Retaining Walls to Overcome Site Limitations
Pushing the boundaries of stability
Occasionally, a project comes along that pushes beyond the boundary of a typical installation. Several recent projects do just that—an amusement park pedestrian bridge, an elevated entrance drive, and coastal bluff stabilization. All involve components intended to contain subgrade materials and improve the human landscape in aesthetic ways. Each project has unusual situational challenges based on project owner design preferences and site limitations.
Building a Legacy
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Designing an appropriate retaining wall entrance for a breathtakingly grand, castle-themed events center can be a challenging task. It must not only meet structural requirements but also must match the architectural aesthetic of its setting. Nicholas Wong and Titan Engineers PC thoughtfully guided the Badanco Holding company, owner of The Legacy Castle in Pompton Plains, NJ, through the conceptual design, appearance, and selection of retaining wall materials. “The owner ultimately chose ReCon Wall Systems as the best structural and aesthetic fit to match the theme of the development,” says Wong, adding that the product met the requirements for both visual appearance and installation practicality.
Before embarking on full-scale design, Titan conducted a feasibility review of the ReCon product to ensure it could meet basic demands. “We verified the product and its structural system would work as intended based on planned loading and site conditions,” explains Wong. “We understood at the beginning that there would be design challenges because of the retaining wall configuration and the developer’s desired vision.” The retaining wall forms the structural framework for the arching entrance drive that ramps up a full story height to the castle’s grand porte-cochère.
The curved approach, gradual elevation, mixed battered and zero-battered faces, and accommodation for both vehicular and pedestrian traffic in a confined space presented complex design scenarios that led to extensive construction plans to clearly explain all the customized nuances of the 10,000-square-foot modular system. “At Titan, we draw the profiles in 1:1 scale so an installer can look at the plans and see what he needs to build in an easily understandable scale,” says Wong. In addition to the basic ReCon retaining wall block, three other block types were called out in the plan set: guardrail block, column block, and capstones. “Some wall portions were gravity; other portions were geogrid. In some places, the wall meets the building structure, so there we needed to make sure the wall was nice and straight,” he says.
“People tend to think of [modular block] as Legos that only fit in certain directions and ways, but we had to customize the modular system to fit the developer’s needs.” The complex design meant a considerable amount of collaboration and communication was needed at the beginning between the civil engineer, the developer, the manufacturer, ReCon, and Titan. “The typical ReCon block form wouldn’t facilitate the desired wall curvature, so the fabricator had to modify his forms to accommodate this particular project,” explains Wong, referring to Northeast Concrete Products. “The fabricator was deeply involved from the beginning of the project.”
Additional fabrication adjustments made possible the installation of light pole columns and protective barriers, for both vehicles and pedestrians, along the top of the retaining wall. Because of the elevated installation, three different barriers were needed along the top surface to prevent any vehicular accidents from spilling over the wall edge and for pedestrian fall protection. With such constricted space, the exterior barriers had to be installed directly on top of the retaining wall, versus traditional installation offset to the interior.
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Since most modular systems dry stack, the fabricator added holes in vehicular barrier block faces to permit installation of grouted rebar to tie barrier blocks together and anchor them to the upper retaining wall courses. “In a vehicle impact, you want to spread as much of the load as possible,” says Wong. “We had to modify ReCon’s guardrail barrier system for this particular project. In this case, we had to reinforce the top of the wall to accommodate impact forces and work very closely with the manufacturer to align all these openings.”
Both exterior walls also support regularly spaced columns to mimic castle battlement architecture, and they house a light on every third column for entrance illumination.
Credit: RECON WALLS
The Legacy Castle in Pompton Plains, NJ
“The main electrical conduit ran underneath the roadway, but conduits were also cast into the column blocks to feed electricity up into the lights,” says Wong.
With so many different block types and customizations, communication became increasingly critical to ensure the block manufacturer—and later the retaining wall installer, Mike Fitzpatrick & Son Inc.—knew what they were bidding. Because of the high level of planning, design communication, and detailed explanation through Northeast Concrete’s shop drawings and the bidding process, no unexpected situations popped up during installation. “We communicated the entire time during the design and construction phases. There were no surprises because of how well everything was planned at the beginning.”
