Claiming New Ground

Sept. 1, 2005

With the prime flatland in most urban centers already consumed, developers are resorting to steeper terrain for new residential and commercial building projects. This is creating an unprecedented demand for retaining walls that not only meet aesthetic values, but also meet the structural requirements needed to hold soil in place and prevent erosion and stabilization problems.

Even though the structural and engineering requirements for some of these retaining walls can be complex, for the companies that build retaining walls, help is generally as close as their computers. Most manufacturers of retaining wall materials provide engineering support and proprietary software to the people who design and build retaining walls with their products.

Design Assistance North of the Border
“The shortage of prime building land in the Toronto area has certainly created a demand for our retaining walls,” explains Bob Morawczynski, a civil engineer and the owner of Regent Landscaping Ltd. in Sharon, ON, which is a suburb of Canada’s largest city. “The projects we are now working on tend to include steeper ground, which means we are building some fairly extensive retaining walls. In Canada, any wall higher than 1 meter [3 feet] has to be Canadian Standards Association–approved and must meet the requirements of the Canadian Building Code. A licensed and certified engineer must also supervise the construction, and although in 99% of cases the soil conditions are suitable, the owner of the property, or their engineer, must provide a soil report.

“For many of our walls, we use RisiStone products,” he adds. “RisiStone is an engineering company that develops retaining wall systems and licenses these systems worldwide. They offer excellent software that the designer or engineer can look to for help when in the planning stages of the wall.”

The RisiStone software Morawczynski refers to is available from the company’s Web site as freeware. The program is twofold: It can analyze and check a design created by the user, or it can function as a design tool, determining the reinforcement that will be required to make the wall stable. The interactive program requires the wall designer or engineer to make design decisions throughout the process. According to the RisiStone Web site, “This allows the user to review the profile design, panel layout, cross section parameters, geogrid placement (if required) and the calculated factors of safety.”

Although the designer has input to every panel, the program takes care of the time-consuming calculations, and when a wall design does not meet the required standards, the software offers hints or options that will bring it up to the desired level. “I use the software for every RisiStone wall that I design,” says Morawczynski. “I have been using their software for over 10 years, and I know that I can trust the calculations.”

In Ontario, Unilock, the exclusive licensee and manufacturer for RisiStone, recently started producing the Sonoma line of segmental retaining wall blocks, and Morawczynski designed the first wall in Canada using them for a project in Whitby, ON, near Toronto. “The beauty of using the Sonoma blocks is that they are a much larger block than most, so on a large wall, we are able to speed up the installation process.”

The standard Sonoma Stone units are 48 inches wide, 7.25 inches high, and 14.75 inches deep (120 by 18.5 by 37.5 centimeters), but they are designed to have the appearance of smaller blocks. “Unilock has developed a clamp for installing the Sonoma blocks, and this really speeds up the installation process,” says Morawczynski. “Using the clamp system is like placing three blocks at once. You can make amazing walls with this product, and the size of the units is perfect for walls and steps. It is a 100% engineered wall, and the calculations are so exact that nothing is wasted.”

Steep Ground in the Pacific Northwest
Bob Stokke also knows about working with steep ground, because just about all of the flatland has been built out. Stokke is the director of land construction for D.R. Horton, a national company that is the largest homebuilder in the US. He is based in the Seattle, WA, suburb of Kirkland and looks after everything from feasibility studies to site construction and the preparation of roads and lots for the company’s western Washington operations, which encompasses an area that stretches from Bellingham to Olympia.

“Here in western Washington, we have to deal with some fairly steep topography, especially since whatever flat ground we had was developed more than 20 years ago,” notes Stokke. “It takes a fairly flat lot to build a house effectively, so we use retaining walls to step the lots, and we also use them to maximize the usable space in the backyards. We tell people that ‘we build neighborhoods, not just houses,’ so we want to provide families with as much usable backyard space for their kids to play in as we can. Retaining walls allow us to do this where the yard might drop off, or it may back up to wetlands.”

Stokke says D.R. Horton uses several types of retaining walls, depending on the circumstances. In most cases, the wall used depends on whether it is a cut or fill scenario. The slope is often cut, and the wall is placed against the cut face. “In these situations, if it is a higher wall, we will use rockery for a natural wall, and if it less than 4 feet in height, we may elect to go with a Keystone wall. The Keystone wall is 18 to 24 inches in thickness, and a rockery is a minimum of 4 to 4.5 feet,” says Stokke. “If it is in a fill situation, we go with Lock+Load wall. This product has only a 6-inch-thick face, which attaches to a counterfort with a stainless steel wire. The counterfort is what provides the stability, and by taking less room, it allows you to gain yard space, in some cases 2 to 3 feet over other retaining wall systems.”

