Shored Reinforced Earth Walls Take on Challenging Terrain in North Carolina
Shored Reinforced Earth Walls Take on Challenging Terrain in North Carolina
In western North Carolina, in the mountainous terrain of the Pisgah National Forest, few roads are straight, fewer are level, and the combination of straight and level is almost non-existent. Highways thread their way through meandering valleys or along folded and twisting hillsides, defining the shortest distance between towns. Some roads, like NC Route 194, just cannot avoid the tortuous curves and terrifying switchbacks imposed by an unforgiving topography, sometimes seeming to trace the alignment of a piece of ribbon candy turned on edge. Though in places barely two lanes wide and with narrow or nonexistent shoulders, NC 194 is the most direct route to travel between Banner Elk, NC in Avery County and Valle Crucis in adjacent Watauga County. Due to its narrow and curving nature, however, by the end of the 2000s decade, NC 194 was in serious need of an upgrade and The Reinforced Earth Company (RECo) was there to help.
The NCDOT's reconstruction goal for NC 194 was safety + widening, but the topographic constraints, both vertical and horizontal, severely limited their design options. Even the barely achievable 3-4 ft. widening they planned, to improve turning radii for larger vehicles and provide some semblance of a shoulder, required retaining walls up to 46 ft. tall and significant cuts and fills to construct them. Normally NCDOT specifies mechanically stabilized earth (MSE) walls when retaining walls are required, but the standard 0.7H aspect ratio (soil reinforcement length ≈ 0.7 x wall height) would not work here without massive and environmentally destructive up- and down-slope excavations. Clearly this was a job for Shored Mechanically Stabilized Earth (SMSE) walls.
A SMSE wall is a combination of a shoring wall (often a soil nail wall) supporting the backslope and an MSE wall constructed against that backslope (Fig. 1). Typically the upper two (or more, if allowed by the slope geometry) layers of MSE reinforcements extend across the top of the shoring wall to integrate the two systems. This composite wall system allows cost-effective and visually appealing MSE walls to be used at sites severely constrained by slopes so steep that a 0.7H-wide platform on which to build the MSE wall is not possible. While each wall type comes from a different designer/supplier, the composite SMSE wall works for one simple reason: the shoring wall supports the soil behind the MSE wall.
An MSE wall is designed as a coherent gravity structure, resisting lateral earth pressure because of its mass and resulting sliding and overturning resistance. Since a shoring wall nearly eliminates that lateral earth pressure, the cross-sectional width of the MSE structure required for gravity stability, defined by the length of the reinforcements, can be significantly reduced. Instead of the normal 0.7H reinforcement length, the Reinforced Earth® component of SMSE walls typically has an aspect ratio of 0.4H to 0.5H. But with two different wall types from two different suppliers, design coordination and careful checking are critical.
The owner has ultimate responsibility for global and compound stability checks and must assure that these checks are performed. Analysis of compound stability through the soil nail and MSE walls requires input from the designers of both wall systems, while those designers must each be responsible for the internal stability of their respective walls. When the MSE wall is founded on a slope, the toe-of-wall embedment depth (Fig. 1) is proportional to the steepness of the slope and must be clearly specified by the owner. And to assure the SMSE wall will perform as intended, all design responsibilities discussed above must be clearly and thoroughly defined in the contract documents.
The Reinforced Earth Company (RECo) has been building SMSE composite retaining walls since before they had a formal name, but a name originated with publication of the Federal Highway Administration's Shored Mechanically Stabilized Earth (SMSE) Wall Systems Design Guidelines (Publication No. FHWA-CFL/TD-06-001) in 2006. The FHWA noted that, in the 2 decades preceding their report, MSE walls had "prov[en] to be reliable, constructible, and cost effective". The addition of shoring technology meant the functionality and value of Reinforced Earth (MSE) walls could be applied to a wider range of projects.
In steep terrain, to widen a road such as NC 194 on the "outside" or downhill side (Fig. 1), it is necessary to excavate a flat bench, below the existing roadway, on which to construct the MSE wall. As mentioned above, the typical bench width is 0.7H, where H is the height of the wall from the leveling pad (6 in. thick unreinforced concrete, cast directly on the foundation grade) to the top of the wall. If the road must remain in service, as was true with NC 194, then the use of shoring maximizes the steepness of the excavation backslope and minimizes undercutting of the roadway. By designing the shoring as a permanent – rather than temporary – structure, money is saved and the shoring becomes an integral part of the total earth retention system.
For this SMSE project, NCDOT specified the combination of a soil nail wall and a vertical-faced MSE wall. The general contractor was Maymead, Inc. of Mountain City, TN, Schnabel Foundation Company (Sterling, VA) was the soil nail designer/installer, and the MSE wall erection subcontractor, Charles Blalock and Sons (Sevierville, TN) selected RECo to supply the MSE walls. Prior to wall construction, the following 5-step design process, mandated by NCDOT, was followed:
Obtain field survey from contractor, develop cross sections.
Determine preliminary MSE wall strip lengths.
Provide preliminary layout and cross sections to soil nail wall designer.
Coordinate submittals with contractor (2 separate design packages, one MSE and one soil nail).
NCDOT reviews soil nail wall submittal and MSE wall submittal and checks global stability.
With Maymead, Blalock, Schnabel and RECo all having a role in the design, NCDOT's review and coordination role was critical to assuring the specified requirements were met and the walls were built successfully.
While Reinforced Earth MSE walls are constructed from the bottom up, soil nail walls are installed in layers from the top down, beginning with a steep but shallow excavation. The excavated surface is covered by wire mesh and shotcreted after installation of each row of nails. Figure 2 shows the drill rig installing a soil nail, with the rows of completed nails and their shotcrete-covered bearing plates clearly visible in the rows above. Once this soil nail wall was completed along NC 194, Reinforced Earth wall installation was routine and produced the neighbor-pleasing results seen in Figure 3.
For projects in steep terrain, where there is no room to excavate a flat bench for wall construction, a shored Reinforced Earth MSE wall may be the answer. RECo has multiple technical approaches to SMSE geometries and can provide design solutions for clear distances between MSE wall facing and face of excavation ranging from as little as 1 ft. to 0.7H or more. But every SMSE wall presents a new design challenge and needs thorough specifications and careful 3-way coordination among the owner’s engineers, the MSE wall designer and shoring wall designer. The NC 194 project was successful because the appropriate stability checks and the needed coordination were provided, giving NCDOT the confidence to specify SMSE walls on additional mountainous terrain projects in the future.