Retaining Walls: What You See and What You Don’t

Jan. 1, 2004

What you see with many retaining walls these days is a fashion statement. No one likes to see just a concrete wall, and the facial elements of retaining walls have changed radically within the past two decades, just as pavers have changed the way driveways look.

Once made almost exclusively of large sections of poured-in-place concrete along highways or shorelines, retaining walls now get all dressed up to satisfy the architects and engineers who specify them, as well as the consumer end user. Designers and contractors now can choose from dozens of types of retaining walls, including various segmental blocks and facial styles. The opportunity now exists to carry a design through many areas of a site, such as from the patio to the garden retaining wall on a residential site.

For the property owner, the only limiting factors are cost and having an architect who has kept up with the latest designs and isn’t afraid to explore design concepts and products that originated outside of his or her own geographic area.

Stepped Shoreline Detail

Horizontal and Vertical Integration
The Lake Michigan shoreline has a long history of erosion. One company in the area, Redi-Rock International LLC, was established in 1999 and has protected many properties along the shore. A subsidiary of Manthei Development Corporation in Charlevoix, MI, near the top of the state’s lower peninsula, Redi-Rock is owned by two generations of brothers, cousins, and sons of the Manthei family.

“Our interlocking blocks average around 2,200 pounds each and have natural rock faces,” says Ben Manthei, president of Redi-Rock. “The face is 4 feet wide and 1.5 feet high. Each goes back into the bank about 2.5 to 3 feet.” The walls are designed to look like natural limestone rock, cobblestone, or split limestone with free-drain agate behind the face of the blocks. “Agate lets water in the soil go down to the bottom of the drain, and the drain carries the water out from behind the wall,” Manthei explains. The business is about half residential and half commercial and includes many projects on Lake Michigan and around golf courses.

One recent project built by MDC Contractors, another Manthei Development subsidiary, was the retaining walls for two high-end homes that a speculative developer had erected on a bluff 200 ft. above Lake Michigan. “The more expensive home sold first for $7.5 million,” reports Manthei. “We did site grading and built retaining walls between and around the homes north of Harbor Springs, Michigan.”

Both homes were built on a steep grade, reached by driving 100 ft. up a winding road. The houses stand 75 ft. above the beachfront.

“With these houses,” Manthei says, “our biggest problem was moving the building materials on the steep access road. Before we built the retaining walls, you could walk out of the homes’ basements across a small patio and to a big drop-off. Our walls were 6 to 9 feet tall. From the house side, you see a natural, rock-faced, free-standing wall 2 feet wide, designed with curves.”

An 18-ft. wall separates the houses and a turnaround area. The cost for grading and constructing the walls with mechanical geogrid connectors was a little more than $200,000.

Sunken Tennis Court
Another challenging project for MDC Contractors was sinking a tennis court for a condominium project in Petoskey, MI. The developer discovered that the planned tennis court would block the view from some of the condos. “We had to build retaining walls around three sides of the court,” says Manthei. “These retaining walls have drain tiles to handle groundwater, rain, or snow.”

The tennis court walls are now 6-8 ft. below the condominium building and 10 ft. from an already constructed garage. Eventually the tennis court’s retaining wall will support some of the garage’s weight.

The garage limited the options on this project, explains Jeremy Manthei, CPC, site foreman for MDC Contractors and son of Ben’s brother Jim Manthei, who started making the firm’s segmental interlocking block about five years ago. “Other types of retaining walls rely on soil tiebacks or geogrids to a higher elevation,” Jeremy says. “Because of the closeness of the garage, it was extremely difficult to dig the footings for the retaining wall. This wall is about 100 feet long and 10.5 feet high. We started 7 feet below the bottom of the garage footings and placed 10.5 feet of soil around the garage.” The cost for this project was about $250,000.

Lake Bellaire Beach
One of Jeremy’s first projects was to shore up an eroding beach on Lake Bellaire. “The house had a big, natural rock retaining wall,” he says, “but every winter the ice on the lake would push the boulders out of the water and onto the yard of the house. A product that will work in our freeze/thaw climate will work anywhere. In warmer climates you have to allow for more permeability. If the concrete contains microscopic voids and freezes, the voids will pop to the surface.

“We built a 3-foot wall with steps to the water about 150 feet long in the water behind the rocks and used geotextile filter fabric behind the wall that allows water to pass through and keeps out sand. We backfilled with sand. This was my first project built in water, so I learned as I went.”

Jim learned he had to run the wall at base elevation to the end of the steps rather than step up the step with the wall. The wall took three days to build and cost close to $9,000. “It is not unheard of, where large blocks are required, to install 600 feet of wall in one day,” says Jeremy.

“On taller retaining walls, we use a geogrid we developed and patented: Mirafi Miragrid 18XT,” says Ben. “We use 300-psi wet-cast concrete mix. When we mix this concrete, the chemicals in the mix harden and dry on their own.”

