In just five days, some county transportation agencies have spanned streams with smaller, single-span bridges and realized a 40% cost savings over conventional construction.
By Chris Von Handorf, P.E.
If almost 30% of our nation’s bridges are “either structurally deficient or functionally obsolete,”1 transportation engineers need to make the most out of limited funding for bridge repair and new construction. Any bridge owner or contractor working under today’s budget constraints will tell you that they need to have their project completed faster, easier, at lower cost, and last longer with minimum maintenance. A relatively new bridge system, GRS-IBS serves all of these project needs and more when used for appropriate site conditions.
GRS-IBS is one of many Federal Highway Administration’s (FHWA) “Bridge of the Future” and “Every Day Counts” initiatives that aims to identify and encourage innovative solutions for shorter project delivery, enhanced safety and increased environmental protection. GRS is also one of several “Accelerated Bridge Construction” (ABC) technologies that is informing and changing the way engineers design bridge replacement structures.
Let’s begin with the ubiquitous engineering acronyms. GRS-IBS stands for Geosynthetic Reinforced Soil Integrated Bridge System. GRS is built by alternating layers of engineered backfill with sheets of geosynthetic reinforcement. The U.S. Forest Service first used gravity walls in the 1970s to stabilize logging road embankments. Although GRS has been used in only the last few decades, the concept of reinforcing soil with organic materials has been around for thousands of years, dating back to straw and mud dwellings.
Perfect solution for smaller single-span bridges
GRS-IBS is adaptable to different site conditions and can be used for new or replacement structures with steel or concrete superstructures. Even with this application flexibility, GRS-IBS is generally used to cross small to mid-sized streams with single-span bridges (lengths ≤ 140 ft and abutment heights ≤ 30 ft). Because GRS-IBS is a shallow design, it is used only at streams and rivers with low water velocities and no scouring issues.2
GRS bridge abutments do not require the elements of traditional designs, including overexcavation for a deep foundation, approach slabs, end walls and bridge bearings. Moreover, GRS-IBS eliminates the “bump” characteristic of older designs: the uneven settlement between the bridge and its approach. GRS crossings transition (integrate) directly from the reinforced-soil abutment to the bridge beams. The bridge superstructure rests directly on the GRS-IBS substructure.
Why GRS-IBS is easier and faster
GRS walls have shallow foundations that allow abutment construction to begin right after excavation and leveling of the underlying soil. Installation of the system’s concrete facing blocks calls for a level and plumb first row.
GRS abutment installation consists of three steps:
1. Laying the facing blocks
2. Placing and compacting the 6-in.-thick granular backfill layers
3. Laying down the reinforced geosynthetic fabric sheets
These three steps are repeated until the wall reaches its final height. A typical crew for construction of smaller, single-span bridge (≤ 140 ft) abutments consists of four to five workers and one backhoe excavator operator. No special worker skills or tools are required, and a walk-behind vibratory tamper and track hoe excavator are the only heavy equipment needed.
To begin, the geosynthetic reinforcement sheet is laid over the leveled soil base. Granular-fill lifts, or layers, are 6 in. high and properly compacted behind the facing blocks. It is crucial that there are no voids or wrinkles in the GRS fabric before backfilling each layer. Alternating layers of fabric, facing block and granular fill continue up to the reinforced bearing bed that supplies additional strength for the superstructure.
A minimum of five reinforced bearing-bed layers is required beneath the bridge beam seat. Bearing-bed fabric spacing is greater, at ≤ 12 in., and grouting and rebar are inserted in the concrete facing blocks. A 2-in.-thick layer of foam boards is placed on the top sheet for the bearing seat. A final layer of fine aggregate brings the height of the GRS abutment to bridge-beam grade and protects the fabric from hot asphalt. Superstructure construction typically commences with installation of precast concrete beams. Once the superstructure is installed, the approach is built. Using a process similar to the GRS design, backfill material and geosynthetic reinforcement are layered to create a strong, stable and flat approach (without the “bump” experienced by drivers crossing traditional bridge installations). Upon completing the bridge approach, the bridge deck and approaches are paved and finished. GRS-IBS construction has been completed in five working days on some project sites.
GRS-IBS is a type of “gravity wall” that completely encapsulates the soil (geothermal sheets meet but do not overlap) and performs like a composite structure, exhibiting a predictable stress/strain curve. FHWA has published a GRS-IBS Interim Implementation guide that can be used by state and local agency engineers for the design and construction of GRS-IBS.
Engineers list 10 advantages from GRS-IBS installations
County transportation engineers who have used GRS-IBS have reported what they view as the system’s most important advantages. Here are 10 GRS-IBS advantages:
1. Flexible and easily field-modified system adapts to different site conditions
2. Faster bridge completion and opening to traffic
(about 5 days)
3. Reduced cost over conventional designs
4. Can be used in poor soil conditions or wet sites with standing water
5. No need for bridge piles
6. Easier to build (no special skills or tools, smaller crew, minimal equipment needs)
7. Increased durability and lower maintenance (fewer parts)
8. Jointless bridge system (integrated substructure and superstructure): no “bump” between sleeper slab and superstructure for improved driving conditions
9. Increased safety due to smaller crews and less heavy equipment
10. Sustainability and support of local economy (local materials and products)
The FHWA and U.S. states report that the GRS-IBS reduces project costs from 25 to 60%, depending on the application. Another advantage to GRS-IBS construction is the use of local, readily available materials including specified backfill material, geosynthetic reinforcement, block facing units, superstructure materials (concrete or steel beams) and paving material (asphalt or concrete). With so many of the required materials produced close to most construction sites, it is easy to see how specification of GRS-IBS is both sustainable and contributes to the local economy.
Acceptance of GRS-IBS is growing
As of July 2012, there have been 74 GRS-IBSs constructed in 26 different states across the country. Additionally, seven of these bridges are part of the National Highway System. It is clear that GRS-IBS technology will soon be a widely accepted bridge construction and replacement method.
ABC, or accelerated technology, is changing our infrastructure rebuilding efforts. Instead of months and months of bridge construction and road closure, we are seeing bridge structures completed and opened to traffic in just weeks. New and replacement bridges are being constructed faster at lower costs and with improved worker safety and structural durability.
With infrastructure budgets shrinking, no assurance of massive injections of federal transportation dollars, and many of our current bridges in urgent need of replacement or repair, GRS-IBS technology could not have come at a better time.
Chris Von Handorf, P.E., is a structural engineer with Hoch Associates in Indianapolis.
1 According to the 2009 ASCE Report Card for America’s Infrastructure
2 Some GRS-IBS applications have used riprap, embedments, aprons or other scour-mitigating constructs for sites with moderate scour potential.
• 2009 ASCE Report Card on
• Transportation Research Board, “Cost Effective and Sustainable Road Slope Stabilization and Erosion Control,” NCHRB Synthesis 430, 2012.
• FHWA “Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide,” Publication No. FHWA-HRT-11-026, June 2012.
• U.S. DOT, FHWA, EDC, “Basic Design and Construction Concepts,” 2012.
• FHWA “Geosynthetic Reinforced Soil Integrated Bridge System Synthesis Report,” Publication No. FHWA-HRT-11-027, January 2011.