Retaining walls have become so much a part of our environment that most people barely notice them. However, professionals realize how much they have changed over the years and what an asset they have become. The ability to provide strength and integrity makes some projects possible that once would have not worked. The increasing desire for aesthetically pleasing walls has led to a blossoming of textures and colors for wall products.
The advancement in construction techniques also permits quick and methodical construction with little variance of results. Each company that manufactures retaining wall products has developed wall systems that tie together every element so that construction can proceed in a straightforward manner. They provide comprehensive design and installation guidelines that make a project progress quickly and efficiently.
Retaining walls are built to replace older, failing walls; as emergency repair after flooding; to allow construction of wider roads or new ones; to provide strength to bridges; and for a variety of other uses. Here are some recent projects that highlight the value and versatility of retaining walls.
Replacing a Failing Wall in Minnesota
Sometimes there is a need for a new retaining wall because a failing wall needs to be replaced. In Plymouth, MN, an office complex with a large paved parking lot had a stormwater retention pond to collect runoff. Three underground stormwater pipes came into the pond, and one outlet was located at the low end. The retention pond was surrounded by a failing retaining wall. The wall had been built 21 years before, and was made of dry-cast segmental retaining wall blocks. The wall was leaning forward, and some blocks had deteriorated. It was easy to tell that further destruction was ahead, possibly leading to damage to the parking lot and obstruction of the stormwater facility.
Replacement of the wall was needed, but there were major challenges to be overcome. The project would have to have a minimal impact on the office complex and the parking lot. No geogrid installation would be possible because it would require more excavation than space allowed. Using geogrids would have required about 78 inches of excavation at the higher points, meaning removal and then replacement of a large portion of parking lot, substantially increasing the disruption for the office workers and dramatically increasing the cost. The retaining wall would have to be designed to handle constant water immersion and the possibility of heavy water load in front of the wall. It would also have to be durable against parking lot deicing chemicals that would run off into the retention pond.
The engineer for the project was Curt Derichs, principal with Civil Design Professionals in Bloomington, MN. “Water is the biggest enemy of retaining walls,” he says. Now he would have to design a wall that would have water against the face. The design would also have to ensure that water did not get trapped behind the wall, causing an early failure.
The choice for this replacement retaining wall was a ReCon gravity wall, which could be installed quickly without geogrid reinforcement. The blocks have the durability needed to last in a retention pond with deicing products.
At the start of the project, the retention pond was dewatered. The pond had to be drained several times during construction. The failing wall was removed, and a leveling pad of crushed stone was put in place and compacted. The base block was placed 45 inches deep. Clean, one-inch stone was backfilled into the spaces between the units to provide free drainage.
ReCon Series 50 units were chosen for this wall. They are manufactured from wet-cast, air-entrained concrete, making them the best choice for an environment of road salts, water immersion, and freezing and thawing. They come in a variety of sizes, including 48 inches wide, 16 inches high, and depths from 24 to 60 inches. They weigh from 1,411 to 3,115 pounds. Forterra Pipe and Precast of Shakopee, MN, supplied the blocks.
For this design, 24-inch, 39-inch, and 45-inch deep blocks were used. Successive block courses were added, with geotextile fabric placed between rows, and the voids were filled with clean stone. The blocks are placed on the lower level and then pulled forward so that the groove slides onto the lower block’s tongue, locking the layers together. A layer of top blocks finishes off the wall.
The wall contractor for the replacement was Matt Barron, owner of Hardscape Construction in Burnsville, MN. He notes that the biggest challenge was the stormwater retention pond, and the fact that it had to be dewatered at the beginning of the project and each time it rained. “Talk to Mother Nature,” he says, “and have her hold off on the rain while the project is under construction. We had nine inches of rain during the two weeks we worked on it.”
The final wall is approximately 5,000 square feet and the maximum height is 10 feet, eight inches. Stan Hamilton, president of ReCon Wall Systems of St. Louis Park, MN, notes that the choice of a ReCon wall to replace the failing one was the least invasive from a construction perspective.