Regardless of the type of wall used, Stokke stresses that proper drainage is very important, especially in the rainy Northwest. “For a rockery wall, we will generally put in a drainage blanket of quarry rock; for the Keystone, we will use washed rock; and with Lock+Load products, we will use 1.25-inch clean rock. We also install an outlet for water, such as a drainpipe at the base of the wall that is tied into the stormwater system.”

Maintaining the Grade in Massachusetts
Jon Pate, a licensed landscape architect with Boston, MA–based Carol R. Johnson Associates Inc., says retaining walls are becoming more prevalent on the company’s projects. The company has approximately 50 landscape architects on staff, and in addition to the main office in Boston, where Pate works, it has a satellite office in Knoxville, TN. The company serves national and international clients, integrating high-quality and cost-effective solutions through the use of environmentally sensitive designs.

“In many areas across the country, development has shifted to more challenging sites, and that has increased the amount of retaining walls that we are specifying in our projects,” notes Pate. “Locally, we are seeing clients and developers purchasing properties with plentiful ledge. These are sites where, 10 to 15 years ago, they would have questioned the development costs.”

Since he’s a landscape architect, aesthetics are important to Pate, who adds that in today’s market, designers have many choices when it comes to specifying aesthetically pleasing walls that also meet structural requirements.

“A good example of a project we are working on right now that requires approximately 35,000 square feet of retaining walls is the Linden Ponds project in Hingham, Massachusetts, which is an independent retirement community being developed by Erickson Retirement Communities,” explains Pate. “This is a 100-acre project that will be completed in several phases over a six-year period, and we will be using many retaining wall types depending on the location.”

Phase one of the project is nearing completion, phase two is under construction, and design work will start later this year for phase three. Pate says a challenging item on this project is that, because it is a retirement community, accessibility is a critical design component. “Our client and project team want all areas of the site to be as accessible as possible, and further, all buildings are interconnected, which means grade transitions are key elements on a tight site where we strive to preserve as much of the existing woodlands as possible.”

Retaining walls at grade transitions between buildings and at existing woodlands help achieve that goal, and Pate adds that there are a number of options available when it comes to good-looking retaining walls, which provides clients with better options at different price points. “At key areas where residents will gather on a terrace or where they might be sitting on or leaning against a wall, we specify a higher-quality finish and more interesting wall. Where retaining walls may not be seen, or only viewed when driving or walking, we use other, less-expensive options. These walls need to solve the grade issue, but we don’t have to specify a high-end wall at a higher cost.”

For some of the walls in places where the landscape architects were creating an impression of craftsmanship and high quality, a concrete wall core with a stone or veneer face was used. At building terraces, they specified a Weymouth granite, which was originally quarried from the site and was supplied from an active quarry across the street from the development. For other walls that would not be quite as high-profile, they went with two segmental retaining wall products from Versa-Lok Retaining Wall Systems. “We used the Mosaic style in areas that needed to be more aesthetically pleasing, and their standard block where few people would see them,” notes Pate.

The Mosaic style breaks the standard form of most segmental retaining walls, as it comes in a tumbled ashlar pattern. “The tumbled blocks give it a weathered look, and the end result is more interesting visually since it breaks up the horizontal lines that are typical of segmental walls,” explains Pate. “They are also available in an earth-tone grey and tan mix, which helps the wall blend into the surroundings, creating a more natural look.”

Soil Nailing in Topeka
For George Schuler, a project engineer with the Judy Co. Inc. in Kansas City, KS, appearance was also very important for a retaining wall that formed part of a shopping center complex in Topeka, KS. “The wall was 20 to 25 feet in height and was used on a steep slope of 3:1,” explains Schuler. “In addition to looking good, the owner of the property also wanted to use the retaining wall to maximize ground usage for the shopping center development, so we used a technique called soil nailing, and then went with a shotcrete-sculpted facing from Boulderscape in San Clemente, California. Our experience with soil nailing, and the fact that we could get within 1 foot of the property line for the retaining wall by using this technique, are some of the reasons they chose us for the job.”

According to the Judy Co.’s Web site, “Soil nailing is a relatively new construction technique first used in Europe to stabilize and construct tunnels. In North America, it is quickly becoming a popular method of soil and rock retention, slope stabilization, and shoring.”

