Sizing Up the Right Wall
Ease of working with materials plays into wall designers’ and contractors’ decisions.
For retaining wall contractors, the long-term goal of a solid retaining wall that will not fail is the primary consideration, but just as important is how the material will hold up to challenges inherent in constructing it.
Weather, tight spaces, poor-quality soils, suitable backfill, and time and budget constraints are among the challenges contractors must address.
High Pointe Commons
A retaining wall project involving High Pointe Commons in Harrisburg, PA, entailed six different walls of more than 24,000 square feet, with the largest wall more than 18 feet high and 710 feet long.
The commercial development of shops, restaurants, and a hotel meant the project was going to be a large endeavor; even so, the elements to be considered were the same as on any project—wall height, length of geogrids, slopes, surcharges and soil conditions, and coordination among utilities, storm pipes, manholes, and traffic plans. To that end, Keller Engineers was able to use a wall-design checklist from Allan Block to ensure all elements were taken into account.
|Photo: STAY-RIGHT PRECAST CONCRETE
A Stone Strong Systems Wall in North Carolina
|Photo: KEYSYONE RETAINING WALL SYSTEMS
A Keystone wall near a Utah rail line
“The challenge with a lot of large projects like that is getting the job done in the allotted amount of time,” says David Kirkpatrick, owner and chief executive officer of K&M Dri-Lay Masonry, who served as part of the project team. “Allan Block is a very user-friendly product from the installer’s point of view. There are no pins to contend with. The installation is sped up versus other block systems.” Kirkpatrick notes that some of the walls on the job site were hundreds of feet long. Using the Allan Block units, “it tends to be a little more forgiving as far as any movement from front to back, keeping the walls straight and looking good. It helps us make a wall look better, dealing with walls that long versus other blocks, the way they’re designed and made as far as the face of the block is concerned.”
One of the retaining walls incorporated 24,500 Allan Block Classic units. The wall has a nominal setback of 6 degrees and a flat top with a heavy traffic surcharge above and a 2:1 slope below. To address slope stability, the wall was designed with a 5-foot bench to provide a deeper rock base and a greater slip arc to resist undermining. Eighteen feet of the 23-foot wall are exposed; the rest of the wall is buried.
In the High Pointe Commons project, most walls had either paved surfaces or slopes above them. The Allan Block design software enabled the engineer to model the wall section with elongated top grids under all surcharge conditions. The flexibility allows the design to match as close to as-built conditions as possible. Lengthening the top grids when paved surfaces are above also minimizes the potential of future cracking in the retaining wall.
Another design consideration is viewing reinforced soil mass and retained soil mass as separate structures working independently. Allan Block points out that a defined construction joint remains because the reinforced soil mass—formed by the Allan Block facing, the infill soil, and the geogrid working together as a single unit—is built with a tighter control on compaction. In contrast, retained soil is typically unreinforced, recompacted site soil built with fewer controls. In bridging the construction joint between the soil zones with extended geogrid layers, such variations as settlement are disrupted and minimize the potential of future cracking of the paved surface.
Wall contractor Dri-Lay Masonry was advised to build the wall and backfill in one-course increments, achieving maximum lifts of 8 inches for proper compaction and verifying with installers what level of compaction was required. High Point Commons’ site was large enough to allow use of onsite soils, the compaction of which could be tested by the engineer.
Kirkpatrick says one of the biggest challenges on the project entailed excessive amounts of rain. “We had floods on three occasions, when construction practices during the project were done to keep the water away from the walls,” he says.
Kirkpatrick also notes that Allan Block has a “very good draining system. When we had some of these large rain events, the water was channeled away from the wall, but when it does get to the wall, you want to make sure the water has a way out.
“The water that did get to the wall was able to get down through the drainage system, out through the face of the wall, and into the collection pipes that are installed. We had no issues with it whatsoever. The biggest problem we had with this project was dealing with the wet soils and excessive rainfall.”
The project taught his company a lot about productivity and planning ahead of time for rain events during construction, he says.
|Photo: INNOVATIVE COMPANIES
A commuter rail line had to pass over Union Pacific tracks, requiring a flyover wall structure.
Dealing with compaction issues and working in compliance with railroad rules and regulations were just a few of the challenges that workers faced two years ago when they began construction of two commuter rail lines in Utah that feature a combined 305,000 square feet of retaining walls.