Storm Damage in Colorado
In September 2013, floodwaters surged along roads and streams in Boulder County, CO. More than 17 inches of rain fell in four days as a low-pressure system camped out overhead. Heavy rain events are frequent in the region, making up a substantial part of the annual rainfall. But this unusual situation dumped much more rain than expected. The area receives 21 inches in average annual precipitation, so this much rainfall over a few days overwhelmed the entire county. Many roads became dangerous or impassable for travelers heading into the mountains west of Boulder.
The streambank next to Lee Hill Road eroded drastically and threatened the roadway. Heavy trucks were driving the road to deliver materials to other flood-damaged areas, so the road needed to be stabilized immediately.
The city and county of Boulder wanted a fast, permanent solution to the erosion of the road. The project had to be completed quickly so that the road would be ready before the fall rains and the spring runoff from melting snow.
Loris and Associates Inc. was tasked with engineering the wall. Dan Beltzer, a civil engineer with the company, says the biggest challenge for this project was lack of time. “We were working against the clock on this, and we needed a wall solution that we could come in and build as quickly as possible.” The company also had to plan for as little traffic disruption as possible.
The usual solution after severe damage like this is to install an emergency fix and then come up with permanent designs that meet aesthetic standards, which are very important to residents of Boulder County. But this time, the county wanted one repair and a wall that could remain in place for years. The wall had to be durable enough to withstand heavy water flows in the frequent heavy rainstorms.
Some common solutions such as riprap or other wall applications were not durable enough. Horizontal reinforcement was not an option because of the proximity of the road.
The choice was a Redi-Rock gravity wall. The wall could be built to fit the geometry of the bank after it had been scoured out by the floodwaters. “The Redi-Rock gravity wall solution fit very well with leaving that slope in place and building a segmental wall as quickly as possible, outboard of that slope, and then flow-filling in the remaining void area,” says Beltzer.
The units installed were 60-inch blocks, each weighing about 3,200 pounds, and 28-inch blocks, weighing 1,200 pounds. The blocks are wet-cast, precast modular units. The weight of the blocks and the stability of the finished wall provide the strength to carry the load from the road above, even with heavy truck traffic. The finished wall reaches as high as 12 feet.
A variety of textures and colors are available from Redi-Rock. Because the look of the wall was so important to local residents, the manufacturer, Signature Stone of Greeley, CO, created a reddish color for the blocks that would match local rock outcroppings. The Ledgestone texture looks like natural rock. Seth Clark of Signature Stone says, “Aesthetics were important to Boulder County. They wanted a product to look as natural as possible, and Redi-Rock was the next-best option to natural rock.”
A grouted rock curtain wall was built 6 feet under the leveling pad. This wall used small riprap boulders about 8 to 12 inches in size. Larger granite boulders about 30 inches in diameter were grouted and placed in front of the Redi-Rock wall to provide extra scour protection from the stream.
A leveling pad of small stone was placed and compacted. The first course of blocks was installed and aligned on top of that. The next courses were installed quickly by lining up the knobs on the lower blocks with the grooves in the upper row. Drain features and backfill were added.
Yenter Companies Inc. of Arvada, CO, installed the Lee Hill Road retaining wall. The Redi-Rock wall provided the stability needed for this important road and allowed flood restoration to continue higher in the foothills, with heavy trucks passing hourly. The project was finished in just 10 days. Officials in Boulder County were extremely pleased with the look of the retaining wall, as well as with the speedy completion.
17 Mile Rocks Road in Brisbane, Australia, needed an upgrade and realignment. New residential development in the area led to much heavier traffic, and the road was not able to handle the increase. The plan was to widen the road and increase the length of a merge lane at the intersection with Duporth Avenue. The design included a retaining wall at the edge of the road, as well as pedestrian pathways and a bicycle lane.
The original specifications called for a geogrid-reinforced, small segmental block design. Stone Strong dealer Concrib Pty. Ltd. proposed an alternative. By using the Stone Strong Big Block system, the amount of excavation needed was reduced, bringing significant savings in time and money. The headquarters for Stone Strong is in Lincoln, NE.
The wall measures 7,534 square feet and is 16.4 feet high at the maximum. The 24-square-foot Big Blocks are 96 inches long, 36 inches tall, and 44 inches deep. They are precast concrete blocks with a void that is backfilled with clean stone. Each unit weighs about 6,000 pounds. This large size means they are big enough to form a wall without the need for reinforcement. Corner blocks and cap units are also available.
