1. Erosion Control
    2. Vegetation Management

    Long-Term Options: Function and Beauty

    Nov. 1, 2003
    Retaining walls are everywhere you look, and more are installed every day. In the past, most were cast-in-place concrete or timber walls, but in recent years, the segmental retaining walls (SRWs) seem to dominate the market. Applications can range from huge highway overpasses to a short wall in a backyard.Advantages of the SRWs over concrete are cheaper cost, the plethora of choices for color and texture, ease of installation, shorter installation time, and unsurpassed strength. Innovations are continuing in the SRW market in both the wall block and the supporting geogrid. Components of an SRW are the blocks, connectors, and reinforcing geogrid. The use of these components depends on the height and design load of the wall. Shorter walls might not require geogrid reinforcement, and some block designs do not need connectors. Blocks are made of concrete, either in a dry process or a wet cast. Design standards for SRWs have been published by the National Concrete and Masonry Association. Many companies also provide design and engineering services. Most dealers of SRWs around the United States and Canada are licensees of the block technology. Having local dealers cuts down on shipping costs and time. California In-Place and GreenIn California, land with an ocean view can sell for $250-$500 or more a square foot. Installing a retaining wall often can make an otherwise unusable portion of a lot suitable for building for much less than that. Much of the California construction is residential. Vinci Pacific of Del Mar, CA, is a grading and excavation contractor. Sean Christiansen, estimator/project manager, describes a project in which a canyon was filled in and walled to provide land for six new houses. The block used was a buff-colored Verdura 40 from Soil Retention Products, with 30 ft. of geogrid placed every 3 ft. “They are time- and cost-effective,” he says. Another advantage is the ability to build wall and fill at the same time. Handplacing the blocks and backfilling took about two months, and Christiansen notes that the construction was a fairly smooth process. Houses now are being built on the site, and some are occupied. Although zoning ordinances in many municipalities allow retaining walls to be no more than 6 ft. high, variances sometimes are granted if a wall is plantable. In this case the Verdura walls were planted by another contractor. A plantable wall can be completely vegetated within a year, adding to the aesthetic appeal of the wall. “Retaining walls are making money for builders,” remarks Dean Sandri of Soil Retention Products in Carlsbad, CA. “It’s not a bad investment.” Sister company Soil Retention Systems has installed more than 3 million ft.2 of retaining walls since 1987; according to Sandri, Verdura blocks are sold to both do-it-yourself markets, such as Home Depot, and large construction and development companies. Christiansen recalls a multiwall system for a landowner. The valley property was plagued by a water table at about 10 ft. deep and an insignificant view. Vinci Pacific contractors built a hill for the home with about 30,000 yd.3 of dirt fill. The lowest retaining wall, built with Verdura 30 and geogrid reinforcement, was 30 ft. high; two other walls, 10 ft. and 5 ft. tall, were built at higher elevations. The total retaining wall space was 15,000 ft.2 The landowner now has a view of a golf course. “The reality is that most of the land left is hillside and requires retaining walls,” Christiansen states. The 2-in. lip on the Verdura block does require a finished slope of about 1:4. The blocks, which come in four basic colors and custom colors, are molded and compacted by vibration and then are allowed to set for a day. The interlock is a 4,000-lb. shear lip. Because of the California location, Soil Retention products have been tested for up to 30-ft./sec. water flow and seismic events. The highest single walls are 50 ft., with tiered walls reaching up to 80 ft. tall. Overflow in Kansas