Although the installer implemented some block adjustments onsite, like additional barrier block drilling for rebar reinforcement, crafting slices of wall veneer to fit curvature gaps, and mitering capstones, most blocks came from Northeast Concrete ready to fit. This meant an easier, faster installation. “The attractiveness of modular retaining wall systems is the reduction of onsite construction labor,” explains Wong. “We kept that approach. Much of the design elements were incorporated at the plant so the installer wouldn’t have problems getting this project in place.”
Wong has observed modular gravity wall systems becoming more popular on sites capable of supporting them. “I think it’s because of the reduction in field labor and equipment required to install these walls. The materials may be more substantial and expensive, but the savings from labor costs can far outweigh that, and they’re flexible enough to accommodate differential settlement and still be both stable and attractive.”
Bridging the Gap
On the Gulf coast of Alabama resides one of the region’s newest entertainment attractions, the Park at OWA. Situated within a 500-acre planned sport, retail, and entertainment complex, the Park boasts a 14-acre artificial lake. One vehicular and five pedestrian bridges connect all corners of the property to a central island, where an amphitheater and additional park facilities are located. One bridge, a focal point of the Park, provides visitors with an aesthetically heightened approach thanks to its triple arches, arching profile, and decorative features.
“The owner wanted something that was very pleasing to the eye,” says Rob Adamson, regional sales engineer with Contech Engineered Solutions. “We considered a form liner and other options to create the decorative headwalls, but they preferred the Keystone Retaining Wall Systems look.” More specifically, the client chose Keystone’s Stonegate Country Manor system with a weathered Old World finish as the final wall material. The system offers three block sizes that can be installed randomly for a less regimented appearance. “Plus, the system had more flexibility to ramp up and over the bridge, and changes could be made more quickly and easily with Keystone than with a precast wall system,” says Adamson.
He says Keystone’s in-house engineering staff provides project owners with an assurance of success, especially if the project has an atypical design. “Any engineer can draw something on paper, but it may not be realistic to build,” he says. “There are other block manufacturers that may make a similar block for slightly less, but when you add the value of the in-house engineering expertise and years of experience designing complicated wall systems, that is what typically sets Keystone apart from the competition.”
For wall heights of not more than 3 feet, Stonegate can be gravity based. At 12 feet wide, 160 feet long, and 17 feet high at its tallest, the arched OWA bridge required layering of backfill and reinforcing geogrid to internally tie together the two outer walls. “When you place the layers of geogrid reinforcement within the backfill zone, the backfill locks into the openings in the geogrid and provides friction and stability,” explains Adamson. “When the weight of the backfill tries to push the wall over, the geogrid counteracts that force and holds the wall up.” The Keystone system uses alignment pins inserted into multiple pre-drilled holes in the top and bottom of neighboring blocks to lock them together. The weatherproof pins make exact placement of the blocks much easier, and each block has multiple pre-drilled hole options to allow for both vertical or battered configurations.
Placing the edge of the geogrid between the blocks and inserting the pins through the geogrid as well mechanically links all components. Geogrid layers were inserted between every other block course, or every 12 inches, and overlapped the geogrid layer extending from the opposite wall. The vertical placement of layers alternated between the two sides so that subgrade could be placed between each. This results in a total of 34 geogrid layers at the bridge’s tallest point.
“A lot of coordination happened between Keystone’s designers and the OWA project engineers to make sure that the look the owner desired could be easily built in the field,” says Adamson, referring to the structural integrity and location of utilities within the top backfill layers. All utilities for the island’s facilities—water, sanitary sewer, and 10 4-inch-diameter conduits for future routing of telecommunication, electrical, and other wiring—delicately cross the OWA lake via the bridge subgrade. “The Keystone designer and the project engineer coordinated to ensure the utilities were set at certain elevations so they didn’t conflict with the geogrids,” says Adamson.