There are a variety of stains for the blocks and four different patterns. For the 17 Mile Rocks Road project, the choice was chiseled granite.
Doval Constructions Pty. Ltd. constructed the wall. The only delays were because of wet weather. Five courses of the blocks were placed on a leveling pad and a foundation row buried about 12 inches.
The addition of the Stone Strong wall enhanced the area and added safety to the realigned intersection. 17 Mile Rocks Road was ready for the increase in traffic.
Containers for Georgia
The Port of Savannah, GA, hosts the largest container terminal in North America. In 2015, the port handled 3.66 million 20-foot equivalent units (TEUs), the standard measure of containerized freight. A TEU is a 20- by 8- by 8-foot container.
Much of the freight that travels through the Port of Savannah transits the Panama Canal. The Canal was recently expanded with wider and deeper locks; this will allow passage of larger ships, known as post-Panamax. They can carry 13,000 TEUs or 260% of what the ships previously carried.
The port needed to be ready for this increase in container traffic, including the roads around the area. Georgia DOT is building a new road, the Jimmy DeLoach Connector, for this purpose. About 8,000 trucks now enter or leave the port daily, causing major traffic congestion. The 3.1-mile connector will hook up two main roads, funneling trucks closer to the port gates. This will relieve congestion on the local roads and shorten travel time for the trucks.
The Jimmy DeLoach Connector design required six new bridges and new interchanges. The design-build contract was awarded to Archer-Western Contractors of Atlanta and its partner, The LPA Group (now Michael Baker International).
The design for five of the bridges called for two tall abutments supported by a mechanically stabilized earth (MSE) retaining wall in front of the bridge seat, which would be supported on piles. Known as a “mixed” MSE abutment, this arrangement means that the MSE wall retains the soil, while the piles support the bridge through the reinforced soil to the foundation.
Timing for the steps of construction was important. Waiting times of 30 days were built into the schedule to allow initial settlement of foundation soils to occur after pouring the bridge abutments. Some of the MSE walls were also undercut 18 inches and backfilled to reduce settlement.
Archer-Western Contractors chose MSE walls engineered by The Reinforced Earth Company of Reston, VA. This type of wall can be constructed quickly with a limited footprint, a great value in this complicated project.
The Reinforced Earth Company has manufacturers around the country. For this project, a precast plant in Newnan, GA, provided the MSE panels. The system includes layers of granular backfill and linear metallic soil reinforcing strips or ladders. The modular precast concrete facing panels are attached to the reinforcing strips. The connectors that tie the facing panels and the reinforcing strips are a combination of a special tie strip embed and a nut, bolt, and washer. The friction of backfill, the strips, and the panels binds the structure together, giving strength, stability, and the ability to distribute the loads evenly.
One of the bridges for the Jimmy DeLoach Connector project crossed over the operating rail lines of the Norfolk Southern Railroad. The bridge was designed to be outside the railroad’s mandatory track clearance envelope. As the retaining wall construction is on the backfill side of the face, work continued without disruption of rail traffic or construction.
The increased traffic through the Panama Canal and into North America through the Port of Savannah will have a shorter path because of the MSE bridge abutments built with products from The Reinforced Earth Company.
Multiple Stakeholders Drive Unique MSE Wall Slope Application
Maintaining the parking lot was a big concern for the Stephen Szabo Salon in McMurray, PA. A Pennsylvania Department of Transportation (PennDOT) project was being constructed around the salon, which had the potential to negatively affect the Szabo business. The building is situated at the corner of Valley Brook Road and Route 19, and customer access was already limited to Route 19. The PennDOT plan called for Valley Brook Road to be widened, a new box culvert to be installed, and modifications to be made to the intersection of Old Washington Road and Valley Brook Road. All of this amounted to significant excavation into the hillside behind the Szabo Salon and substantial loss of salon parking space. Ultimately, a slightly larger parking lot with enhanced traffic flow was proposed. These enhancements were achieved through the installation of a mechanically stabilized earth wall, constructed with Keystone Compac III Victorian segmental retaining wall units.
