In the tight-margin business of grading and excavation it never hurts to have a competitive edge-a special niche in the marketplace that allows you to leverage your earthmoving talents, crews, and equipment into more profits. Using new or existing techniques and materials to build unconventional retaining walls might offer one way to do that.
In some cases, that might mean simply tweaking your current way of working. In others, it might involve using your skills and resource in an entirely new fashion. Either way, it will probably require viewing your work from a different perspective.
Ross Tortorigi owns Ross Environmental and Civil Contractors, a Birmingham, AL, firm that now specializes in the construction of athletic fields and sports complexes. He remembers the time he built a mechanically stabilized earth (MSE) wall with a gabion facing.
“There was no problem doing the take-offs for the job,” he says. “However, I was used to reading plans for flat construction projects. This one required me to think vertically. Every day was a learning experience for me. However, I had a very experienced crew to do the job. I just orchestrated the work.”
Here’s a closer look at this and other types of retaining wall projects. They might spark an idea you can use to profit from your earthmoving expertise by building earth retention structures that offer strength, beauty, or both.
Putting a Green Face on a Wall of Rock and Steel
Most contractors don’t associate a soft, green look with rock-filled gabions. However, as illustrated by the gabion-faced MSE wall that contractor Tortorigi built nine years ago in Vestavia Hills, AL, rocks and vegetation can coexist to produce a retaining wall with an aesthetically pleasing face.
The 350-foot-long, 48-foot-high wall was built as close to vertical as possible to provide space for a one-story addition to an assisted living facility at the top of a slope. A gabion-faced MSE wall consists of backfill; a horizontal anchor mesh, which extends into the backfill; and a gabion basket front, which retains the backfill on that face of the wall. Such a structure allowed the use of locally available clayey-sand soil for backfill and could be constructed in a relatively short time-three months.
Welded wire gabions and welded galvanized, PVC-coated wire mesh anchor panels, made by Modular Gabion Systems, a member of the C.E. Shepherd Co. LP, in Houston, TX, were selected for the project. The 12-gauge wire mesh panels, with 3-inch-square openings, are an extension of the gabion, explains George Ragazzo, general manager of Modular Gabion Systems.
“The mesh panel eliminates any concerns regarding connections between the gabions and the horizontal anchors and increases structural integrity,” he says. “Also, it provides greater passive resistance, which makes it feasible to use locally excavated soil instead of imported granular material for backfill.”
Construction Details
Tortorigi’s company, Ross Environmental and Civil Contractors, constructed the wall. The crew placed an 18- to 24-inch-deep layer of limestone at the bottom of the area to be backfilled and another 24-inch-thick layer behind the backfill to prevent groundwater from seeping into the backfill. They installed an 18-inch course of limestone between the backfill and the first tier of gabion baskets. The backfill was compacted to standard specifications with a sheepsfoot roller. A vertical riser pipe carries runoff water from the roof of the building and the surrounding soil surface through the backfill, discharging it below the elevation of the toe of the wall.
The contractor built the gabion baskets onsite in continuous 300-foot lengths from rolls of 12-gauge PVC-coated welded wire mesh with 3-inch-square openings. The gabion baskets were placed at a 4.8-degree batter (3-inch setback on 36-inch-high gabions) and filled with graded limestone. The anchor mesh capped each lift of gabions and extended into the soil from 6 to 33 feet, depending on vertical location.
The crew constructed the first 18 vertical feet of the wall with 18-inch-high gabion lifts and the remaining 30 feet of wall with 36-inch-high gabion lifts. They used 3-foot spiral binders to connect adjacent mesh panels.
The project, built during an unusually wet winter, was completed over a five-month period. “During part of the time, it rained about every three or four days,” says Tortorigi. “However, actual working days totaled right at three months. Other than that, the main concerns with the project were achieving proper compaction and drainage. However, since I’m familiar with those aspects, they weren’t a problem.”
After the project was completed, the project owner planted vines at both the bottom and top of the wall. Over time, these vines have gradually spread along the wires of the gabion baskets, as intended to camouflage the stark rock wall with a face of natural green.
More information is available at www.gabions.net.
