GRS-IBS Case Studies
Researchers at the Federal Highway Administration (FHWA) knew simple bridges could be built better, faster, and for less money. Officials in Defiance County, Ohio, were ready to test that theory. By using Geosynthetic Reinforced Soil (GRS) in an Integrated Bridge System (IBS), FHWA and Defiance County launched a revolution in bridge construction.
Defiance County Proves a Simpler Bridge Can Be a Better Bridge
Bowman Bridge, Defiance County, Ohio
Traditional bridge construction can be slow, expensive, and complex. The FHWA knew there was a better way. They teamed up with local officials in Defiance County, Ohio, to prove it—and a revolution in bridge construction began.
FHWA worked with Defiance County officials to test a new way to build bridges: the geosynthetic-reinforced soil (GRS) integrated bridge system (IBS). They not only succeeded in building a bridge for less money and time; they went on to build 18 more bridges with the GRS. By the end of the 2010 construction season, an additional 3 bridges will be built.
The Bowman Road Bridge in Defiance County was built in just 6 weeks. Construction time could have been reduced to less than 3 weeks, if two separate labor crews had been available to build both abutments simultaneously.
This represents a radical departure from typical construction time for a conventional bridge, which would have stretched into months.
Time Reduction Factors
Many factors contributed to the time reduction. There was no need to pour concrete, which can take significant project time. Instead, precast concrete box beams were placed directly on the GRS abutments. No approach slab was needed; GRS was compacted directly behind the bridge beams to form the approach way. Materials are readily available, a benefit of the generic nature of the system.
Costs were also radically reduced. Defiance County realized a cost savings of nearly 25 percent on its first bridge support project. These savings came from more than just the reduced labor costs that result from shorter construction time and simpler construction. GRS bridge abutments also require fewer materials.
The cost savings extend beyond construction and labor expenses. Maintenance costs have been reduced. In one example, a bridge in St. Lawrence County, NY realized a 60% savings.
The basic building blocks of the integrated bridge system are composed of geosynthetic reinforced soil. Geosynthetics are often used in civil engineering works and related fields because they are capable of producing strong supporting structures.
Construction is much simpler with GRS IBS. GRS construction uses basic earthwork methods, primarily for fill and compaction, along with sound, general (non-specialized) construction practices.
Elimination of Roadway and Bridge "bump"
Instead of using cast-in-place concrete for the abutment walls, the Bowman Road Bridge engineers used split-face cinder blocks (modular concrete blocks) to face the abutment. Underneath the GRS abutment, they used a Reinforced Soil Foundation (RSF) over the clay subsoil. The RSF uses GRS to provide embedment and a larger bearing area. GRS consists of layers of compacted granular soil alternated with sheets of geotextile fabric reinforcement to provide support. With GRS IBS, there is no need for a deep foundation. The construction is jointless and has no approach slab. This has the added benefit of eliminating the "bump" typically experienced where a bridge abuts the roadway.
Road and bridge settle together–integrated approach
Safety was also increased. GRS is durable. A GRS abutment can have an ultimate capacity of more than 25,000 lb/ft2 depending on the aggregate and strength of reinforcement. When a GRS IBS bridge is constructed according to FHWA guidelines, it has been shown during a full-scale shake table test that a GRS abutment can withstand a 1.0g ground acceleration.
Yet another benefit is the smaller environmental impact of GRS IBS construction. There is a smaller construction footprint, since there is no need to install a deep foundation or cast-in-place concrete.
Michael Adams, a FHWA research geotechnical engineer, was a guiding force in the Defiance County story. As he describes GRS IBS, "The more I investigated it, the more it impressed me. Every time we did experiments, it exceeded our expectations."
Overview of Bowman Road Bridge Performance
The performance of the Bowman Road Bridge and the other GRS IBS bridges has been outstanding. All have performed well and have required no maintenance to date. They have also saved the county significantly on construction costs.
FHWA and Defiance County proved that this new technology works. For transportation agencies whose budgets are insufficient to meet bridge construction demands, GRS IBS could be the long-awaited for answer.
Completed Bridges in Defiance County, OH