The project team included SLC Commuter Rail Constructors as the design-build contractor, Innovative Excavation as the wall contractor, Parsons and HDR Engineering as the civil engineers, and Terracon as the geotechnical engineer.
The team helped the Utah Transit Authority (UTA) build two commuter rail lines—the FrontRunner North Commuter Rail Line, which runs from Salt Lake City north to Ogden, and the FrontRunner South Commuter Rail Line, which runs from Salt Lake City south 45 miles to Provo. Construction of the North line involved building 5,000 square feet of retaining walls, and the South line 300,000 square feet of retaining walls.
The commuter line had to go over Union Pacific Railroad tracks because of property restraints. The flyover wall structure is a 45-foot-high, 48-foot-wide, 3,000-foot-long approach wall for the Union Pacific Railroad crossing, says John Taylor, general manager of the retaining wall division of Innovation Excavation.
The FrontRunner South project involved single-phase slip joint construction using locally manufactured Keystone Retaining Wall Systems products, high-productivity installation, wick drain installation, and lightweight backfill, all designed for low maintenance.
Initially, UTA was entertaining several two-stage systems, Taylor says. That would have involved large flyover walls over the Union Pacific Railroad track in two-stage construction to alleviate foundation settlement concerns.
Once excavation was completed, a flexible wire mesh mechanically stabilized earth (MSE) wall would have been installed and allowed to settle. The second stage would have involved installing a permanent concrete veneer, panels, block, or other product of UTA’s choosing in front of the wire mesh wall.
The design and construction of the Keystone wall eliminated that two-step process, minimizing costs and staging time.
“We teamed up with Keystone, knowing we could probably do this with just a single-stage wall and save them some money,” says Taylor.
The combined efforts of design, engineering, lightweight backfill, and Keystone modular wall units meant a single-stage wall could be built, eliminating the wire mesh product and installation as well as the settlement time period.
Once excavation was completed, the Keystone walls were quickly installed. Taylor indicates that the productivity was better than expected.
“The flyover section was very unique,“ he says. “Over the existing Union Pacific tracks, we had to construct a structure of 45 feet in the air with the Keystone walls back to back. We had our geogrid laying one over the other as we built the wall up. It’s a very unique design to change the lateral pressure on the bottom of the wall.”
|Photo: STAY-RIGHT PRECAST CONCRETE
This apartment complex property needed 25-foot-high walls with 40,000 square feet of block.
The backfill adds stabilization without weight. The most common backfill is the more readily available dirt or granular fill. The general contractor and engineers believed such standard fill options would have been too much of a load at the bottom of the wall, so the project team came up with the idea of using a lava-like rock material that weighs half as much as dirt or granular fill.
After overcoming some initial compaction issues, the team figured out how to get a consistent rock size and shape, creating a backfill that would be easier to handle than dirt because of its weight, would deliver better load-bearing properties, would be less expensive to acquire and transport, and would allow construction to continue during winter as dirt retains moisture, clumps, and freezes in cold weather.
“It went really well through the winters,’ says Taylor. “In other parts of the project, we had to deal with rain, snow, and wet soil. There was some extra work to keep that soil dry and covered up.”
Key to the success of the project was the installation of slip joints every 50 feet in the taller sections of the flyover walls, permitting the wall to flex in smaller sections to minimize the settlement issue.
Because large retaining walls and poor foundation soils can lead to settlement issues, the project team designed and installed a wick drain system under the walls. Excavators drilled 80 feet into the ground at predetermined intervals, feeding 6-inch-diameter wicking material into the holes to accelerate clay soil drainage from the large fill surcharges. Wicking water out of the ground faster and draining it away helps accelerate settlement and reduces long-term settlement duration prior to use.
“They had predicted the whole thing was going to settle roughly 12 to 18 inches, and to date, it has settled only about 4 inches with the design and the way it was constructed,” Taylor says.
Another challenge to overcome was the fact that the commuter rail line runs over the Union Pacific Railroad track. During construction, workers had to comply with the Union Pacific’s rules and regulations in addition to UTA’s rules regarding safety, noise, and work hours, occasionally having to wait until trains passed to resume work.