Engineered Wall Systems
When a retaining wall is built as part of a large development project, work often proceeds in stages. Gary D. Pritt, general manager of New Dimension, a contractor in Mint Hill, NC, used products from RidgeRock Retaining Walls in Charlotte, NC, to build 80,000 ft.2 of retaining wall in the first phase of Charleston Places at Vallantyne, an upscale residential townhouse community in Charlotte. “We’re now building the second phase, which has an additional 25,000 square feet of retaining wall,” says Pritt. “Charlotte is on a hilly sandy-clay soil plateau.

“We had to blast rock to construct a wall 30 feet wide and 45 feet deep. The project cost about $1.5 million just for the retaining wall. It took us about six months to build phase one.”

Stone was blasted to permit installation of a geogrid. “We crushed the stone on the site and backfilled it in. There have been no drainage problems. Twenty-five degrees Farenheit is cold here, and there is not much frost. The site is on private property, so there has been no traffic problem.”

Pritt also used RidgeRock at Hunter Crossing in Charlotte, where his firm had to move temporary holding ponds while building a 25,000-ft.2 retaining wall about 1,500 ft. long and up to 35 ft. tall.

“We had to start construction before the ponds were removed,” he says. “The trick was to build one wall, move the pond, and then build the other wall across where the pond had been. We had to undercut to build the walls. The project cost about $300,000 and took about three months. The only thing out of the ordinary was diverting the water and containing it.”

The owners of RidgeRock Retaining Walls launched their company in 1979 as a reinforced-soil business and added retaining walls in 1998. “We are developers, marketers, and the patent holder for the Ridgerock Retaining Wall system,” says Jim Martin, P.E., one of the owners.

Working Around the Weather
In cold climates like Minnesota, freeze/thaw cycles present special challenges for wall construction and drainage. “We do interlocking retaining wall installations between freezes. It is our challenge in our climate to make sure retaining walls stay in place,” explains Clint D. Ernst, president of Custom Retaining Walls and Landscape Corporation in Rochester, MN. “We build our walls on the original grade. Walls are only as good as what they sit on. In our climate, where the ground is frozen part of the year, it is critical to have good drainage from behind the wall. Otherwise moisture captured behind the wall will freeze. Frozen water expands and will push on the wall, forcing the wall in just a few years to begin to fail.

“There is an automatic setback when we install walls so that when the thaw occurs, the wall is pushed forward. When a retaining wall is properly drained of any moisture caught behind the wall, it will drain rather than freeze.”

Concrete products, he says, should be from the same run so they will look the same and have the same humidity level and other characteristics.

“We hold up bigger walls with geogrid and geotextiles laid underground into the hillside. The longer the length of the wall, the longer the grid length.” Other challenges, he notes, include working on hillsides, as they present traction problems for backhoes and lack level surfaces for skid loaders with tracks. Getting large trucks close to the site to haul in materials also can be difficult.

“This whole area is compacted. When the ground is not properly compacted and the grid works its way up as the ground settles, the grid will start to pull back from the wall, and the wall can fail. It can fall back into the hillside, and gaps will develop in the wall.”

Custom Retaining Walls is an employee-owned company that works primarily in southern Minnesota using products from Rockwood Retaining Walls Inc. of Rochester, MN. Rockwood traces its history to Rochester Silo, a farm-structure company founded in 1914. The Price family purchased the firm in 1949 and later changed the name to project the evolution of its business into the manufacture and sale of retaining wall systems.

“We use the same equipment to make retaining wall systems that they used to make farm structures,” says Rockwood’s Greg Ernst, who is Clint Ernst’s brother. “The segmental wall industry’s sales are growing 10 to 15% a year. Industry providers are private and secretive, so no one knows exactly what the real figures are, but a good guess is that 120 million square feet of retaining walls a year are sold.”

Three Key Principles
Clint Ernst cites three key principles for retaining wall construction:

  • A good base
  • Good compact backfill
  • Good drainage throughout the wall

    The entire wall has to be a big drain, he says, so that any individual piece of wall can serve at some time as a drainage point.

    “On large projects, engineers and contractors should always know how water moves across neighboring properties and how this runoff will affect their client’s project,” Clint notes. “In addition to rain runoff, all nearby springs-even long-buried springs-and the course of their runoff should be identified. With springs, installing plenty of drain tiles and rocks to avoid water retention and drainage problems is important.

    “We always recommend that our clients hire a certified engineer, who hopefully will see the site before designing their projects. Anyone can design a wall you’d like to see built, but it takes an expert to consider soil patterns, runoff, and how much geogrid is required. Understanding the land’s previous use also is important, especially what might be buried on the site.”