Driven-In-Place Earth Anchors Simplifies Stabilization
One alternative to stabilizing embankments when property lines or other constraints make it impossible or impractical to excavate the site for geogrid, geotextile, or other soil-reinforcing materials is to install grouted earth anchors across the failure plane. That requires special equipment to drill holes for the steel rods and to pump in grout to secure them in place. Even then, this process often leaves excess grout on the site that can require cleanup. Screw anchors are another alternative. But they too require special equipment and crews to install.
The Manta Ray driven tipping plate anchor can offer a more contractor-friendly option. Made of hot dip galvanized ductile iron attached to an anchor rod, it features a rectangular metal plate at one end, which flips into place to secure the anchor in the soil. Rather than being augered or torqued in place, it is driven into the ground using hydraulic or pneumatic equipment, like a hydraulic breaker, vibratory compactor, or jackhammer.
This system can be combined with segmental concrete blocks or other fascia to produce a strong yet attractive soil retaining structure.
No Digging or Drilling
“Installation is simple enough that most grading and excavating contractors can do the work themselves,” says Mike Jennings, director of sales for Foresight Products LLC in Commerce City, CO, manufacturer of Manta Ray and Stingray engineered earth anchor systems.
“You don’t have to dig or drill holes to install them,” he says. “Unlike other anchoring systems, Manta Ray actually compacts the soil around itself for a clean, safe, simple installation.”
Once driven to the proper depth, he explains, an open center hydraulic jack grabs onto the anchor rod attached to the anchor head and pulls on it, rotating or tipping the plate, like a toggle bolt. The hydraulic jack continues pulling until the anchor reaches the required holding capacity, which is measured by a gauge on the jack.
“Each anchor is immediately proofed to the exact capacity required,” Jennings says. “No other system offers this feature.”The load capacity of the Manta Ray anchor is based on the size of the anchor and soil conditions. The smaller anchor models are designed for harder soils or where lower loads are required, while the larger anchors are used in softer soils.
In the hardest soils, a 4-inch pilot hole can speed up installation. For added load capacity, the anchors can be grouted in place. Some models can be installed in rock formations with low rock-quality designation.
A Versatile Tool
Manta Ray anchors can be used to reduce the cost of various types of retaining walls, Jennings notes. In these cases, he recommends installing them at least 6 feet behind the failure plane, after proof-testing, with a minimum of 4 feet of overburden.
A Hybrid Block Wall
By minimizing excavation for MSE walls, the anchors allow segmental concrete block walls reinforced with geogrid to be installed where excavation is not possible.
At a residential site in Bloomington, MN, Manta Ray anchors were used as tie-backs to reinforce the soil in constructing two concrete segmental retaining walls where site conditions prevented excavation for installing geogrid. One was about 4.5 feet tall, the other about 6 feet tall. This approach provided a faster, less expensive solution than the two other alternatives-a cast-in-place concrete gravity wall or a sheet pile wall.
A Cost-Effective Repair to a Sheet Pile Bulkhead
In addition to offering a more cost-effective alternative to driving sheet piles to bedrock for preventing overturning, this anchor system can be used to repair existing sheet pile structures. For example, over 850 Manta Ray anchors were used to rebuild a failing sheet pile seawall bulkhead at Lake Tahoe, CA, which threatened more than 100 homes and properties. Steel cable tie-backs between the bulkhead and concrete deadmen had failed, causing a progressive overload of the structure and remaining tie-backs.
To repair the bulkhead, a compact excavator, equipped with a custom-mounted Manta Ray Anchor Driver attachment, was positioned on a narrow walkway above the bulkhead. The excavator reached over the edge of the bulkhead to drive the anchors through holes that had been cut in the sheet pile. Careful alignment of the anchors was critical for driving the anchors in between existing concrete deadmen, fence posts, storm drain culverts, and deck support posts. This was done without disturbing the homeowners’ properties.
More Applications
Timber walls. Here, the anchors replace conventional timber deadmen to provide permanent structural integrity with minimal excavation.
Concrete walls. Manta Ray anchors can be used as stabilizing tie-backs to reduce the costs of repairing concrete and stone gravity walls.
Gabion walls. This anchor system allows gabions to be installed on steep slopes. It’s a faster, easier alternative to traditional anchoring methods such as grouted or screw anchors.
“The Manta Ray is installed so that after the anchor is placed in the embankment, the other end of the rod protrudes into the empty gabion basket,” Jennings says. “As the basket is filled, the stones help secure the plate on the end of the anchor rod for added strength in stabilizing the slope. As a result, you can build a gabion wall with less setback and still provide the needed overturning stability.”