Because the Keystone Compac Straight Face Units were manufactured and supplied by Amcor Masonry Products in North Salt Lake City, which also provided the Tensar geogrid soil reinforcement, there was a savings in freight costs.
“It was a great project,” Taylor notes. “When I see the aerial photographs now that it’s all in place, it’s one of those deals where you stand back and you’re amazed that you’ve been involved in it.”
North Carolina Apartments
For a sprawling 20-acre apartment complex in Wakefield Hills, NC, large blocks from Stone Strong Systems were used to provide a retaining wall. The project required 100,000 cubic yards of cut and fill. The wall was up to 25 feet high, and 40,000 square feet of Stone Strong block was used in the fill sections, says Rod Hoffman, a business development manager for Stay-Right Precast Concrete who also oversees construction of Stone Strong Systems walls.
|Photo: TMS CONSTRUCTION
A road-widening project cut into a homeowner’s yard.
“The fill areas were so large as far as the depth, the width, and the height of the wall that, as we progressed, what made the difference in using Stone Strong was the fact that we could just place the dirt behind it, put the gravel in the block, and then continue up,” notes Hoffman.
Only the bottom part of the wall needed geogrid, he says. Additionally, the same machine used for grading and the cut and fill was used to place the wall. “It allowed us to bring the whole job up evenly, section by section, instead of building an entire wall,” says Hoffman.
In contrast, he says, “When a small block wall gets set up, you do one small section, and you need specific fill material—something with more stability to it. We were able to use the dirt that was on site. All of this made a huge difference for us. It helped the job progress.”
Another benefit was that as the workers neared the top of the wall, the last 9 feet did not require geogrid fabric. “That means you have the top 9 feet where you can drill fence posts in, put in utilities like electric, gas, telephone,” says Hoffman. “You use trenchers and don’t have to worry about cutting that top layer of fabric, which is all that holds the small block wall in place.”
Hoffman says drainage was not an issue on the project. “On a small segmental block wall, you have to be very careful because [a large rain event] can wash the block out. They’re very small and they can be moved by a small amount of water. With these huge blocks of over 8 feet long and 3 feet deep, there’s no way the water was going to move them, so there was never a concern,” Hoffman says. “Luckily, with several years on the project now, we can say it’s never failed, never had trouble. Some of the challenges in the grading and utility business were taken away by using a large block wall.”
The apartment complex borders wetlands. “The area needed to be protected, so it was very important that there was no chance the wall could fail. With the heavy rains like we’re having now, you have to be real careful about wall or slope failure,” Hoffman says.
“Stone Strong does its job,” he says. “There’s no chance somebody is going to cut it with a trencher and cause it to fail later. You can bring in the block in large quantities easily in a day. You can set thousands of square feet of wall because the blocks come in 24-square-foot sections and you just stack them like building blocks. It’s filled with stone, and the stone in the middle drains the water away from the wall to make sure it doesn’t build up behind the wall. With a large cavity in the middle, it can move a huge volume of water instead of it building up in the wall.”
Customizing Color in Issaquah
At another apartment complex in Issaquah, WA, Lock + Load Retaining Walls’ steel- and fiber-reinforced concrete walls were used to help create a wall that in some parts reached 40 feet high.
The 17-building apartment complex is part of a larger development known as Issaquah Highlands.
Part of the mass grading of the project involved construction of retaining walls. Lock + Load was selected for the wall facing. The actual wall construction was MSE, incorporating the Lock + Load panels.
The Lock + Load panels needed to meet specific architectural requirements for the Issaquah Highlands, which has an architectural committee that reviews all of the construction that takes place, says Ray Coglas, principal with Earth Solutions NW. “They need to meet certain criteria for aesthetics,” he says. “For this project, the Lock + Load panels were fabricated specifically with a dye that gave them kind of a Southwestern tan color, which isn’t typical but can be done.”
The wall also incorporated a geogrid reinforcement and considerable drainage elements, says Coglas, whose company provided the engineering, observation testing, and oversight during the construction. The wall was completed in October 2010.
