Affordable, versatile, and easy to install
BY JOHN BIONDO AND STEVEN KNAPP
Wire-formed walls might not be considered as aesthetically pleasing as some retaining systems, but they offer a cost-effective alternative for engineering a grade separation or pressurerelief wall on a budget. With new facing options and generally a significant price advantage, these "soft," mechanically stabilized earth (MSE) systems are an attractive alternative to cast-in-place walls.
Wire-formed retaining walls, such as the SierraScape System from Tensar Earth Technologies, have been in commercial use for about a decade, long enough to build a substantial track record of performance in a wide range of applications. The system is essentially an extension of the geosynthetic- reinforced slope technology introduced in the early 1980s. As the name suggests, it features a facing unit made out of galvanized or non-galvanized wire that sometimes is connected to a geosyntheticreinforcement layer (typically UV-stabilized geogrids).
However, the facing unit is not a gabion, which is a retaining system that has been in use for centuries. Although both systems can incorporate wire at the face, the two systems’ similarities end here.
While a modern gabion is a wire basket, a wire-formed wall includes an L-shaped facing component that is open at the back.
Also, a gabion wall typically relies on gravity to resist lateral loads, while a wireformed wall, on the other hand, gains its strength and stability from the geosynthetic material installed in the associated MSE structure.
The two systems also vary somewhat in appearance. While both can incorporate stone at the face, only a wire-formed wall with a polypropylene geotextile filter or a turf reinforcement mat at the facing can be filled with on-site material and naturally vegetated to blend with its surroundings.
This feature is advantageous when a structure abuts a stream or wetland, or where the structure is being used as part of an environmental restoration effort.
Wire-formed walls as tall as 50 feet often are considered an alternative to cast-inplace foundation structures on commercial and residential sites, landslide repairs, or road-widening projects. They can offer cost savings resulting from the system’s weight and the simple installation process.
A typical wire-formed system, for exam ple, consists of welded wire mesh facing units, polypropylene and high-density polyethylene (HDPE) reinforcement geogrids, and a connection element for tying everything together. Better quality components are made of materials that are not affected by hydrolysis, chemical degradation, UV degradation, or stray current.
The components are ideal for harsh environments such as transportation or site-development projects. They also are light enough to be hand-carried or moved using the type of compact equipment most contractors already own. This portability offers a significant advantage on projects where site access is limited or complicated by topography.
Unlike cast-in-place structures, wireformed walls do not require complex forms, experienced labor, or heavy equipment for installation. Unskilled laborers can grasp quickly the fundamentals of installation and become proficient workers with just a few hours of on-the-job experience.
This simplicity is one reason that more segmental retaining wall (SRW) contractors are adding wire-formed walls to their service portfolios.
In many ways, installation is not that different from building an SRW. After establishing a level base course of gravel, construction basically consists of stacking the facing units, unrolling and pinning the geogrid reinforcement, and connecting the components together. Often, contractors can use general embankment fill or on-site soils to backfill the wall structure, and workers can use hand-operated equipment to compact the fill. Hand-operated compaction equipment normally is recommended within 3 feet of the face to avoid facial-alignment issues. The ability to recycle on-site materials in many cases yields significant cost savings for fill materials and disposal costs.
Some SRWs and all cast-in-place walls have a slight edge when it comes to face slope. Nevertheless, wire-formed walls allow facing angles of 70 to 90 degrees.
This near-vertical face keeps wall footprints smaller and increases the amount of usable land behind the structure.
Installers and geotechnical engineers also are fond of the system’s design flexibility.
Wire-formed walls can accommodate radiuses and inside or outside corners with little modification. Additionally, they can bridge existing structures, such as culverts, rock formations, and utilities, without burdening the schedule, budget, or technical skills of the installation crew, which improves productivity and reduces costs.
Productivity is further enhanced by the system’s broad tolerance of adverse weather conditions. With wire-formed walls, construction can continue during conditions that would halt most other projects. A good crew can install hundreds of square feet of wallface per day, even under lessthan- ideal conditions.