Geotechnical engineers design soil nailing projects, where essentially a top-down construction method is used and a bench of between 4 and 6 feet in height is excavated. Holes are then drilled into the excavated face and pumped full of a ready-mixed grout. The nails, which are normally continuous threaded steel bars, are then inserted into the grout-filled holes. The steel bars, or nails, are long enough to penetrate the failure plane of the excavation. The concept is to stabilize the soil by creating a grouted mass that the surrounding soil will act upon in friction. Once the grout sets, the protruding nails are fitted with a steel plate that transfers forces from the wall to the nail, and horizontal and vertical drain strips are installed to control seepage and eliminate hydrostatic pressure buildup. Reinforcing steel and a temporary shotcrete facing is then applied, and the process is repeated until crews reach the bottom of the wall. Finally, the permanent face is shotcreted using a trailer-mounted concrete pump. For temporary projects, such as shoring or behind a cast-in-place concrete finish, the shotcrete can be left rough or troweled. For permanent walls, it can be sculpted and stained so that it blends into the natural surroundings.

 “For the shopping center project in Topeka, we did all of the reinforcing work and supplied the pump for shooting the shotcrete, and Boulderscape came onsite with their crew to do the sculptured finish and staining,” says Schuler. “In terms of cost, the method we used worked out less than other methods such as cast-in-place, and aesthetically it was far more pleasing to the owner, who even got to pick the color and finish he wanted.”

Engineering for Height in New Jersey
Pat McCrindle of McCrindle Paving Systems Inc. in Collingswood, NJ, says most of his customers are well educated and know what they are looking for in the way of finishes before he even meets with them. “Here in New Jersey, we have a large number of manufacturers of blocks for segmental retaining walls, and they spend money on advertising and promotion, so when a customer calls us, they quite often know what type of look they want. We will try to steer them in the right direction, and we are not afraid to say no if we don’t think something will work, but we have lots of excellent options to choose from. The ICD StoneWall Select product is an 8-inch by 12-inch block that is substantial and ideal for lager walls, while the EP Henry Coventry Wall is also popular as it has the look of a natural stone wall. There are also products available locally from Allan Block, Unilock, Bellgard, and others. It is really just a question of aesthetics and what the owner wants.”

McCrindle says that in some cases, the material choice is narrowed down and dictated by the purpose of the wall. “Certain types of blocks are better for specific walls. For larger walls, we have to have soil samples so that we can determine a suitable backfill for the wall, and we have to determine the slope behind the wall. We have to look at the actual use of the wall and determine if it will have a fence on top of it and if the wind will push or pull it. In addition, we look at drainage issues to relieve hydrostatic pressure. This can be handled in a number of different ways, including the installation of drip lines with face outlets to let the water release, and stones behind the wall with a 4-inch perforated pipe at the base of the wall.”

A recent project McCrindle took on was in northern New Jersey for the owner of a professional sports team. The wall, which was up to 18 feet high in some places, included 5,000 square feet of Mesa block from EP Henry in Woodbury, NJ. “This wall took approximately 5,000 blocks, and took 10 working days to complete,” notes McCrindle. “It was designed for reinforcement, and we hired an engineer to work with us to ensure it did the job. With the right engineering, you can design a wall for almost any purpose. In the past, we have built walls that were engineered so that semi trailers could drive on top of them.”

Joe Pillari, an owner, partner, and vice president of Pillari LLC, works primarily in northern New Jersey, but also takes on projects in Connecticut and southern New York, including the southern reaches of New York City. As a landscape architect, Pillari says the core of his business, which he started in the mid-1980s with his brother, Bruno, is landscape construction with a focus on segmental retaining walls. “About one-third to almost one-half of our business is retaining wall construction,” explains Joe Pillari. “About 15% of that retaining wall business is residential and the balance is commercial.”

For Pillari, the structural considerations are most often the important part of the design, especially on some of the larger walls the company has built. “The maximum height we have done to date is 42 feet, but we are designing a wall right now in Jersey City that is going to be 43 feet high.”

On most projects, Pillari works with Keystone Retaining Wall Systems. Some projects might require an engineer who specializes in the type of wall being designed. “The first thing I look for when working with an engineer is job-site compatibility. For specific projects, that may involve using a geotechnical engineer with experience in things like rock encounters or soil compaction techniques. For the most part, on simple grade-change walls, we will work directly with an engineer from Keystone. We look at things like the slope, the cut, the end use, and the soil, and then find an engineer for the job.”