Streams and shorelines. In a manner similar to that used to build steeper gabion walls, the anchors can be used to secure Reno mattresses on steeper channel sideslopes, he adds.
Manta Ray anchors also can increase stability of cellular concrete revetment matting in streams and on shorelines. “The anchors install easily through the openings in the individual blocks,” Jennings says. “This prevents lifting of the mats and erosion under extreme flood conditions.”
Foundations. Another application involves the use of the anchors to add an uplift force to minimize the amount of concrete needed in building a foundation. For example, Jennings notes, using the Manta Ray anchors to provide 50,000 pounds of lifting capacity to the soil would eliminate 50,000 pounds of concrete.
More information is available at www.earthanchor.com.
Bigger Blocks Reduce Labor Costs
If you’ve been shying away from taking on segmental retaining wall projects because of the amount of hand work that conventional concrete block walls require, you might want to take a serious look at a less labor-intensive alternative-much larger blocks, like those made by ReCon Retaining Wall Systems Inc. Unlike typical dry cast blocks, which weigh about 80 to 100 pounds, the ReCon blocks, made from wet-cast, air-entrained concrete, weigh about 1,400 to 2,500 pounds, depending on size. These heftier blocks can help improve profit prospects by allowing you to better utilize your grading and excavating equipment, such as excavators, loader-backhoes, and skid-steer loaders to handle the larger blocks, while reducing labor requirements.
“These big blocks are more conducive to a grading and excavating contractor’s operations than the small blocks,” says contractor Matt Barron of Hardscape Construction in Burnsville, MN. He’s built numerous retaining walls using both conventional blocks and the larger ones. “Using the big blocks, we can reduce crew size by about 20% compared to the small blocks and still achieve the same amount of production,” he says.
In addition to his retaining wall construction company, Barron is also a part owner of ReCon Retaining Wall Systems, which is based in Minneapolis, MN. While the face of conventional segmental wall blocks typically measure 18 inches long and 8 inches high, the ReCon blocks measure 48 by 16 inches. The blocks, available in depths of 24, 39, and 45 inches, feature a face with a natural stone-like texture. Using an interlocking tongue and groove design, the blocks can be assembled to form a straight or curved structure with a 3.58-degree batter. Lifting insert loops and fork pockets on the back of the blocks simplify handling and installing them with equipment.
An Alternative to MSE Walls
One of Barron’s projects illustrates some of the advantages of this large block retaining wall system. The big box retail construction project near Minneapolis called for a 10-foot-high, 600-foot-long retaining wall to stabilize the embankment of an existing nearby county highway. However, county regulations prohibit disturbing road rights of way for commercial development projects. That ruled out excavating the slope to reinforce the embankment behind a conventional segmental concrete block wall with geogrid. The much heavier ReCon blocks offered a way to build a gravity retaining wall, eliminating the need to construct an MSE structure behind it.
“Typically, the height of a retaining wall built with smaller blocks is limited to about 4 feet, without using geogrid to reinforce the soil behind it,” says Stan Hamilton, president of ReCon Retaining Wall Systems. “Because of the mass of the large blocks and their unique locking system, they can be used to build walls as high as about 14 feet without reinforcement, depending on site and soil conditions. That saves the time and expense of excavating and replacing soil to reinforce soil with geogrid.”
The dimensions of the blocks and tested shear and geogrid pull-out performance allow the blocks to be used to build professionally engineered, reinforced walls higher than 14 feet, too, he notes. “This provides retaining wall performance not generally available with natural stone walls,” Hamilton says.
Less Costly Than Concrete
The large block system also offered a less expensive alternative to a poured-in-place concrete wall on this project.
Barron used a Komatsu PC200 excavator to excavate for the foundation of the wall and a Bobcat T250 compact track loader to stage the blocks. Both machines were used to set the blocks in place and backfill the wall as it was built.
“We built the wall for about $25 to $30 per square foot of face less than a poured-in-place concrete wall with a cantilevered footing would have cost,” Barron says.More Advantages
The ReCon blocks offer several other benefits:
A natural look. The blocks are available with faces in a granite or limestone finish or a smoothface surface with a chamfered edge and sandblasted surface. Once built, the walls are stained to produce a natural stone appearance.