    Craig Crossing Park in Lenexa, KSCraig Crossing Park in Lenexa, KS, contains a retention pond that experiences periodic overflow from the creek. Mowing to the edge of the pond was difficult, and the city wanted a natural-looking wall built to stabilize the pond area. The area is a favorite recreational site with hiking and biking trails nearby. The original design designated natural stacked limestone. Ben Manthei, managing partner of Redi-Rock headquartered in Charlevoix, MI, states that one reason Redi-Rock walls ultimately were chosen for the project was their design and coloration. Courses of block were laid, with the final course set back about a foot, allowing animals and people to easily step out of the pond. By staining the block a cream color and applying a texture to the face and top of the blocks, the Redi-Rock was made to resemble natural limestone. “The city liked it because it stacked straighter and looked like real stone,” explains Gerald Heckman of St. Joe Concrete Products in St. Joseph, MO, the supplier of the blocks. The cost of Redi-Rock was comparable to natural stone. The blocks are produced with a wet-cast process and stain is applied in the color chosen by the customer. The texture came from special forms. Each block weighs 2,200 lb., making them among the largest blocks in the industry. Manthei says, “The integrity of a wall system is based on the size of the rock.” The size of the blocks also decreases the amount of site preparation. If the foundation is good compacted soil, the site is ready for block placement. For Craig Crossing Park, the area was marshy, so a layer of 1-in. stone was spread under the blocks. Because geogrid is not required, less property is required for tiebacks and walls can be set close to property lines.The Craig Crossing Park project covered 2,400 ft.2 and incorporated 417 blocks. The area has been flooded and drained several times with no change to the integrity of the wall. Ron Tebbenkamp of Dennis Johnson Construction, contractor for the project, recalls, “It went exceptionally wellHeckman notes a growing interest in this type of wall construction, citing recent inquiries from cities and counties. One advantage is the lack of setbacks for a retaining wall. One project was a retaining wall next to a cemetery, where land could not be disturbed. Other inquiries have come from agencies needing to reline ditches and wetland areas. Blending In When homeowner Mary Coleman bought an old estate perched in the hills near San Jose, CA, she had the house that was built into the hillside mostly torn down and rebuilt into a 7,700-ft.2 residence. Although the yard had been nicely landscaped, including retaining walls, when the original house first was built 30 years ago, crews used the tennis court for parking during reconstruction, and because the fence was down, deer ate much of the old landscaping. Coleman hired Lisa Costello of Two Hands Unlimited in Watsonville, CA, to rebuild her landscape, including adding new drainage areas and a new retaining wall.Costello, who holds both engineering and landscaping licenses, chose AB Stone by Allan Block of Edina, MN. Costello was certified as a Master Wallbuilder by Allan Block through a course she attended. She notes that the company also provides free engineering services, and many clients like that despite her engineering expertise: “It gives them a second opinion.” For Coleman’s yard, Costello needed to replace a retaining wall against a vertical cut. The footing was 24 x 24 in. filled with 0.75-in. drain stone. A geogrid was placed up about 3 ft. Both measures were “overkill,” according to Costello, because the blocks would have withstood the load alone. The block used was the AB Stone, handplaced with a 12° setback. The finished wall is about 5 ft. high x 100 ft. long.“I’m quite happy with it,” Coleman says of the look of the finished wall, noting that the color and texture of the block blend well with her house and the area. “No issues at all.”Tall in Atlanta, GAA recent project in the Atlanta area produced the tallest SRW in North America. David Pitre, engineering representative for Dallas, TX – based Pavestone Company, explains that two land bridges were built to provide access from a church to a parking lot. One of the structures required a retaining wall 62 ft. high.The area bordered a wetlands area, so disturbance and fill had to be kept to a minimum. The product chosen was Anchor Landmark, an SRW design with a positive connection system. A polymeric locking bar and the channel and locking flange provide the load capability. Pitre notes that a common use for SRWs in the Atlanta area is covering old timber retaining walls. Many of these timber walls are at least 10-15 years old and deteriorating; SRWs can be placed over the timber with minimal work and time. Commercial development in Atlanta and Denver has made these cities prime sites for SRWs, according to Pitre. “It’s a huge and growing market.”Biking in WisconsinAn SRW used in the town of Mequon, WIThe town of Mequon, WI, wanted a 6-mi. section of bike trail, including the five bridges that were necessary in some segments of it, to reflect the community’s rural heritage. Great Lakes Marine Contracting usually installs poured-in-place concrete bridges, but for aesthetic purposes and at the request of a citizen committee, the company decided to use SRWs as bridge abutments for this project – in this case, Mesa Retaining Wall Systems from Atlanta-based Tensar Earth Technologies. “The main point for us here was the connection strength between the blocks and the geogrid,” explains Aron Jarr, product manager for Mesa Retaining Walls at Van Der Vaart Inc., the supplier.The block chosen was the natural-color Mesa standard unit, 8 in. high x 18 in. wide x 11 in. deep nominally, and Mesa 3 and Mesa 4 geogrid. Materials were required to meet Wisconsin Department of Transportation standards. The blocks were handplaced by a two-man crew from Great Lakes Marine Contracting, and five rustic-looking wooden bridges were built on top of them. The walls are 10 ft. high, and about 3,600 ft.2 of block were used. “The application was pretty straightforward,” Jarr recalls. “No problems.” John Huck, City of Mequon engineering supervisor and project manager, remarks, “The bridges look fantastic!” Historic MinnesotaA historic project in Minnesota required special handling. The Minneapolis Park and Recreation Board had acquired Mill Ruins Park, formerly the site of a flour mill. The site is on the National Register for Historical Places, and special care was needed for the restoration. A tailrace from the old mill required a retaining wall, and the board wanted it to have the look of old stone to fit the historical atmosphere. Versa-Lok Weathered Standard Retaining Wall System was chosen because of the look of natural stone. The contractor, Martin Lake Contracting Inc. of Stacy, MN, faced some challenges with this project. After starting a leveling pad of crushed gravel and sand, the crew realized that the nearby Mississippi River and adjacent lock and dam were causing flooding in the work area. Riprap was placed in the tailrace bed to prevent further erosion. A soil-separation fabric was installed with extra gravel to avoid washout and provide a good base for the first course of block. Subsequent courses were built up and interlocked with nylon/fiberglass pins. “The pinning system gives us more flexibility in keeping the wall straight,” explains Brad Koecher of Martin Lake Contracting. Versa-Lok’s solid blocks also mean there is no need to fill blocks, saving time. Other Versa-Lok walls were constructed in the area, include a parking-lot wall and a river inlet. The look of the block has added to the historical feel of the project. Canada StonesA project for Chrysler Canada in Brampton, ON, required curving, high-visibility walls. A new access road was required to replace a slope and berm. Because the site would be visible to the public, Chrysler wanted an attractive wall. In addition, a stormwater management area at the foot of the wall had to remain in place.Bob Morawczynski of Regent Landscaping, the contractor on the project, used Risi Stone Systems’s Pisa 2 stone, together with Mirafi 2 geogrid, to form the curved walls; for similar projects Morawczynski uses Stratagrid with Pisa 2 stone to provide additional support when connecting curving and straight walls. A total of 6,000 ft.2 of retaining wall was installed. A crew of two compacted 2,000 yd.3 of dirt behind the wall and added a drainage zone to prevent the buildup of hydrostatic pressure. The entire project took the crew nine days to complete in September 1994. “It was quite an interesting wall because of the curves,” comments Morawczynski. “Beautiful!” He adds that, after almost 10 years’ time, the wall still is in perfect condition. Allison Uher of Risi Stone Systems in Thornhill, ON, explains that the solid block doesn’t have to be filled, making the system stronger and speeding up installation. The integrated tongue-and-groove design provides self-sloping placement and strength. Risi also provides design services.Risi Stone Systems are available in a variety of sizes from 18 lb. to more than 1,700 lb. “Good for a large range of applications,” relates Uher.