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Take the leap of faith
By Jessica Bies
Introduction
More than seven years after the Federal Highway Administration (FHWA) and local officials in Defiance County, Ohio, built their first geosynthetic-reinforced soil (GRS) integrated bridge, county engineers throughout the U.S. are starting to recognize the benefits of using GRS bridge technology.
Still promoting widespread application of this technology, Mike Adams, a geotechnical engineer with the FHWA and one of the leading researchers, developers, and promoters of GRS bridge technology, has rallied several county engineers to his cause. Among these is Toby Bogart, highway superintendent for St. Lawrence County, N.Y., and an early adopter of the cost-saving techniques.
Canada-bordering St. Lawrence County, the largest county in New York state, is home to 202 bridges, half of which Bogart hopes to convert to GRS. Constructing bridges in as few as 10 days and saving an average of 50% on each, Bogart’s success highlights the considerable benefits of utilizing GRS bridge technology.
Background
GRS technology uses alternating layers of compacted fill and sheets of geotextiles to provide support for integrated bridge systems. Perfected by the FHWA at its Turner-Fairbank Highway Research Center (TFHRC) in McLean, Va., GRS bridge technology made its debut in the U.S. when the Colorado Department of Transportation (CDOT) constructed the Founders/Meadows Parkway Bridge in 1999 on footings supported by GRS, eliminating the use of traditional deep foundations or piles.
Considered an experimental project, the Founders/Meadows bridge took advantage of FHWA research but was constructed before GRS bridge technology had been sufficiently tested. Designed to incorporate monitoring programs, the bridge served as a test case for GRS bridge performance and was built in 12 months.
In the meantime, FHWA’s Adams had been studying GRS at the TFHRC. In 1996, Adams broke the world record for load capacity and applied pressure on a GRS structure. During the next several years, he traveled the U.S. performing experiments in GRS and encouraging engineers to do the same.
A prototype, built in the summer and fall of 2005 in Defiance County, Ohio, gave Adams and the FHWA the chance to work with county engineer Warren Schlatter and streamline the new bridge-building techniques. The Bowman Road Bridge was built in six weeks and cost 25% less than a conventional bridge, serving to highlight two of the main benefits of GRS bridge technology: lower costs and shorter build times.
Following the success of the Bowman Road Bridge, Defiance County proceeded to use GRS technology for the construction of several more bridges. By the end of 2011, Schlatter’s crews had built a total of 23 bridges and reduced construction times to two weeks.
In 2010, the FHWA selected GRS integrated bridge systems as one of several technology innovations for accelerated deployment as part of its Every Day Counts initiative. Every Day Counts promotes the use of ready-to-go innovative technologies proven to shorten project completion times, enhance roadway safety, and improve environmental sustainability.
How it works
GRS technology is well-suited for single-span bridges less than 36m (120ft) in span. Almost 80% of bridges in the U.S. have span lengths between 21-27m (70-90ft) and many of these bridges could be supported on GRS. There is great potential for its use as a convenient cost-saving measure, particularly by counties facing budget cutbacks.
Using techniques developed by the FHWA, GRS abutments can be constructed with readily available materials and workers implementing the technology do not need to be highly skilled. Building a GRS mass or abutment is a three-step process. First, a row of blocks is put into place. Second, a layer of fill is compacted to the height of the facing blocks. Third, a layer of geotextile is extended between the rows of blocks to connect them to the GRS mass.
The 1-2-3 process is repeated until the wall height is reached. Precast concrete box beams are placed directly on the GRS abutments without a concrete footing. The bridge structuring has no cast-in-place concrete.
“There are two necessary factors to assure good performance of a GRS mass: good compaction with quality granular fill and close reinforcement spacing,” said Adams.
Another important aspect of the GRS bridge is the absence of an approach slab. GRS is compacted directly behind the bridge beams to form the approach way. Asphalt pavement is used to cover the bridge and the approach way, creating a gradual transition from the road to the bridge and assuring that they will settle together without forming a bump.
Who’s doing it?
In June 2009, Toby Bogart became a county engineer in New York state’s far-north St. Lawrence County. During his interview, then highway superintendent William Dashnaw asked Bogart what he knew about GRS bridges, saying that he would like to try one. Bogart admitted that he had not heard of them.
Without a specific bridge in mind for replacement, Bogart got in touch with the FHWA and Mike Adams. Bogart remembers being skeptical. “There’s no way I can put this on soil and fabric and expect it to stay there,” he says, recalling his thinking at the time. But after talking with Defiance County’s Schlatter and hearing about his success with the technology, Bogart was ready to try it out.
He got the chance less than four months later in October 2009. One of the bridges in St. Lawrence County was marked with a prompt interim action (PIA) safety flag. According to Bogart, they had 24 hours to either repair the bridge or shut it down. After seeing how much the bridge had deteriorated, the county decided to replace it using the GRS system.
The bridge replaced in October 2009 was the first of 12 that St. Lawrence County has now built. The county received an innovative bridge research and development grant from FHWA to build one more in 2012.
The allure of GRS bridges lies in their low-cost adaptability. In Defiance County, Schlatter has reported savings of 40% and he said his crews are now able to build a complete bridge in about two weeks. In St. Lawrence County, N.Y., Bogart has reported savings of almost 50% and estimates that almost half of the county’s 202 bridges will be converted via GRS.
In addition, Bogart has been able to adapt the system to work around pre-existing concrete scour protection structures by building new abutments several feet behind the older ones. This allows him to avoid extra permitting and ensures the construction crew won’t disturb the waterways. “We really enjoy the flexibility these bridges offer us,” Bogart said. “We’ve really honed our skills.”
Spreading the word
Since tackling his first GRS bridge, Bogart has traveled around the U.S. speaking for FHWA at Every Day Counts summits and seminars. He also speaks locally, attending local bridge seminars with NYSDOT and presenting in front of the NYS County Highway Superintendents Association. “Most are really enthusiastic. They don’t seem to have that extreme doubt in their head,” Bogart said.
According to the Bogart, the high success rates associated with GRS bridges is what ultimately wins engineers over. “They’re amazed at the money savings and the [short] time frames,” he explained. For Bogart, GRS bridges are another weapon in the arsenal against crumbling infrastructure during a time when budgets are getting tighter and tighter.
His advice for a county engineer considering using GRS is to simply try it. “Take the leap of faith. It’s worth it.”