“The urethane form liner used to make the blocks can be fashioned to produce different textures,” says Barron. He took advantage of this feature on one historical preservation project to match the face of a large-block railroad bridge abutment project to the original stone construction. “The blocks were made to look exactly as we wanted,” he adds.
Large scale. In addition to facilitating the use of grading and excavating equipment to construct the walls, the shear size of the large block faces adds to the aesthetic value of some projects. Barron tells of one 15-foot-high, 200-foot-long big-block wall he built to stabilize the steep slope of a lakeside home. “With their massive size and limestone-like texture, the wall looked like it was built with large pieces of quarried limestone,” he says.
Greater durability. “The large wet-cast blocks are generally less porous than the smaller dry-cast blocks,” Hamilton says. “As a result the large blocks stand up to harsh environments, like road salt spray and repeated freeze/thaw cycles. Also, the concrete mix can be varied to further enhance durability of the blocks.”
More information is available at www.reconwalls.com.
High-speed Technology Provides Faster Fix for Steep Slopes
A device that uses compressed air to shoot small-diameter steel rods into the ground at 220 miles per hour offers a faster, non-invasive alternative to typical soil nailing practices for reinforcing steep slopes.
Soil nailing, itself, is gaining popularity as a more cost-effective alternative to conventional earth retention structures such as cantilever walls and gravity walls. Usually measuring about 4 to 5 inches in diameter and up to 20 feet or more in length, traditional soil nails bolster the shear strength of in-situ soils and their ability to remain stable at steep angles. The length of the nails, depth and angle of placement, and spacing are based on an engineered design specific to a given site. Once the nails are installed, the slope can then be protected from erosion with a mat, wire mesh, segmental concrete block, or other facing material.
High-speed Technology
Traditional soil nailing involves drilling 4- to 5-inch-diameter holes into the slope with a small, track-mounted hydraulic, rotary, or percussion drilling machine. After filling the holes with grout, the nails are placed in the holes and the grout is allowed to set. From start to finish this process can take about three or four days, depending on the size of the project.
However, geotechnical engineers Bob Barrett and Al Ruckman from Soil Nail Launcher Inc. of Grand Junction, CO, have developed an excavator-mounted unit that installs the nails in a fraction of this time. Their device uses high-pressure compressed air to drive 1.5-inch-diameter, 18-foot-long rods into slopes at high velocities.
The patented Soil Nail Launcher is based on declassified technology originally developed by the British military in the late 1980s as a secret weapon to shoot canisters of nerve gas as far as 7 miles.
The shock wave generated by the high-speed projectile as it enters the slope displaces soil at the tip of the rod and limits any abrasion to the nail as it enters the earth, Barrett explains. Once in place, the soil particles collapse around the rod to provide high pullout resistance.
This technique can be used to repair failures of existing slopes and to construct new steepened slopes, he notes.
Because the device is mounted on an excavator, it can reach over or up the sides of slopes, in between trees and other obstacles, and under overhead obstructions to eliminate the need for excavation and full road closures.
“There’s no disturbance to the slope, no waiting for holes to be drilled or for the grout to dry,” says Barrett. “Normally, it takes about an hour to install three soil nails when drilling holes and grouting the nails in place. In that same time, the Soil Nail Launcher can insert about 12 rods.”
Saving Time and Money
The company’s Soil Nail Launcher services include engineered designs for each project.
The savings in time and labor using this technology can add up to substantially lower slope stabilization costs, Barrett reports. Some examples:
The Soil Nail Launcher was used to repair a failing pile wall, where overhead power lines hindered installation of new piles at an Ohio site. Just moving the overhead lines to accommodate a pile driver would have cost about $700,000 and the entire repair job would have taken about a month, Barrett estimates. “We installed 200 soil nails, completing the job in four days at a cost of $150,000,” he says.
In Colorado, erosion along a mountain highway left a guardrail and shoulder in danger of imminent collapse. Using the Soil Nail Launcher to insert the rods and steel mesh and shotcrete to protect the slope, Barrett and his crew completed the repair in about 10 hours, during which one lane was closed to traffic. This work was done for less than half the cost of a conventional solution, such as retaining walls, piling, or excavation.
Launched nails were used instead of drill-and-grout soil nailing to stabilize a 1:1 slope where a 100-foot-tall Tennessee highway embankment had failed. This cut the estimated time to complete the project from 30 days to five days, minimizing traffic backups and saving more than half a million dollars, Barrett notes.