The project required satisfying requirements from a variety of stakeholders: the salon owner, the neighborhood and municipality, and PennDOT. Original plan proposals from PennDOT indicated a reinforced soil slope was to be installed; however, this limited the parking expansion and traffic flow for the Szabo Salon. Additionally, the municipality and neighborhood had concerns over the look of such a large reinforced slope, reaching heights up to 36 feet. These concerns, combined with the PennDOT standards, drove the decision to pursue a different site solution. The Peters Township Planning Commission agreed to allow a variance for the height of the wall, and the design engineers set out to find a design that would address all concerns and requirements.
Anne Duggan, president of Kevcon Inc. of Cheswick, PA, says her company was approached to provide a potential solution that was not only structurally sound, but also aesthetically pleasing—meeting all of the project’s needs. A solution was proposed to use Keystone units in a configuration that would meet PennDOT reinforced slope criteria, provide a larger parking area with better flow for Szabo Salon, and still allow the road project to proceed as planned.
The Keystone Compac III Victorian segmental retaining wall system, with fiberglass pins, was chosen because of its attractive face design and proven structural performance. Each unit has a chamfered edge on three sides and a natural stone look that adds to the appearance of the wall. Each unit is 8 inches high, by 18 inches wide, by 12 inches deep, and weighs about 72 pounds. The blocks are made of concrete with a minimum compressive strength of 3,000 psi at 28 days, and a maximum absorption of 8%. Using the smaller-sized blocks for the segmental retaining wall, in lieu of a large-block system, proved more cost-effective for this project and allowed for easier installation. The system also allowed for faster, more efficient construction. Bauer Company Inc. of Worthington, PA, manufactured the Keystone units.
During installation, geogrid reinforcement was extended into the soil behind the wall to make an integrated structure that provided the wall system strength. A challenge arose with a catch basin that was situated behind the area for the wall installation. The catch basin had to remain in place. According to Duggan, installers resolved this by wrapping the geogrid around the catch basin, so that a longer geogrid or a harness was not necessary. Drains were placed in the wall to allow the water to leave.
This project was the first segmental wall installation by Gulisek Construction LLC. Duggan says they did a great job. Clayton Stahl, president of Gulisek Construction, says the “easy, repetitive method of construction helped keep production rates steady and on schedule.” He adds that the company held a safety meeting and training for the six to eight workers on the project. The method of construction allowed them to continue work even during the winter months.
Stahl includes some lessons learned for anyone else who is starting a similar project. “Consider access and material staging areas prior to starting the wall. It can be built as fast as you can get the aggregate backfill from your stockpile to point of placement.”
The finished wall system reaches 36 feet in height at its highest point. The wall has five close-set tiers for a total combined length of 1,202 linear feet. The total wall area is 7,434 square feet. There is a 3-foot horizontal setback between walls, to comply with PennDOT specifications of a 1:4 batter for reinforced slopes.
The stakeholders were pleased with the resulting wall. The Szabo Salon owner still had a parking lot, the neighborhood and municipality were happy with the look of the Keystone Compac III Victorian wall, and PennDOT specifications were met.
Points to Consider
The professionals who designed and constructed these projects have advice for anyone who is starting similar projects:
- Know exactly what is required of the retaining wall you are planning. Calculate the load it will carry, the height needed, and any site-specific issues that need to be addressed.
- Decide whether the space is appropriate for reinforcing with geogrid or not. Also determine the size of blocks that will work best.
- Consider the space needed for access, storing aggregate supply, and staging vehicles.
- The most important steps are building a good base, compacting backfill sufficiently, and providing good drainage behind the wall. Remember that water is the worst enemy of a retaining wall.
- Plan for weather. Construction during winter or rainy times can be more complicated, and possible delays should be worked into a schedule.
- Follow manufacturers’ installation guidelines carefully.
- One of the advantages of today’s retaining wall products is the aesthetic appeal. Use that to the upmost.
Retaining wall projects can be tremendously satisfying. You may be able to drive around your area of the country and see walls you built that are preserving space or supporting bridges and roads, and know that your project adds to the strength and integrity of those structures—and looks good, too.