The site’s coastal plain geography and its typical mix of soft sandy and organic soils meant the entire structure, including its three arches, required concrete pilings to ensure adequate support. Although the bridge will primarily serve pedestrians, the bridge had to be capable of accommodating vehicles including park administration vehicles, facilities maintenance vehicles, and possibly emergency vehicles including fire trucks. “Most of the soils in the bridge locations tended to be loose sands with a fair amount of clay content, so these bridges had to be supported by concrete pilings,” says Adamson, referring to the eight pillars that form the structural foundation of the bridge. A 10-inch reinforced concrete piling forms the interior core, with Stonegate blocks wrapped around its exterior to form the finished 30-inch column. Light posts installed on the top of some pillars add nighttime illumination for bridge users.
Fitting the Keystone blocks around the arches involved customizing each adjacent block by using wet saws onsite to cut a standard block down to the appropriate size, angle, or shape. In roughly four months from December 2016 through March 2017, the site was prepared, the foundation poured, and the entire bridge constructed. The overall project schedule was extremely fast. The bridge design was completed up front, but other project changes that affected the bridge had to be adjusted during the construction phase.
Adamson refers to this back-and-forth, almost design-build coordination as the biggest project challenge and attributes the project’s success to good communication, even with the periodic changes. He and Tod Green, a bridge consultant and fellow professional engineer with Contech, partnered on project coordination with Keystone designers, OWA project engineers, and McInnis Construction. “There was constant coordination and really good communication between all parties to make sure changes didn’t affect everyone, and if they did, all the designers made the updates and changes on their end.”
Photos: Park at OWA complex in Alabama
The project wasn’t without a few minor weather-induced hiccups. Roughly one hour southeast of Mobile, and 20 minutes from the coast, the project site happens to be in a historically rainy region. “The Mobile area is one of the wettest places in the country, and we got plenty of rain that winter,” says Adamson. “It caused delays and a few project washouts requiring mid-construction repairs and reinstallations.” Weather aside, Adamson notes the amount of construction as a minor challenge. “There were several contractors working simultaneously around the bridge site, each trying to get their portion completed on schedule. Good coordination and communication was a necessity.”
Slip Sliding Away
A US Geological Survey study completed last year on Southern California beaches indicates that more than 30% will be completely eroded down to bedrock by the year 2100 via a sea level rise of 1 to 2 meters. California’s geology and geographic position make its shores a prime location for beach bluffs, which help maintain adjacent beaches through sediment contribution and sand replenishment, but as people have been drawn to build homes and businesses along the ocean, these many bluffs have nowhere to retreat without impacting human interests. In Pismo Beach, CA, Danny Cohen and J.C. Baldwin Construction Co. recently tackled a handful of bluff stabilization projects including a city property, two hotels, and a private residence all within 10 miles of each other.
“The geography, topography, and geology combine to make the conditions more conducive to bluff erosion that can affect structures,” explains Cohen, also indicating that the more significant recent storm events the region has experienced seem to be accelerating erosion. “Sometimes the material at the base erodes through wave action, undermining the slope that continues to slough off and cave. Stronger storm events with higher and stronger wave action lead to greater erosion.”
Cohen and J.C. Baldwin specialize in geotechnical applications including tieback and soil nail bluff retention systems intended to protect the bluff surface from wave action and prevent further weathering. Tiebacks typically involve drilling a 6- to 8-inch-diameter hole, from 50 to 150 feet deep, horizontally into the bluff face, then inserting a bundle of flexible steel cables anchored by grout into the hole and tying the bundles to a shotcrete surface applied 1 foot to several feet thick on the bluff face. A soil nail application uses a similar installation process and components but replaces the bundled tieback cables with a single piece of inflexible steel bar.
“The number of tiebacks or soil nails is based on the proposed wall height, required loading, bluff structure, and its soil type,” says Cohen. The bluff soils and conditions also dictate the drilling depth—weaker, less structured material typically requires deeper installation. The recent Pismo Beach projects all utilized tiebacks. Cohen explains that the decision to use tiebacks versus soil nails can be dictated by design calculations—tiebacks can typically support greater loads—or by the design engineer’s preference.
To be completely successful, bluff protection must extend deep and high enough to provide an adequate range of protection. “It has to be high enough to protect against potential waves and be deep enough to prevent potential scour,” says Cohen. “Not always, but typically, you can find a bedrock material to embed the bottom of the wall into to prevent scour.”
PHOTOS: J.C. BALDWIN CONSTRUCTION CO.