More information is available at www.soilnaillauncher.com.
Cellular Confinement S.tructure Blends in Naturally with Surroundings
When construction of a town-style shopping center in Lyndhurst, OH, required a long, high earth retention structure, project developers wanted a retaining wall that satisfied both the engineering demands of the job and the aesthetic concerns of the community.
The steep walls, about 400 feet long and up to 25 feet tall at the highest point, were needed to provide adequate space for a parking lot within the property lines of the project. Because it was adjacent to an historical estate noted for its architectural significance and expanse of wooded grounds, nearby residents championed a structure that would blend in with the estate’s natural setting.
Both the structural and visual requirements were met by constructing vegetated MSE walls using a cellular confinement system, which included a fascia planted with wildflowers.
This type of structure features individual sections or layers of interconnected cells, typically 6 inches high. They are filled with soil, gravel, or other onsite material and stacked to form a steep faced wall that minimizes erosion and is structurally stable under its own weight and known externally imposed loads.
“In addition to providing a green slope, the cost of these walls was significantly less than other alternatives, such as a poured-in-place concrete wall,” says Tom Letizia, technical sales manager for Meredith Brothers Inc. The company is technical representative and distributor for the Geoweb cellular confinement system used on this project.
Site-friendly Features
Made by Presto Products, the Geoweb system is an engineered, polyethylene, honeycomb-like cellular structure that improves the performance of dirt, gravel, and other infill materials. In addition to such applications as supporting loads and controlling erosion on slopes and channels, it’s also used to stabilize steepened slopes on change-in-grade construction projects.
In addition, the mass of the cellular confinement system permits the use of relatively short lengths of reinforcing geogrid or woven geotextile in building the MSE wall, he notes. As a result, it requires less excavation for reinforcement than with other types of fascia, such as conventional segmental blocks.
A cellular confinement system also offers contractors a productivity advantage over conventional segmental concrete blocks when building the fascia for an MSE wall. “A 6-inch-high, 8.8-foot-wide Geoweb section weighs only 7.5 pounds and provides a 4.3-square-foot face,” he says. “Placing the six or eight conventional blocks required to produce a similar-size face area would require much more time and labor.”
Of special importance on this project was the ability of the Geoweb system to support the growth and establishment of vegetation on the face of the structure.
Each layer of the cellular confinement system is set back as the structure is built. The exposed cells, filled with topsoil, create a terrace to capture rainfall and provide ample space for root systems to support the growth of vegetation planted in the cells. What’s more, the impervious vertical face of each layer prevents excessive, non-natural evaporation of ground moisture to help sustain vegetation, Senf says. The web sections are available with textured, non-reflecting faces in a tan, green, or reddish-sand color.
Construction Details
The two sizeable walls for this project were built in about two weeks of actual construction time during the late fall of 2004. “The crew had never seen the Geoweb system before,” says Letizia.
Each 6-inch-high Geoweb section measures eight cells (8.6 feet wide) and three cells (2 feet deep). To aid construction, a spreader frame, made onsite of two-by-four lumber held together by carriage bolts, was used to hold each section in the desired shape as it was placed. Adjacent sections were held together with heavy-duty galvanized staples.
Each cellular confinement layer was set back 1.5 inches to produce a 1:4 batter.
A polyester geogrid, provided by Synteen Technical Fabrics, extended from the face of the wall back 18 feet into the slope. It was installed across every three or four layers of Geoweb sections and compacted soil lifts.
Skid-steer loaders, trackhoes, and loaders were used to backfill the cells with medium-size (less than 1-inch) aggregate. This aggregate was compacted with a handheld plate compactor, while the soil lifts behind each layer of the cellular confinement system were compacted with a vibratory roller.
The outer rows of cells were then filled with topsoil and planted with a mixture of wildflower seeds.
Except for the exterior face cells of the Geoweb sections, all cell walls contain perforations to aid backfill lateral drainage and root growth. “The edges of the aggregate also protrude through these perforations to create greater frictional interlock between the cell wall, infill material, and underlying soil layers,” says Letizia.
Letizia has observed the earth retention structure a number of times since it was completed. “It’s performing as expected,” he reports. “I’ve seen no settlement and the vegetation has become well established. The developer built a park near the wall for residents of the area. Once, I saw a buck deer next to the wall, despite all the development in the nearby area. That was pretty heartwarming to see.”
