Geosynthetic Reinforced Soil Structures Can Carry The Load
Four years ago, Federal Highway Administration (FHWA) research geologist Michael Adams successfully smashed the world’s record for load capacity and applied pressure on a geosynthetic reinforced soil (GRS) structure with the 5.5-meter- (18-foot-) tall bridge pier that he designed on the grounds of the Turner-Fairbank Highway Research Center (TFHRC) in McLean, Va.
Since then, Adams has been busy making GRS converts across the nation. He has been involved in several
major experiments in Colorado. Subsequently, in Las Vegas, he conducted another experiment in which he broke his own record for the applied pressure of a GRS structure. Adams and his FHWA colleagues have also added to their project at TFHRC, constructing two full-scale abutments/embankments to complement their bridge pier. Adams’ next stop is Amherst, Mass., where he and a crew plan to build another four piers this summer.
At TFHRC four years ago, the bridge pier designed by Adams was loaded to 9,800 kilonewtons (2.2 million pounds force). The pressure applied was 930 kilopascals (135 pounds per square inch).
In Las Vegas this past winter, Adams’ experiment — dubbed the Vegas Mini-Pier — was loaded to an equivalent stress of 1000 kilopascals (145 pounds per square inch). The vertical and lateral strain to the Vegas Mini-Pier at that pressure was about three percent. Adams has designed other GRS structures with vertical and lateral strain as low as one-half of one percent.
"We've learned that it works, it looks nice, and it’s easy," Adams said with a chuckle. Pointing toward the full-scale abutments at TFHRC, which were constructed in attractive earth-tone colors and patterns, Adams asked rhetorically, "Would you rather look at that or at a concrete wall?"
Then, turning a bit more serious, he noted that the performance of the GRS structures is very predictable.
"They’re very predictable if you build ’em like I build ’em," he said. "You can calculate how much the wall will displace vertically and laterally for a given applied pressure. We always knew [that the structures were sound and reliable], but we’ve confirmed it."
Yet, Adams’ pride in the performance of his GRS structures is tempered with disappointment. Not in the technology, the workmanship, or the results, but in the fact that the highway engineering industry has not exactly rushed to embrace the use of GRS for earth retention or load-bearing applications.
GRS for bridge support requires only free-draining road-base soil that packs well and geosynthetic reinforcement. Many such features can be constructed very quickly — from a few days to a couple of weeks. As for the geosynthetic reinforcement, it is simply a series of polymer sheets or grids or metal strips. It doesn’t much matter; the reinforcement simply needs to be durable.
Adams loves to talk about a 1996 experiment done by the Colorado Transportation Institute in which ordinary bedsheets were successfully used as soil reinforcement to support an impressive stack of concrete Jersey barriers.
"You don’t need any specialized equipment or material," said Adams. "In the Colorado experiment, they got their reinforcement at JCPenney’s! We are finding out that it’s the spacing of the reinforcement that controls performance — not necessarily the strength."
A Simple, Effective Technology
To Adams, the simplicity of GRS is its beauty. He loves to point out that GRS has been in use in one form or
another for thousands of years, dating back to ancient Mesopotamia. It irks him that not everyone is as excited about GRS as he is. "The way I see it, there’s a reluctance to advance the state of practice in the highway industry," Adams said, shaking his head. "It’s a darn shame that the civil engineering industry doesn't adopt [a greater number of] different technologies. It’s confusing to me, but there’s not much incentive to adopt change in the highway industry."
Adams concedes that many people might not understand how GRS can possibly work, especially with the load and pressure he has applied to his structures. So he explains the process:
- Start on a level surface.
- Lay a base structure of common, but good quality, cinder blocks.
- Dump fill into the base structure.
- Compact the fill.
- Lay a sheet of reinforcement.
- Continue alternating layers of block, compacted fill, and reinforcement. Referring to the bridge pier at TFHRC, Adams noted, "Everybody who looks at it thinks it’s externally supported. But it's internally supported. The reinforcement confines the soil."
"You don’t need to pin the block; that’s what a lot of people don’t understand," Adams said. "You use friction to hold the blocks together and limit lateral earth pressure." However, the pins help to keep the blocks aligned.
Adams said that it is critical to correctly space the layers of reinforcement in a GRS system. To him, proper spacing is more important than the strength of the geosynthetic reinforcement itself.
"You can do so much with the spacing. Proper spacing makes the composite [soil and reinforcement] stronger," Adams said.
Colorado Embraces GRS Technology
At least one group of engineers — those at the Colorado Department of Transportation (CDOT)— speaks Adams’ language and speaks it quite fluently.
Shortly after the construction of the bridge pier at TFHRC, Adams traveled to Colorado for an experiment at CDOT’s Hanuana maintenance facility in Denver. The experiment was a joint effort of CDOT’s Reinforced Soil Research Center at the University of Colorado at Dever and TFHRC.
A CDOT maintenance crew built a rectangular prototype abutment, a rectangular pier, and an oval pier along Interstate 70. The soil fill was a common road base, and the reinforcement was polypropylene sheets — the same as used to build the bridge pier at TFHRC.
The abutment and piers held up beautifully after being deadloaded with the weight of 125 Jersey barriers — a half-million pounds (226,800 kilograms).
