clover - not only does it fix nitrogen into the soil but also the wildlife like it for forage."
Why does Chafin prefer excelsior to, say, straw products? "Aspen is relatively cheap and abundant. Plus, excelsior products have a rougher surface than does straw. Running water moves more slowly over excelsior, increasing the drag on the water and increasing the soil's infiltration rate. Excelsior is much more absorbent than straw. It sucks water up like a sponge and holds moisture in the root zone far longer than straw does, which aids in germination and plant health.
"That 'holding in moisture' really helps in the summer months," he adds. "Spring and fall are the best seeding and planting times, but since this spring was so wet, we had to wait until summer to grade and plant, which is much drier. The excelsior blankets kept the hydroseeded areas moist for rapid germination.
"Some people think the old way of just putting down straw on broadcast seed is sufficient," Chafin chuckles. "We typically don't get called in to many jobs at first, either to sell or to apply products. But those folks will call us after they've tried something else - two or three times."
Author's Bio: Janis Keating is a frequent contributor to Forester Media, Inc. publications.