Tieback and soil nail systems have become more common in the area as previous protection methods using boulder and riprap revetments at the base of the bluff have become less favorable and have begun failing more frequently. A stack of 2- to 8-ton boulders would often be strategically placed, sometimes in conjunction with a geotextile fabric, along the toe. “You’d have to stack different sizes to fit a certain way so there’s as little voids as possible, locking it all in,” explains Cohen. Revetments can be stacked only so high and can make beach access difficult, and even the largest boulders movable by equipment can be relocated by wave action. “A big wave can move a couple tons of rock very easily. They used to be fairly common, but are now considered an emergency repair when something needs to be done quickly.”
Although bluff retention systems can also impede beach access, if considered during the design process, access can be maintained or added by building it into the structure. “The coastal commission doesn’t like bluff walls because they want them to erode and replenish the beaches, but if you’re going to put in a system, they like that system to have public access,” he says.
J.C. Baldwin frequently works with Boulderscape of San Juan Capistrano, CA, to provide architectural finishing of the bluff retention systems. By adding color amendments and textures to match the surrounding bluff environment, the new stabilization can blend into the landscape. Like environmental artists, Boulderscape can mimic geologic formations by using custom trowels, texture mats, and specialty stains. “Typically, the structural tieback or soil nail system is installed and the architectural section is added to it later,” explains Cohen. “The surface is intended to be mainly cosmetic, but it does form part of the wall, so it adds thickness and some stability—it’s roughly 80% cosmetic and 20% stability—so it’s included in the overall stability calculations.”
One of the Pismo Beach hotel projects required installation of a scaffolding system, roughly 40 feet high and 100 feet long, above an existing boulder revetment to access the bluff face and drill 130 tieback holes between a series of existing 3-foot-diameter concrete piers embedded in the face. “When trying to auger a 65-foot, 6-inch-diameter hole horizontally into the bluff, you don’t always get a perfectly straight line,” says Cohen. While installing the 15,000-square-foot architecturally sculpted concrete, his crew did intercept one pier, but after adjusting the auger angle, successfully completed that particular hole. They also created a new beach access, which added miles to the public California Coastal Trail.
Cohen and his crew follow construction plans provided and developed by an independent design engineer. The plans indicate the location and angles—horizontal and vertical—of installation for each tieback. “Usually the walls follow the contours of the bluff, so there are curves and the tiebacks can sometimes intersect each other,” says Cohen. By making small adjustments to the horizontal and/or vertical angles at which a tieback enters the bluff, crews can avoid conflicts. “When you have 45 or 50 tiebacks holding up a wall, moving one a few inches or adjusting an angle a few degrees typically doesn’t make much difference,” he explains.
Aside from bluff stabilization, J.C. Baldwin installs tieback and soil nail retention systems for a variety of other applications, including residential and commercial lot expansion, landslide stabilization, and spaces too tight for conventional block retaining walls. In one particularly tight situation, the company installed a system roughly 3 feet from an existing house wall. To get equipment in and have space to auger, they had to gut the wall contents and drill between the wall studs and framing members.
Cohen admits that retention systems aren’t suitable everywhere. “They don’t work immediately along property lines because the tieback or soil nail extends into the adjacent property. Also, there are limitations to what concrete can support—as the wall gets thicker, tiebacks get longer, and at some point, it becomes economically unfeasible.” J.C. Baldwin has installed retention systems up to 50 feet in height, and although higher systems are possible, this is roughly the maximum height for the wall to remain economically feasible. Cohen recommends working with an engineering firm experienced in these systems to ensure proper design. “We’ve had a few outrageous designs come in because they didn’t fully understand the concepts and construction issues, and overdesigned because they weren’t certain about a few things.”
Thorough, detailed communication seems to be the common theme for smooth retaining wall and slope stabilization. At The Legacy Castle, Wong claims “the key to project success was the upfront, initial communication between each group.” As a result, he says, he wouldn’t change anything about the entire process. Similarly, weather conditions are the only change Adamson would have made at the Park at OWA. Solid communication can be tedious and time-consuming, especially in incredibly complex design situations, but upfront efforts can yield huge results when it comes to quality installation, staying on budget, and client satisfaction.