"The CDOT experiment was the showcase of an international conference on mechanically stabilized backfill technologies, and I was a keynote speaker," Adams said with obvious pride in his work. "I’m part of a paradigm change. This [the abutment and pier in Denver] really showed the potential of the technology — the versatility of it."
In the last few years, Adams has also advised engineers in other parts of Colorado. In Blackhawk, he assisted in prestraining a couple of bridge abutments — squeezing GRS soil mass beneath the abutment. (The bridge pier at TFHRC was also prestrained.)
In Grand County, Colo., the local highway department put GRS technology to use and built four huge walls to widen a road on steep mountain terrain.
Colorado engineers are the country’s leaders in GRS technology, Adams said.
The Experiment in Denver
In a report on the Denver experiment in which two piers and an abutment were constructed using GRS and
subsequently loaded to a half-million pounds (226,800 kilograms), engineers at the University of Colorado at Denver wrote:
- Construction of the piers and abutment was "rapid and simple."
- Piers and abutment successfully supported a load equivalent to 230 kilopascals (33 pounds per square inch), which was more than expected.
Crews built the piers and abutment in a 3.53-meter- (11.5-foot-) deep pit. The oval pier and the abutment were 7.6 meters (25 feet) tall, and the rectangular pier was 7.3 meters (24 feet) tall. Engineers designed the rectangular pier slightly smaller to facilitate a different experiment at a later date.
The piers and abutment were built in six steps:
- Dig the test pit.
- Prepare the geosynthetic reinforced foundation.
- Pour and level a thick concrete pad on top of the foundation.
- Lay concrete facing blocks.
- Place and compact a layer of backfill soil.
- Top each layer with geosynthetic reinforcement, and then repeat the layers until completion.
The Abutments at TFHRC
Early in 1999, FHWA’s Geotechnical Research Team, using GRS technology, completed two full-scale approach embankments and abutments for a single-span bridge at TFHRC. The abutments consist of alternating layers of compacted gravel and woven polypropylene reinforcement confined within a wall of dry-stacked modular blocks. The 152-mm to 203-mm (6-inch to 8-inch) layers of granular fill are separated by thin sheets of geotextile fabric. The GRS bridge abutments are designed to accommodate superstructures up to 22 meters (72 feet) long and 5.5 meters (18 feet) wide with girder depths up to 1.8 meters (6 feet).
The abutments were designed according to Adams’ strong belief, supported by research, that the spacing of reinforcement layers is more important than the strength of the reinforcement itself. They feature weaker, but more closely spaced, reinforcement layers between the compacted fill layers.
The first round of testing of the GRS abutments took place in October 1999 and was a joint effort between TFHRC and the New York State Department of Transportation. The test involved the loading of a prestressed concrete box girder. The girder, one of five beams provided by NYSDOT, was taken from a decommissioned bridge in Saratoga County, N.Y. The bridge was constructed in 1961 and was in service for 37 years.
The girder, which spans 21.9 meters (72 feet) with a cross section of 1.2 meters by 0.85 meters (4 feet by 2.75 feet), was placed on the geosynthetic bridge abutments. The girder was loaded until it cracked significantly. But the GRS bridge abutments performed successfully. Even though the bridge beam was supported directly by the GRS abutments, no settlement of the abutments was recorded.
GRS structures continue to "break all the rules," Adams said with a grin.
Ideal Uses for GRS Technology
As enamored as he is of GRS technology, Adams is clear on the point that it is not appropriate for some uses.
"It’s great for a simple overpass, but it’s not for [a bridge over] a stream where a severe flood could occur," because flooding can weaken a GRS structure.
"A good setting for GRS is in an urban park, on a scenic road, on a back-country overpass," Adams said. For that, "it’s perfect."
- Kanop Ketchart and Jonathan T.H. Wu. "Performance of Geosynthetic-Reinforced Soil Bridge and Pier Abutment," Mechanically Stabilized Backfill, Balkema (publishers), Rotterdam, The Netherlands, 1997.
- Michael Adams. "Geosynthetic Bridge," brochure published by the Federal Highway Administration, Washington, D.C., December 1999.
- Michael T. Adams, K. Ketchart, A. Ruckman, Albert DiMillio, Jonathan T.H. Wu, and R. Satyanarayana. "Reinforced Soil for Bridge Support Application on Low Volume Roads," Proceedings, Seventh International Conference on Low Volume Road, Transportation Research, No. 1652, 1999, pp. 150-160.
- Michael T. Adams. "Performance of a Prestrained Geosynthetic Reinforced Soil Bridge Pier," Proceedings, International Symposium on Mechanically Stabilized Backfill, Denver, Colo., Balkema Publishing, Rotterdam, The Netherlands, pp. 35-53.
Maria Koklanaris is a free-lance writer. She has been a journalist since her graduation from The Pennsylvania State University in 1986. She has been employed by The Hamptons Magazine on Long Island, N.Y.; The Connection Newspaper Group in Fairfax County, Va.; The Washington Post; the Associated Press wire service; and The Washington Times.
For more information about the geosynthetic reinforced soil pier at TFHRC, see "Geosynthetic Reinforced Soil Piers: A Bridge From the Past to the Present" by Doug Rekenthaler in Public Roads, Winter 1997, pages 43-51. This article is available from the Public Roads electronic archives on the TFHRC Web site (www.fhwa.dot.gov/research/tfhrc/).