“It was a fairly tall wall in some places, but we still approached it the same as any MSE wall in evaluating internal and external stability, and evaluating the underlying soil characteristics,” says Coglas. “In this case, the only element that could affect the wall performance was primarily groundwater conditions in upgrading of the wall area, but that was mitigated through a fairly substantial drainage system we put in as part of the wall construction.”
|Photo: TMS CONSTRUCTION
The ReCon gravity wall system required
no geogrid reinforcement.
Weather was another consideration, as the project was moving into early autumn.
“We had to schedule around rain events and plan accordingly as we got to the later stages of construction,” he says. “The initial wall construction was done in late August and weather was pretty good, but toward the end we accelerated the construction so that we could beat the main part of the wet-weather season.” He adds, “During the winter, we were able to confirm our drainage systems were working as planned, and we were glad that we took the steps that we did.”
Anchoring the Slopes
John Easom, territory manager for the state of Delaware and the Eastern shore for ACF Environmental, says his company is frequently using MSE walls in which a wire-formed basket, wrapped with geotextile and embedded in the soil, gives the wall its form. Soil anchors are used for a safety factor on the walls.
“We use a lot of Presto Products’ Geoweb Cellular Confinement for slope protection and wall applications with Manta Ray earth anchors,” says Easom. “I’ve done several of them in the state of Delaware where the soils were eroding and needed to be stabilized.”
Using equipment from Foresight Products, which provides the earth anchors, the anchors are load-tested to specific engineering requirements, says Easom.
“The anchor will be holding some sort of linear reinforcement like a pipe that we wrap the geotextile around and then build the wall,” he says. “Typically, geotextile grids of a certain length have been used between layers, packed into the soils, and then the soils on top of them give the support so you won’t have a tip-over or a wall fail. The anchors allow us to build walls in more confined spaces.”
Project teams need a vibratory hammer for the installation, Easom notes.
“The biggest market for these is sea walls,” he says. “All of those guys have a pile driver to drive the pilings for their sheet walls and the vibratory hammer for the anchors. Some contractors use a standard 90-pound jackhammer and air compressor, which Easom says can be difficult “because you can’t get enough pressure behind it. I try to use either a small skid loader with a chipping hammer on it or an excavator with a vibratory plate on it.”
Maintenance is not of concern with the anchors, Easom says. While most clients tend to take care of the retaining walls, nonetheless, “once the anchor’s attached to the base of the wall, then you’re good to go for a lifetime,” he says.
The learning curve in using the Manta Ray anchors is relatively quick, notes Easom. “I can have a contractor’s crew trained in one work day to do the anchors. Sometimes it’s two or three days to get the anchors driven, depending on the number of anchors, and some questions may come up along the way for some tricky situations—for example, if the load testing is not what they want—so I always stay around to make sure we get them through those times.”
As for working with various soils, Foresight offers a list of soils and anchors applicable to each soil type, Easom says.
“If you get into loose, sandy soils like I have in the downstate shore regions, then I have to use a wire-based anchor, which is typically a Manta Ray or Stingray,” says Easom. “In the clay soils, like in the northern part of the state, we use a Manta Ray 1, MR-1, which is the 7-inch-based standard anchor. Sometimes we use smaller Duckbill anchors if we get into rocky soil, but they have many different styles of their anchor and a complete chart on their website to tell you which soils or what load capacity that anchor will work in.”
Easom says sometimes soil will need to be imported for suitable backfill, “but most of the time when you’re building these walls, it’s because the soils are poor and they’ve failed,” he says. “Sometimes they have to be dug out, the anchors driven in, and then good soils are imported in to finish the project.”
Using Manta Ray anchors is less expensive than other choices because there is less labor involved, Easom says. “Manta Ray anchors are typically only used in situations where nothing else will work,” he says. “The beauty of them is you don’t have to excavate a lot of soils behind where you’re working. You’re driving from the face of the soils that you’re going to build the wall around and you don’t have to do that excavation in the back, so they’re usually their benefit is cost savings in allowing you to do a job that you typically couldn’t do any other way.”
As for severe weather events, such as when Hurricane Irene was threatening the East Coast in late August, Easom says while some parts of the retaining wall may be undermined, not so with the anchors.
“Once an anchor’s in the ground, it’s in there for life,” he says. “When there’s a difficult situation and you need a good product to prevent tip-over or failure of a wall, anchors are the way to go.”
