By Michael Adams
Three years ago, the Federal Highway Administration (FHWA) teamed up with local officials in Defiance County, Ohio, to test a new prototype geosynthetic-reinforced soil (GRS) integrated bridge system. Built in 2005, the prototype GRS bridge on Bowman Road in Defiance County is a success story for the county as well as the FHWA.
FHWA sought to investigate the feasibility of using GRS to build simple bridges better, faster, for less money, and to use the experience to develop guidelines for GRS bridge construction and to support widespread application of the technology.
Defiance County sought to lower costs while maintaining performance. Not only did it succeed in building the Bowman Road Bridge for less money and time, but due mainly to the pioneering efforts of the county engineer, Warren Schlatter, and his team, the county went on to build a total of 11 bridges, to date, using the FHWA GRS integrated bridge system. Table 1 provides a complete listing of the 11 GRS bridges constructed in Defiance County, and the impressive record of structural performance as of December 2007.
In the summer and fall of 2005, FHWA provided guidance and abutment design plans to Defiance County to build the Bowman Road Bridge using GRS technology. 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 6 weeks, compared to a typical construction time of several months for a conventional bridge. The construction time for the bridge could have been reduced even more, to less than 3 weeks, if 2 separate labor crews had been used to build both abutments simultaneously.
For the Bowman Road Bridge, instead of using cast-inplace concrete for the abutment walls, split-face cinder blocks (modular concrete blocks) were used to face the abutment.
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 (8in.), 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 designed wall height is reached.
Precast concrete box beams were placed directly on the GRS abutments without a concrete footing; the bridge structure 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 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 smooth, bump-free ride for drivers traveling on and off the bridge.
The Bowman Road Bridge project was significant in terms of the height of the abutments, span length, and complex geometry. A cross section of the integrated abutment is shown in Figure 1. Photos of the abutments during construction are shown in Figure 2 and Figure 3. Photo 1 shows the completed Bowman Road Bridge.
FHWA instrumented the bridge so that performance monitoring could be conducted over a duration of 2 years. The results of that monitoring have come in, and they indicate successful bridge performance as expected.
GRS innovation and performance
Building on its initial success, Defiance County has demonstrated a unique willingness to adopt new technology and prove its value in real-world applications. While FHWA provided direct support and design guidance for the Bowman Road Bridge, the county independently decided to use the GRS integrated bridge system to construct 10 additional bridges. Much of the credit for this single-handed jump start of the GRS bridge technology belongs to Schlatter, the county engineer.
One of the most significant obstacles to widespread application, and a major challenge for FHWA, is to overcome the reluctance of transportation agencies to try or adopt a new technology such as the GRS integrated bridge system.
Conventionally, most simple bridges are supported on a pile cap abutment with 2:1 side slopes. In most cases, the piles are driven to rock or very stiff soil. However, these simple bridges are often time consuming and relatively expensive to construct, and many transportation agencies have insufficient budgets to meet the demand to build new or replace older, functionally deficient bridges.
As a result, there is a huge backlog of bridge construction projects nationwide. The GRS integrated bridge system, one of several FHWA initiatives linked to the Bridge of the Future program, is aimed at providing an easy and costeffective way to build simple singlespan bridges 70-90ft long.
With 11 bridges under its belt, Defiance County stands out as a leader and an inspiration for other transportation agencies to adopt the GRS bridge technology. GRS technology, developed initially by the U.S. Forest Service and the Colorado Department of Transportation, and later refined by the FHWA for bridge support applications, is a relatively new technology and should not to be confused with the established mechanically stabilized earth (MSE) wall system technologies.
There is a clear distinction between the MSE and generic GRS designs, and this distinction needs to be identified and understood to give the next generation of wall designers the option to build affordable walls and abutments to meet the demand for these structures for highways and bridges. There is a reluctance to change or acknowledge alternative design methodologies that do not follow current MSE design methodology.
GRS bridge systems are built without MSE design guidance; rather, the GRS design methodology is based on the performance of several full-scale experiments and production abutments built in the private sector. Although MSE and GRS are similar in the use of modular block face elements, the technologies differ significantly in terms of the frequency and type of reinforcement layers used, and as such, the design rules for MSE cannot be used for GRS.
Figure 4 shows the reduced bearing area of the beams. Figure 5 shows the compaction of approach fill behind the beams to integrate substructure into superstructure to create a smooth, jointless bridge transition.
The preferred type of reinforcement in a GRS structure is woven geotextile because it is economical and easy to install. A flexible sheet of the reinforcement textile can be placed either at every layer or every other layer, depending on the application.
In addition, the facing blocks in a GRS system can be a lightweight splitface concrete masonry unit (CMU) frictionally connected to the GRS mass between the sheets of geotextiles, without the need for a vendor-approved mechanical connection system. Also, a GRS abutment can be designed without substantial consideration of reinforcement creep, lateral loads at the face of the walls, pullout, and the recommended MSE base-to-height ratio 0.7 with a 2ft wall embedment.
Understanding the differences between GRS and MSE, as well as other standard bridge construction techniques is an important first step in advancing the state of the practice and assisting transportation agencies to realize the benefits of this technology.
Defiance County bridges
With the success of the Bowman Road Bridge, completed in October 2005, Defiance County took the initiative to use the GRS technology for the construction of 10 more bridges during the 2006 and 2007 construction seasons (see photos 1-11). By using the GRS integrated bridge system, Defiance County saved approximately 25% in construction costs, and nearly half of the normal construction time, for each bridge.
Currently, bridges are rarely supported on shallow foundations because of the fear of excessive settlement and scour. The FHWA GRS integrated bridge system is tailored for singlespan bridges. These simple bridges are more tolerant of settlement than multispan structures. The system is designed to compensate for post-construction settlement; the bridge, abutment and approach are supported on the same foundation system. The bridge is designed for uniform settlement between the sub and superstructures.
All of these bridges cross water and were designed to withstand flood conditions. All of the bridges have experienced floods, as evidenced in the photos on pages 14-15. The abutments were constructed on the reinforced soil foundation encapsulated in a sheet of geotextile.
Solid red CMU block was used at the base of each abutment wall. A riprap slope was built up to the height of the red block to protect the stream channel from scour near the bridge. The event of scour would be indicated by exposed red block. The rock was sized for the stream velocity of 15ft/s.
With a 5-man construction team, and depending on logistics, a typical bridge can be built in 2-3 weeks. One of the Defiance County bridges was built and opened to traffic in 10 days. Unlike conventional methods of bridge construction, building a GRS mass is less dependent on fair weather and more forgiving with adverse field conditions
The experience and performance of the bridges in Defiance County demonstrate the advantages, in time, money, and convenience of utilizing the GRS bridge technology.
Table 1 summarizes visual inspections of all the bridges, completed in December 2007. The table indicates that all of the bridges are performing exceptionally well, without any current need for maintenance.
One bridge, Glenburg Road, developed a slight transverse pavement crack partway across the road at the beam approach interface. The interface crack can be attributed to a less than optimal width of the bearing area for the bridge seat, 18in. instead of 24in. Erroneously, wooden posts, instead of steel H-sections, were used to hang the guardrail. Driving numerous thick wooden posts through multiple layers of heavy geotextile was disruptive. The thickness of the asphalt layer is not known and, ultimately, may be the cause of the crack.
Regardless, according to Schlatter, all of the bridges would have a pavement crack at the beam interface soon after construction if the bridges were built on stubby pile cap abutments.
The performance of these 11 bridges demonstrates that this GRS technology is an efficient alternative to conventional bridge construction, and the FHWA plans to continue developing the technology for more widespread applications.