Lessons in large retaining walls


    Large, engineered retaining walls are a staple of many civil engineering projects today. Mechanically stabilized earth (MSE) walls can use metal strips or geosynthetics such as geogrids for reinforcing soil backfill; large-block gravity walls without soil reinforcement may be appropriate where space is restricted and wall heights and earth pressures are not excessive. The following project summaries highlight recent use of these three engineered solutions.

    Triangle Expressway, Durham County, N.C.
    The Triangle Parkway, a 3.4-mile stretch of new roadway from I-40 at NC-147 in Durham County, N.C., is a design-build project being constructed by ST Wooten Construction, with RK&K as the lead engineering consultant. Phase one opened to traffic on Dec. 8, 2011. Once Phase Two (The Western Wake Freeway) of the 18.8-mile project is completed, portions of 147 and I-540 will become the first toll road in North Carolina. The tolling will be all electronic, with no toll booths, and will allow for toll collection with electronic transponders or license plate photo-recognition.

    The project requires five new bridge structures, four of which consist of Reinforced Earth MSE abutments and wingwalls. There is also one stand-alone MSE retaining wall on the project. The Reinforced Earth Company (RECo) provided design and supply of the MSE walls on the project, including custom corner bent panels, pilaster panels, and precast parapet units. RECo also provided wire face MSE wall materials for use in construction phasing of one of the bridges.

    Project aesthetic requirements include brick facing, MSE wall pilasters, and corner bent panels that mimic the pier columns. In addition, road names are imprinted in the wingwall parapets. To allow for a brick veneer on the MSE wall facing, RECo provided plain MSE panels with dovetail slots. Once the settlement period for the wall was complete, full-size brick was laid up and connected to the MSE facing with dovetail anchors. Since MSE walls are flexible and built on top of soil, the final brick facing also needed to provide some flexibility. To that end, vertical joints were placed in the brick facing at 10- to 20-foot (+/-) intervals, aligned with the facing panel joints, and then covered with backer rod and tinted sealant. Under-drains and weep-holes were provided to allow for any water to drain away from the wall.

    To achieve the aesthetic requirements for the top of wing-walls, RECo developed a custom 4-foot, 8-inch parapet unit to provide sufficient space for roadway lettering. Since the parapet unit effectively retained nearly 4 feet of fill, reinforcing strips were attached to the back to provide for stability.

    To match the 24-inch vertical pilaster coursing specified for the bridge pier columns, RECo designed special 6-foot-tall MSE pilaster panels and corner pilasters. The corners were fabricated in an adjustable form to allow for a range of acute and obtuse corner angles.

    In addition to the architectural challenges of accommodating brick fascia, pilasters, and the parapets, there were several geotechnical challenges on the project. For the Hopson Road Bridge, the existing foundation soils consisted of weathered and sound rock. To minimize rock removal, RECo utilized the recommendations of Terre Armee International (RECo’s international sister company) and the Federal Highway Administration (FHWA-NHI-10-024, section 6.3) to allow for benched excavation into the existing foundation materials and variable reinforcing strip lengths. This method of construction may be used when the MSE wall foundation material consists of rock or other stiff/dense soils with STP values greater than 50. The placement of reinforcing strips in a stepped configuration results in an overall MSE volume with a resisting mass similar to one built with reinforcing strips of the conventional 70 percent of the wall height. The internal stability of the MSE wall at the reduced reinforcing strip location is achieved by adding reinforcing strips (where necessary) to satisfy pullout and rupture strength requirements.

    In addition to accommodating the variable reinforcing strip length requirements at the Hopson Road Bridge, RECo designed for potential lateral loads induced by the integral abutment piling. Reinforced Earth is an ideal construction material for retaining walls at pile-supported abutments because the bolted connection allows for reinforcing strips to be skewed at abutment piling locations.

    Cherry Precast of Winston-Salem, N.C., fabricated all of the precast concrete MSE wall materials. Bonn-J Construction installed the MSE wall facing panels, pilasters, corner pilasters, and parapet units, as well as the wire face MSE walls for phasing of the NC-54 bridge structure. Hunt Valley Construction provided the stain work for the walls and bridge structures.
    Information provided by The Reinforced Earth Company

    Sea-Tac International Airport parking, Seattle
    Seattle’s Sea-Tac International Airport is the 18th busiest airport in the country, and the need for convenient parking is a growing concern. To help provide needed parking, one of the area’s leading parking services, MasterPark Inc., sought to turn unusable land near the airport into a valuable $20-per-night parking lot. Five retaining walls as tall as 33 feet reinforced with geogrid were needed to build up the site and create a level and stable area for the parking lot.