John Mercuri owns Mercuri Landscape Contractors in Innisfil, ON. Among his company’s many services is installing engineered segmental retaining walls. Mercuri favors Siena Stone from Unilock, which manufactures paving stones for various purposes. “We also have engineered smaller units reinforced with geogrid,” Mercuri says.
Every job presents it own challenges, he notes. “A few years ago, we ran into some concrete hidden in the sub-base and found an existing wall with concrete footing. The wall was 100 meters long. We ended up having to break up 100 meters by three meters of footing, which required having to get down into the soil to be able to re-engineer the base back up again. Then there were also water challenges. It was on the side of the bottom of an embankment. We ran into some groundwater issues; we were breaking up the existing footings and re-engineering the base while pumping water out of the area.” The job, conducted for a commercial client, required an additional two weeks and had cost overruns.
Tight working conditions are a frequent a challenge, Mercuri finds. “We try to address it with the smallest, yet biggest possible, machine we can use,” he says. “If we have access from the top of the wall, then we’ll mobilize on top of it and swing things down. Or, if we have to maintain access on the bottom side while we’re working, we try to fit the right equipment to the job. There are solutions out there.”
Mercuri says he uses Siena Stone because of Unilock’s engineering and design capabilities. “They’re very user friendly,” he says. “If we have a project that comes through our channels, we can ask them to design something for that application, and they’re really good about that. We tend to stick to that particular brand for that reason.”
Mercuri came upon one job where another manufacturer’s wall had been specified, but Mercuri believed it wouldn’t be an appropriate choice.
“There were limitations as far as setbacks and how much space we could occupy at the top of the wall by the time it’s set back,” he says. “The wall was built on a two or three percent setback right from the beginning, so it was almost tilted. It was easy to bring in the right product and show the client how to go in the right direction.”
Making It Look Good
Two years ago, a road-widening project in Minnetonka, MN, required a cut into the adjacent yard of a homeowner. Shady Oak Road was being widened, and building a needed retaining wall with geogrids would have cut even further into the yard.
Driving sheet piles to attach retaining wall block was initially considered, but bedrock was encountered when the piles were driven, and sufficient embedment was not achievable without major excavation of the bedrock.
The solution was a ReCon gravity wall system with no geogrid reinforcement. It also provided an aesthetic function in that its rustic texture matched the look the city was trying to achieve and also matched other sections of the project where cast-in-place liners were being used.
TMS Construction served as the wall contractor.
Company owner Todd Schmidt says there were some challenges with respect to the steepness of the project and the sandy soils. That, in combination with the desire for a wall appearance that fit with the surroundings, is what led the county engineers to their decision. The ReConwall reached a maximum height of 10 feet, 8 inches.
Shannon Schultz of Brennan Construction, a general contracting and construction management company in Pocatello, ID, turns to Boulderscape when his company’s clients need a wall that performs well and also looks good. Such was the case in a recent job, where a retaining wall measuring 800 feet long and about 25 feet high was built for Portneuf Medical Center in Pocatello.
The working area was tight; the wall was behind a parking garage that allowed only about 6 feet of working space.
“We had to do a vertical cutoff of the soils and we had no way of supporting the wall, so we bored into the existing soils, grout-pumped into the bore, and used a wire mesh. It was all anchored together to support the shotcrete that was sprayed on the wall,” Schultz says. “They wanted a decorative finish over the top of that, and that’s where Boulderscape came in.”
Schultz worked with Boulderscape to build scaffolding to provide the finishing touches.
“They did a perfect job,” he says. “I was very impressed with their professionalism. I had two new construction projects with them there, one of them last year and the other one three years ago.”
Making Way for a Stadium
In Papillion, NE, the Omaha Royals’ AAA baseball team needed a new stadium to replace its longtime stadium that closed in downtown Omaha in September 2010.
Sarpy County broke ground on the new $26 million, 6,000-seat stadium in 2009. The budget included $20 million for the stadium and $6 million for infrastructure for a family entertainment district, including hotels, shopping, restaurants, and recreational activities.
An access road for the 30-acre development crossed through wetlands, requiring special environmental consideration. Engineers initially considered grading a gradual slope from the roadway down into the wetlands, but instead opted for a design using architectural culverts and headwalls.
