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Building the bridge of the future with GRS technology

August 1st, 2006 / By: / Feature, Reinforcement

Ohio bridge builders use geosynthetic-reinforced soil technology.

Build the bridge of the future better, faster, and for less money using geosynthetic-reinforced soil (GRS) technology for abutments.

Instead of a conventional bridge, supported on a pile-cap abutments, GRS technology uses alternating layers of compacted fill and sheets of geotextile reinforcement to provide support for the bridge. GRS abutments can be built with readily available material using common construction equipment, without the need for highly skilled labor. They are also extremely durable. “They can even perform very well in earthquakes, if constructed properly with closely spaced reinforcement,” said Mike Adams of the Federal Highway Administration (FHWA). “The reinforcement spacing controls the performance of the GRS mass.”

Researchers at the U.S. Forest Service and the Colorado Department of Transportation (CDOT) pioneered the early development of this technology. FHWA then worked with CDOT to further refine it. In addition, FHWA built and tested several full-scale GRS structures at its Turner-Fairbank Highway Research Center in McLean, Va.

“While this type of abutment isn’t right for every bridge, the technology is well-suited for single-span bridges of less than 36m (120 ft). It is not advisable for water crossings where the potential for scour is critical,” said Adams. About 80% of bridges in the United States have a span length between 21-27m (70-90 ft). A significant portion of these single-span bridges could be supported on GRS.

Saving time and money

During the summer and fall of 2005, FHWA provided guidance and abutment design plans to Defiance County, Ohio, to build the Bowman Road Bridge using GRS technology. The bridge opened to traffic in October 2005.

This was the county’s first time using the technology to build bridge abutments. “After seeing a presentation on the technology and meeting with Mike Adams of FHWA to learn more, we modified our plans from using traditional abutments to building ones with GRS,” said Warren Schlatter, Defiance County engineer.

To become more comfortable with the techniques, the county initially used GRS for retaining walls, to reduce the cost of culvert construction, before building the Bowman Road Bridge.

The innovation of using GRS paid off, as Defiance County realized a cost savings of nearly 25% on its first bridge-support project. The bridge was also built in six weeks, compared to a typical construction time of several months for a conventional bridge.

“Significant savings were realized in both time and costs,” said Adams. The construction time for the bridge could have been reduced even more, to less than three weeks, if two separate labor crews had been used to build both abutments simultaneously.

For the Bowman Road Bridge, instead of using cast-in-place concrete for the abutment walls, split-face cinder blocks (modular concrete blocks) were used to face the abutment.

How it’s done

Building a GRS mass is as easy as 1-2-3: a row of blocks, a layer of compacted fill to the height of the facing blocks—20cm (8 in.)—and then a layer of geotextile. Each layer of geotextile is extended between the rows of blocks to connect the block to the GRS mass.

The 1-2-3 process is repeated until the wall height is reached.

“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.

Precast concrete box beams were placed directly on the GRS abutments without a concrete footing. The bridge structuring has no cast-in-place concrete.

The bridge also does not have an approach slab, but rather GRS was compacted directly behind the bridge beams to form the approach way and to create a gradual transition from the roadway to the bridge.

Asphalt pavement was then placed on the bridge and approach without a conventional joint system at the bridge ends. The intent was to allow the bridge and the adjacent road to settle together, providing a bump free, smooth ride for drivers traveling on and off the bridge.

FHWA instrumented the bridge so that performance monitoring can be conducted during the next several years. “It is performing very well,” said Schlatter. “There has been very little movement or settling, and no pavement cracking.”

Defiance County is planning to build seven more abutments this year using GRS. “One of the biggest advantages for us is convenience,” said Schlatter. “There are many construction advantages over using concrete, including the ability to build the abutments in all kinds of weather conditions.”

For more information

To learn more about GRS technology or the Bowman Road Bridge, contact Mike Adams at FHWA, 202-493-3025 (email: mike.adams@fhwa.dot.gov). Additional information on the Bowman Road Bridge is also available from Warren Schlatter of Defiance County, 419-782-4751 (email: dce@defiance-county.com).

Source: U.S.DOT/Federal Highway Administration

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