    Correcting Failed Projects
    Custom Retaining Walls is sometimes called to correct failed retaining wall projects. On one occasion, the contractor for a new office-warehouse complex in Rochester installed several hundred linear feet of a 20-ft.-tall modular wall.

    “They neither hired an engineer nor followed my recommendations,” Clint says. “They did not install enough geogrid and used nonengineered fill [fill of unknown origin]. The first winter, the wall failed.

    “The owner asked us to fix the wall. First we hired an engineer. Then we told them we were going to take down the entire wall and rebuild using Rockwood products. Very little could be salvaged.” New construction started from scratch. All of the fill was removed. The site was taken back to grade rock-3-4 ft. underground.

    “Following the engineer’s direction, we built a good base, compacting each level as we built the wall. The project cost the client twice what it should have” because it had to be rebuilt, he says. “Good product was used in the original wall, but the installation was wrong.”

    A “Landscaped” Appearance
    Appearance of a retaining wall is important on many projects. Vinci Pacific Corporation, a general engineering contractor in Del Mar, CA, was the subcontractor for DPR Construction on construction of a new facility for Pfizer Inc. in the Torrey Pines area of San Diego. “We excavated the basement of one building to connect to a new building, and we had to build a retaining wall to extend the back of the building overlooking a canyon,” explains Sean Christiansen, project manager and estimator for Vinci Pacific.

    Vinci Pacific used a “plantable” segmental wall system from Soil Retention Products Inc. “The system is quick to install and is 100% landscape,” says Christiansen. “Some people don’t like to look at concrete. You may have the appearance of a steep slope, but because of the vegetation, you can no longer see the concrete.” Common plantings in such walls include ivy and rosemary.

    Soil Retention Products is owned by Jan Janssen, who also owns Soil Retention Systems Inc., a segmental retaining wall installation company, and Toy Rentals Inc., which rents and sells special equipment for block production and retaining wall installations.

    Solutions for Site Development Challenges
    Site development using “engineered systems” solutions is a specialty of Tensar Earth Technologies Inc. (TET), an Atlanta, GA – based company that offers several earth retaining wall, steepened slope, roadway soil reinforcement, and foundation stability applications to solve geotechnical problems in the development of difficult sites.

    Since its inception in the 1980s, TET has worked on more than 25,000 projects worldwide, says Peter E. Larkin, P.E., the firm’s business development manager. The parent company of TET, The Tensar Corporation, develops and manufactures high-performance, soil-reinforcement polymeric structural geogrids, which TET sells as components of its engineered systems. Tensar geogrids, made of high-density polyethylene and polypropylene, are designed on a site-specific basis to provide the tensile strength and length of service required for each application.

    TET generates much of its business in working closely with various state departments of transportation (DOT), local departments of public works, and general contractors and subcontractors bidding on projects in the public DOT and private marketplace. “We provide our clients with complete systems design, construction materials supply, and onsite start-up technical assistance,” Larkin says. “The challenges faced today by designers and developers are greater than ever before, as the cost associated with site development increases. The sites that are easy to develop are all but nonexistent, and owners and investors demand economical solutions for complex engineering challenges.”

    Building Near Traffic
    In winter 2003, Tyler Gillis, owner and project manager of Sound Retaining Walls in Tacoma, WA, built a retaining wall along the city’s San Mortiz Road. The road ran up a hillside, and because it was the only means of access to an elementary school and an adjoining subdivision, Sound Retaining Walls had to cut into the embankment to widen the road without ever closing it to traffic.

    Timothy Theis of Tim Theis Engineering in West Linn, OR, the structural engineer on the project, selected GravityStone from WestBlock Systems of Fircrest, WA, because of its narrow structure, which minimized disruption of the embankment and the existing road. Construction was about $18/ft.2 for a 500-ft.-long gradient rising 50 ft. in elevation.

    “We specialize in building retaining walls,” says Gillis. “A greenbelt exists between the developer’s property and the school. Local planners designed the road to accommodate future traffic and local development, and the project plans called for adding a sidewalk. The designers also had to incorporate a substantial setback based on the greenbelt and a very tall wall that was required to carry traffic.

    “The owner of the general contractor called me and asked if we would design and build his wall. I talked to the civil engineer we work with regularly, Timothy Theis, and obtained a geotechnical earth study. The local soil had good drainage. We would have had problems if the local drainage was poor.”

    With their soil questions answered, Gillis and Theis designed a retaining wall, priced it, and presented their design to the general contractor, who in turn presented it to the owners. “Our proposal cost roughly $90,000 to build, which was about 40% less-$50,000 to $70,000 [less]-than the next best solution,” Gillis says. “The GravityStone modular wall is architecturally pleasing, which satisfied both the developer and the local school board.

    “Construction went easily. We installed interstate-quality guardrails on top of the wall to keep people from driving off the side. That was important because with this wall system you can’t hammer guardrails into the top of the wall.”