In general, wire-formed walls accommodate most soil types and design challenges, but they are well suited particularly to sites with exceptionally soft soils. Unlike "hard" retaining wall systems, wire-formed walls accommodate differential settlement conditions quite readily without failing. This is one reason that the system is attractive in regions where seismic loading and earthquake movement are concerns.
The system’s flexibility means that, unlike concrete, its wall face won’t crack or potentially lose structural strength. Long term maintenance, therefore, is usually less of a concern.
Wire-formed walls also are proving to be a cost-effective alternative to the types of large, cast-in-place foundations traditionally used to support soil and building loads.
When used for pressure relief, the system enables geotechnical and structural engineers to address below-grade, soil-separation concerns with less cost and complexity.
The system is appropriate for rehabilitation work or new construction. In either case, wire-formed walls can be used to eliminate lateral soils loads. It offers commercial developers an option for building below grade with less cost, while achieving results that are equivalent structurally to conventional methods. In most cases, a relatively inexpensive masonry wall can be substituted for a massively reinforced structure.
To better estimate how a wall will perform on a site, it always is a good idea to consult an expert long before construction begins. A geotechnical engineer who understands MSE technologies, for example, can play an invaluable role in defining options and guiding the decision-making process. It also is advisable to consult a system’s manufacturer.
Most companies have extensive inhouse experience evaluating site conditions and designing appropriate MSE structures. Their field representatives can be helpful in picking the most appropriate system for a specific site.
Wire-formed walls offer a variety of facings, from the natural look of stone or aggregate, to greener, vegetated and bioengineered options. This flexibility enables a wall designer to address the appearance preferences of project owners, architects, land-use officials, and other interested parties who may have strong opinions about the structure’s aesthetics.
A vegetated wall is particularly effective for an environmentally sensitive area. As plantings on the wall begin to mature, the structure increasingly will blend into the surrounding scenery. And, unlike timber structures, a wire-formed wall does not contain chemicals that could be toxic to aquatic and riparian creatures such as frogs, turtles, and salamanders.
John Biondo is the Northeast manager, Segmental Retaining Walls for Tensar Earth Technologies, Inc. He can be contacted at firstname.lastname@example.org. Steven Knapp is a freelance marketing writer. He can be contacted at email@example.com.
Wire-formed wall versus cast-in-place
Design concerns: A project for British Columbia’s Ministry of Transportation involved widening a 1-kilometer roadway section from two to four lanes. The original design called for constructing the new lanes as much as 6 meters below the existing lanes.
Solution: To reduce costs and simplify construction, a value-engineered design solution was proposed and accepted.Wire-formed walls (and geogrid-reinforced MSE retaining walls with precast concrete panel facings) provided an earthquake-tolerant system that enabled the project contractor to construct the new lanes at the same elevation as the existing lanes.
Results: The installed walls saved more than $1 million compared with the original project design.
Wire-formed wall versus gabion
Design concerns: A Wal-Mart site in Knoxville, Tenn., needed more usable land to turn an existing store into a Supercenter and to accommodate a larger parking lot. The original design called for constructing a 45-foot-tall gabion wall to protect nearby wetlands.
Solution: A wire-formed wall was value-engineered to reduce site costs and to improve aesthetics.
The structure was built with a 70-degree facing angle to accommodate the parking lot expansion.
Results: The installed wall provided an estimated savings of 60 percent compared with the anticipated cost of the original design. Equally important, the system was praised for the preservation and aesthetic value it brought to an ecologically challenging project.
Wire-formed wall versus counterfort
Design concerns: A last-minute tenant change provided an opportunity to add a multiplex theater to a development under construction in Millbury,Mass. A cast-in-place wall was considered for the theater’s rear wall but was ruled out because of expense.
Solution: A wire-formed wall was designed to provide grade separation and pressure-relief.
The 34-foot-tall wall was erected approximately 3 feet behind the theater. It was strong enough to retain the lateral earth, as well as the building loads, created by a single-story retail strip located at the top of the wall structure.
Results: The structure enabled the owner to build the theater’s rear wall using conventional masonry block—a much more affordable solution.