Dan Thiele, president of Thiele Geotech, says the objective was to minimize the footprint through the wetlands. The solution: hybrid gravity/reinforced headwalls.
Rather than creating a 3:1 slope and destroying or disturbing all of the wetland, the vertical headwall helped save a substantial amount of wetland, Thiele notes.
A total of three culverts required six headwalls, which abut the culverts and stand approximately 30 feet from the centerline of the roadway on either side.
The project’s civil engineering firm and the contractor each had previously worked with large precast modular wall systems, favoring the blocks’ mass and stability in headwall applications. Redi-Rock’s Ledgestone was chosen for the project based on cost competitiveness, and while aesthetics wasn’t the primary consideration, it led those on the project team to affirm that choice.
Unlike large-block precast systems of the past, the Redi-Rock Ledgestone is designed with a texture that offers a natural, blended-in look for the headwalls. Although they look different, Ledgestone blocks have the same dimensions and design capabilities as other Redi-Rock blocks.
The design of the headwalls was a hybrid of reinforced and gravity structures. The civil engineering firm of Lamp, Rynearson & Associates Inc. required the top 7.5 to 9 feet of the headwall be a gravity structure. With the headwalls standing 28.5 feet high, the bottom portion of the headwalls needed to be reinforced. Thiele says creating a geogrid-free zone in the upper part of the walls helped prevent potential conflicts with utilities that would be installed under the roadway.
Thiele designed the bottom portion of the headwalls using Redi-Rock 28-inch reinforced blocks. Before constructing the walls, Linhart Construction installed leveling pads of compacted stone to ensure the walls’ stability. The crew installed 12 courses of Redi-Rock blocks, or 18-foot-tall walls reinforced in every course with geogrid extending 18 to 20 feet behind the walls.
Following Thiele’s specifications, the crew backfilled, using sand, and compacted after each course of block and geogrid were installed. The remaining six to seven courses used Redi-Rock 41-inch gravity blocks to allow the top portion of walls to be built without geogrid.
The headwalls feature both inside and outside curves built with standard Redi-Rock blocks. No cutting was required.
The headwall portion of the project was completed in about a month. The second phase included Redi-Rock Ledgestone columns and freestanding walls as well as black metal fences atop the headwalls.
Defending Against the Lake
A homeowner outside of Cleveland, OH, was in danger of losing his home because the cliff face—20 feet away from the home—was starting to erode. The top 8 to 10 feet of the cliff is all topsoil, with solid clay 5 feet beneath that and shale deeper down.
Greg Norton, owner of NCS Construction Services, says that while the softer shale flakes off, it gets harder as it gets deeper. The lake below eats away at the base of the cliff, but the groundwater at the top does most of the destruction, starting first with the soil, then washing away the clay, followed by large pieces of rock.
With little room for geogrid, the project engineer opted for anchor bolts to secure the geogrid to the bluff. With a 2.5-inch drill bit, holes were drilled 6 to 7 feet into the cliff face, with anchor bolts cemented into the borings. A steel bar was attached to the anchor bolts and the geogrid attached to the steel bar to secure the wall to the cliff face. The existing seawall was used as the footing.
The concrete wall extends upward for 17 feet, and an 18-inch cap on top rests atop a ledge cut from the cliff face. A Versa-Lok wall rests on top of the cap. After the footing was completed, a series of three decks was built that jutted out from the cliff and was connected to a pier on the water by zig-zagging stairways. The retaining wall was built behind the deck and stairway structure.
The Versa-Lok wall, measuring 65 feet wide and 27 feet tall, begins at 13 feet above the water level. It was built in three tiers with a slight curve to follow the bluff face.
The nature of the bluff face was such that the lower two-thirds was very steep, nearly vertical, notes Chris Andrassy, the civil engineer. “The rock transitions to soil at the top, so it starts to fall back,” he says. “That allowed us to bench into the existing face, so each segment has its own footing.”
Some 3,000 Versa-Lok standard units in a blended pattern of 80% brown and 20% gray were used. A special conveyer belt system lowered the blocks down the cliff face one by one while a chute alongside the conveyer transported the backfill aggregate.
Norton says Versa-Lok’s flexible slot-and-hold pinning system was “perfect” for the application and is also aesthetically pleasing.
Carol Brzozowski specializes in topics related to stormwater and technology.
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