Geosynthetic reinforced soil piers: a bridge from the past to the present.
The brainchild of research geologist Michael Adams, the prestrained GRS pier is offered as a low-cost, quick-construction alternative to the elaborate, high-priced reinforced concrete bridge piers that constitute the support structures for most of the developed world's bridges. But the technology itself, Adams is quick to note, predates even those aforementioned Indian mounds by several thousand years.
"I have been told that this technology was first used in ancient Mesopotamia," says Adams. "The only difference is the Mesopotamians used reeds to reinforce the soil" rather than the plastic sheets or metal strips common in today's GRS applications.
But despite GRS's 5,000-year headstart, it has been slow to catch on with contemporary engineers who remain wedded to more conventional concrete pier construction technology. "Some of the bridge piers being built today employ 1940s technology at its finest," says Adams, noting that comparable structures employing GRS can be constructed quickly. Comparing the cost is difficult because it depends on the location and the size of the bridge.
Adams says GRS has been slow to catch on because users don't understand the technology. In addition, many engineers are dismissive of such overtly simple technologies, especially those that predate the Roman Empire. But Adams points out that the simplicity of GRS is one of the technology's strongest suits. Construction of a GRS pier requires very little specialized equipment; is comprised of common road base; takes only a week or two to construct, depending on design loads and applications; and is extremely durable.
"There's no snake oil here," says Adams. "It's cheap, durable, and environmentally benign." And if that's not all, it can probably weather many of the earthquakes that topple today's bridge piers.
A Cookbook Approach to Bridge Support
The test pier at TFHRC is constructed of alternating, 0.2-meter-(8-inch-) thick layers of compacted road base and sheets of woven polypropylene fabric. Vertical steel rods, which are required only if there is a need to prestrain, jut from the top of the pier. Split-face cinder blocks are stacked around the pier as an architectural containment system. Each course of blocks acts to contain each lift of fill. The facing blocks function more as an aesthetic facade than as load-carrying elements.
The GRS pier was constructed on a reinforced soil foundation (RSF). The design of the RSF was based on research at TFHRC. The RSF was also constructed with compacted road base and a geosynthetic reinforcement. The pier was centered on the RSF to ensure uniform settlement.
Prior to the pier's record-breaking test on July 25, 1996, Adams and his colleagues prestrained the structure using hydraulic jacks. The jacks were used to squeeze the soil mass between concrete pads on the top and bottom of the GRS pier. The jacks and concrete pads were all bolted together with the vertical steel rods.
The team loaded the pier several times, and, aside from some tension cracks in the facing blocks, the pier stood tall with very little lateral or vertical deformation. It was the first time a GRS structure was load tested to such an amazing amount - a remarkable achievement for Adams and his colleagues.
The pier was capable of supporting even more load, but the test was terminated because of the cracks in the facing system. It must be noted that the cracks occurred well beyond any intended service load that might be placed on a pier in actual use.
Its incredible load-bearing capabilities aside, there are a number of significant reasons today's engineers should take a second look at GRS piers, says Adams, who makes no secret of his passion for the technology or his conviction that it is the next big revolution in bridge construction. In five minutes, Adams ticks off a half-dozen reasons to choose GRS over more conventional technologies.
"GRS is ideal for quick replacement situations," he says, "because you don't need a lot of special equipment or specialized labor to build the thing. It's a real cookbook approach - a generic design." With a common back hoe, a small crane, a hopper bucket, some garden tools, and a vibratory plate tamper (hydraulic loading equipment is also needed if prestraining is required), a team can have a GRS pier up in relatively short order.
The cookbook philosophy isn't just limited to the tools or the cooks, either. Using common road gravel, a suitable geosynthetic reinforcement, a conventional facing system (segmental wall or cinder blocks), and, if necessary, prestraining rods and pads, all of the ingredients for a GRS pier are readily available, inexpensive, and easy to work with.
A prestrained GRS pier or abutment offers additional benefits. Prestraining serves to consolidate the fill material - "tensions the reinforcement" in the parlance of Mike Adams. It proof-tests the structure, and it limits post-construction settlement of the reinforced soil composite. It may reduce the differential settlement problem and cost common to many bridge abutments in the United States. This is appreciated by anyone who drives onto a bridge deck and feels the telltale bump identifying the demarcation from where the approach embankment has settled differently from the bridge abutment. Perching the abutment on a reinforced soil embankment should offer a smoother transition between the embankment and the bridge deck.
GRS: A Solution to the Big One?
GRS is also very durable, says Adams. In fact, current research indicates geosynthetics could last thousands of years, a talent for which GRS's originators, the Mesopotamians, had few peers.
But perhaps GRS's most intriguing promise lies in its potential ability to withstand earthquakes. If recent earthquakes have told us anything, it's that current bridge construction technologies - even those touted as "earthquake-resistant" in such temblor-conscious places as Northridge, Calif., and Kobe, Japan - are not doing the job. But GRS has many properties that lead Adams and his colleagues to believe that the technology can withstand serious seismic activity that more conventional piers cannot.
"By its nature, GRS is much more ductile than concrete and, therefore, very energy-absorbent," says Adams. "It's not as rigid as concrete and can absorb ground vibrations." A significant problem with concrete is its propensity for transferring lateral loads to a bridge's superstructure. The geosynthetic soil composite, on the other hand, may act as a base isolation system, which may protect the bridge better than a traditional system. Including vertical prestraining rods offers additional protection because the rods restrain lateral swing during quake motions.
But proof, as the cooks (and engineers) say, is in the pudding. GRS's ability to foil earthquakes probably won't be proven until it is used more frequently in earthquake zones (or anywhere else, for that matter). Although GRS technology is used today for road support, approach embankments, earth retainment walls, and the rare bridge abutment, it remains for the most part relegated to test piers like the one at TFHRC. But if Adams and other GRS proponents have their way, GRS will see a lot more action in the years to come. Outdated design guidelines for GRS are being revised by the Federal Highway Administration. These new guidelines should do much to facilitate the use of GRS in highway construction.
To be sure, GRS isn't ideal for every bridge-building assignment. For example, GRS piers won't work well in "scour zones," which are sites near rivers or other locations where the pier can be undermined.
But GRS should be considered an alternative to a deep foundation design. GRS piers and abutments can serve as quick, temporary bridge supports and can be used in remote locations that are inaccessible to heavy and specialized equipment or concrete. Also, the GRS support system is well-suited for a low-volume road built by a local or county highway department with a limited budget and without much equipment.
Aside from such exceptions, the many promises inherent in GRS technology may indeed bear out Mike Adams' belief that it will be the next great revolution in bridge engineering - a revolution 5,000 years in the making.
Michael Adams will make a keynote presentation about the results of his research on GRS at the International Symposium on Mechanically Stabilized Backfill in Denver, Colo., Feb. 6 to 8, 1997.
Doug Rekenthaler is a freelance writer-editor. His experiences as a writer-editor include cub reporter covering Capitol Hill and Pentagon news beats; managing editor responsible for 12 newsletters covering a wide array of communications technologies; founder of the multimedia industry's first daily fax news service; and corporate communications manager for America Online In'c., the largest commercial on-line service in the world.
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|Date:||Jan 1, 1997|
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