A recent design/build project Pillari completed in Atlantic Highlands, NJ, used soil anchors. The engineer on the job was William McHeffey of MC Engineering in Red Bank, NJ. The project involved removing a failing timber retaining wall and replacing it with a Keystone wall, but because of space restrictions, excavation for a reinforcement zone was not possible. “To get around the working-space limitation, we went with Manta Ray soil anchors, which were hydraulically driven to create a tieback system,” explains Joe Pillari. “Schedule 40 galvanized steel pipe was bolted to the threaded anchor rods, and geogrid was wrapped around the galvanized pipe and pinned to the Keystone block. This provided the connection between the wall and the soil anchors.”

Reinforcing With Geogrid
For Chad Stockman of Stockman Landscaping in Jefferson City, MO, geogrid plays a key role in many of the segmental retaining walls he constructs for clients. “We have a lot of hills in the area we work in, and if you are at the bottom of the hill, water is going to run into your retaining wall,” explains Stockman. “The amount of drainage is one of the considerations that help determine how much geogrid we install on a specific project. Another is the location of the wall. If the wall is next to a concrete driveway that will have a car parked on it, we have to look at the weight of the car and what that will do to the retaining wall if we don’t install enough geogrid.”

Stockman explains that the amount of geogrid is also determined by the height of the wall. “For every few feet of vertical height in the wall, we will put in a layer of geogrid, which is held in place by rocks and soil. The stronger the wall needs to be, the farther back from the wall we will go with the goegrid. We have to analyze the drainage and the usage of the wall, and that determines how much geogrid and how many layers we will install.”

In addition to Stockman Landscaping, Stockman owns Mid-MO Block and Brick, a block plant that produces segmental retaining wall blocks under license to Pyzique Wall Systems, VyKing Wall Systems LLC, and MaytRx Wall Systems. “All three of the blocks we produce are for segmental systems,” explains Stockman. “They are all different shapes and sizes, so one of the first things we do when we meet with a potential client is ask them what size the wall will be and what it is being used for. If a wall is 3 feet or under, we will generally suggest the Pyzique system, which are smaller blocks weighing about 25 pounds each. If the wall is over 3 feet high, we go with the VyKing or MaytRx products, which are larger blocks. The VyKing blocks weigh approximately 75 pounds, while the MaytRx is a totally different system using about six different sizes that can be randomly placed, providing a less uniform look. For a more natural look, we can tumble the Pyzique and the MaytRx blocks.”

Tunneling Underground in Florida
At the Villages, a privately owned planned retirement community in Florida that covers 32 square miles and encompasses 16 golf facilities, including Nancy Lopez and Arnold Palmer courses, contractor Associated Construction Products Inc. (ACP) was faced with the challenge of creating 20 underground tunnels that form part of the golf course paths. The walls of the tunnels had to provide structural integrity, while having an “Old World” look that would match the rest of the development.

Larry Falls, the founder and owner of Tampa and Orlando, FL–based ACP, suggested a GravityStone system from WestBlock Systems in Tacoma, WA. “The GravityStone system is a hybrid concept that uses a modular base and a reinforced wall at the top,” says Falls. “We used a combination of GravityStone products, including Fat Face, Thin Face, and Anchor/Junction Blocks to complete the walls in the tunnels. To achieve the Old World, time-worn look the client wanted, we tumbled the Thin Face blocks.”

Timing was also an issue for Falls on the golf course project. “As you build a wall, you are backfilling and compacting the soil,” he explains. “If you use concrete walls, you have to wait 28 days for the walls to cure, which slows the process down and means you can’t build headwalls for traffic to travel over, hook up utilities, or open up the area for use until the concrete dries. With the GravityStone system, you backfill as you build the wall, and as soon as you have placed the last block, you are ready to start landscaping.”

Falls started ACP in 1986 as a one-man operation. Today, he has upwards of 70 employees, and although most of his work is in Florida, the company has completed work in the Bahamas and other areas of the Caribbean. In addition to retaining walls, ACP handles geosynthetic liners and other erosion control products, along with pavers and concrete panels.

Falls also owns Segmental Designs, which manufactures the GravityStone products under license to WestBlock Systems. “We are very involved in the marketing of the products to engineering and construction companies,” adds Falls.

Regardless of which system contractors and retaining wall companies use, most agree that there is no cast-in-stone method. Every project is different and the engineering parameters are unique on every project. But for professionals in the industry, there is always a solution, and the companies that supply them with products are willing to help find it. 
About the Author

Bill Tice

Author Bill Tice is based in Blaine, WA.