    Goodfellow Bros. of Maple Valley, Wash., the general contractor, and Sound Retaining Walls of Tacoma, Wash., selected Strata Systems to provide the geogrid reinforcement for the complex array of walls. The retaining wall design specified Stratagrid with embedment depths up to 26 feet. Goodfellow Bros. had to excavate 35-foot-tall back-cuts and haul the soil offsite to allow for reinforcement and construction of the walls. Then, as Sound Retaining Walls constructed the segmental walls, the soil was brought back to the site for reinforced backfill. Segmental retaining wall construction, backfill, and compaction of the reinforced fill are always a coordinated effort. Since the backfill was stored offsite, both contractors had to make concerted efforts to coordinate the construction of the walls to optimize construction efficiency.

    A total of 18,000 square yards of Stratagrid SG200, SG500, and SG600 was installed behind the 19,000 square feet of segmental retaining walls. The installation was completed by the contractor without any special equipment, saving time and money.

    Five MSE segmental retaining walls as tall as 33 feet and reinforced with Strata Systems’ Stratagrid geogrid were used to build up a site for a parking lot near Seattle’s Sea-Tac International Airport. Photo: Strata Systems

    This site was once considered unusable because of the dramatic grade change and the cost of development was not feasible. Today, with the economy of utilizing soil reinforcement technology, this site is now a valuable asset to MasterPark.
    Information provided by Strata Systems


    A 19.5-foot tall, 700-foot-long, large-block gravity Redi-Rock retaining wall eliminated the need for excavation that would have removed trees that buffered neighbors from a new road. A Ledgestone texture and color and wall-top fence match the urban design context of the area. Photo: Redi-Rock

    Ronald Reagan Drive, Cincinnati
    The Federal Bureau of Investigation (FBI) is relocating its division headquarters from downtown Cincinnati to a high-profile site adjacent to I-71 and planned mixed-use development in Sycamore Township, Ohio. To accommodate the anticipated number of employees and visitors to the site for the new FBI building and other planned development, a new public access drive was designed from Hosbrook Road to the development sites.The new roadway (called Ronald Reagan Drive) is located between existing residential neighborhoods and a major office building. The terrain along the new access road included some significant topographic challenges. To minimize earthwork and impacts to the existing wooded buffer between the adjacent residential neighborhoods and the development, it was decided to construct a 19.5-foot-high, 700-foot-long retaining wall along the south side of the new roadway.After evaluating the costs and impacts of a variety of wall systems including conventional cast-in-place cantilever walls, drilled soldier pile walls, and conventional segmental walls with geogrid reinforcing, the design/specifying engineer, URS, contacted Redi-Rock to explore the potential use of its large-block retaining wall system. Following close coordination between URS, Redi-Rock, and the township, it was determined that the Redi-Rock system could meet the goals of the project better than the other systems.”Sycamore Township chose Redi-Rock because we had a limited amount of space; we did not want to cut back behind the wall for tie-ins and we didn’t want to lose trees that were buffering the neighbors from the project,” explained Tracy Kellums, superintendent for Sycamore Township.Dave Wormald, P.E., of URS Corp. explained further: “One of the primary reasons we chose the Redi-Rock system was because we wanted to design a gravity wall to reduce the amount of excavation required with minimal disruption behind the wall as well as creating an aesthetically pleasing appearance.”To reach the required 19.5-foot height with a gravity wall, URS utilized several unique components of the Redi-Rock system to achieve a custom solution for the project without the need for geogrid reinforcement or anchors. URS used the Redi-Rock propriety analysis software to evaluate a variety of wall cross sections to arrive at the hybrid design for the finished wall.First, URS specified 3,420-pound Redi-Rock 60-inch base blocks for the bottom six to eight courses of the wall. Just one course was buried. Then, URS specified several courses of 9-inch setback blocks throughout the wall to vary the batter. “We incorporated the 9-inch setback blocks to increase the resistance to overturning forces without the need for geogrid reinforcement,” Wormald said.For the higher courses in the wall where the loading was less, the blocks transitioned to 41-inch Series blocks and 28-inch Series blocks to achieve the full height of the wall. Compacted, free-draining aggregate backfill and leveling pads were used. Combining a variety of Redi-Rock blocks allowed the wall to achieve its goal height while minimizing excavation and impacts to the trees. Plus, the gravity solution saved the township time and money on installation.An additional benefit was the aesthetics the system offered. In the last several years, the township had erected several conventional concrete retaining walls faced with stone veneer. These walls were located within a half mile of the site, and the township wanted the walls on the access road to match the appearance of the other smaller walls in the area. Ornamental fence that matches fencing used by the township for nearby streetscapes was mounted at the top of the wall to reinforce the urban design context of the area.In total, the 8,000-square-foot wall was installed over the course of 25 days, but many of those days were rain delays, so it actually took many fewer days to install. Redi-Rock blocks install quickly like giant Lego blocks using a piece of heavy machinery and a small crew.

    Information provided by Redi-Rock