    WestBlock Systems is an early 1990s spinoff of WestBlock Products, an Oregon-based manufacturer of concrete block products. The company makes several categories of products:

    • Landscape products, including NurseryStone and ChelanStone, are relatively small and generally used in commercial and residential walls shorter than 4 ft.
    • Crossover products, such as SahaleStone and StoneWall, are designed for use in engineered or reinforced walls; their scale and face also work well in shorter landscape walls.
    • Barrier systems are used for above-grade structures, such as fencing and privacy walls.
    • Engineered segmental retaining wall systems, or GravityStone, can be designed and built as modular gravity or mechanically stabilized earth (MSE) systems.

    When designed as a modular gravity system, GravityStone uses interlocking concrete units that can expand into the soil. “A modular interlocking system’s concrete components are assembled into cellular grids. The base of the modular system is normally narrower than the base of an MSE system, and the cells narrow as you build from bottom to top, so the structure requires less space behind the wall face,” explains Jim Hammer, president of WestBlock Systems. “An advantage of GravityStone is that it requires a minimum of excavation. There is an inherent advantage in using GravityStone in cut embankments where rock or means of embankment have to be removed for the wall structure to be erected. GravityStone also can be assembled as a double-sided, above-grade barrier wall that extends continuously from the retreating wall within the same unit.”

    Seismic activity is a great concern in many parts of the country. When the Nisqually earthquake occurred in Washington on February 28, 2001, several WestBlock retaining walls were located near the epicenter 5 mi. from downtown Tacoma. “The earthquake measured 6.8 on the Richter scale,” notes Hammer. “They all had zero damage.”

    Golf Course Specialists
    Since 1995, Darryl Burkett, president and project manager of Riverwalls Ltd., has targeted the golf course industry to obtain work stabilizing eroding shorelines. “I’ve developed a machine that protects the greens from my equipment,” says Burkett, “a machine that uses a hammer and sits on a patent-pending trailer-mounted crane that I can safely take on fairways and greens. The crane holds an 800-pound vibratory hammer parallel to the ground and eliminates the need to erect a scaffolding that can damage a green.

    “Golf course erosion occurs when the course fills lakes with water and draws the water out several times a day to water the greens,” he explains. “We use whatever retaining wall building materials our clients specify.”

    Many top-designer golf courses can’t be changed or renovated without the designer’s approval. This is especially true of Jack Nicklaus’s courses.

    Since 1998, Burkett has worked on shoreline restoration for a Jack Nicklaus signature course in North Barrington, IL. Built in 1989, the course has lost 5-11 ft. of shoreline around the lakes on several featured holes over the past 10 years.

    “They looked at several methods and couldn’t find anyone who would promise to bring mechanized equipment in without damaging their existing turf,” Burkett says. “They heard about my success on Cantigny Golf Course in Wheaton, Illinois, and asked to see my equipment. I showed them different equipment from what they had seen, and I asked them about using steel sheeting. They were impressed.

    “In February of 1997, we did some sample test runs. Then they gave me a $250,000 contract, and we installed eight-gauge steel sheeting manufactured by Superior Piling of Ridgeview, Illinois, on some of their most difficult shots. We drove the sheeting down to the substrate.”

    To get the desired steel sheeting penetration, Burkett used his crane-mounted vibratory hammer and installed a continuous waler (a piece of steel that is welded continuously to the back of each piece of steel sheeting) to weld each piece of sheeting halfway between the water and the top of the grade with the Manta Ray Earth Anchor System manufactured by Foresight Products LLC of Commerce City, CO.

    “Anchors are installed every 10 feet. I use Manta Ray Earth Anchors with stainless steel rods. The anchor size depends on the soil conditions of the compacted soil. I also bend the steel rods around the waler. Then I backfill from the water line to the mud line with washed gravel, and I cover the gravel with filter fabric. We fill the remainder with dirt, sod, or seed to meet the course grades.”

    Burkett says the worst possibility is to have a wall fail. If it fails from the top, it can be fixed, but if the sheeting pushes out from the bottom, this is not possible. “Then you have to reexcavate and rebuild and install a proper tieback system.

    “Gravel used in the wall acts like a big curtain drain,” he explains. “We torch or cut a weep slit in the bend of the sheeting to add extra drainage wherever it is required. Occasionally water drains through the bunkers or greens. I sometimes install drainage directly through the sheeting. Different textures are installed in front of the sheeting – sometimes vinyl or steel. The sheeting stops erosion and helps to reclaim lost land.

    “Sometimes golf course boards of directors or architects ask me to reclaim 7 or 8 feet of lost land. To do this, I install I-beam pilings 4 or 5 inches below the water line and then set outcrop rocks on these beams and build a decorative stone wall.”