Rock County, in southern Minnesota, closed an antiquated timber bridge that carried County Road 55 over a regional railroad. The bridge structure needed to be replaced, but the construction methods and materials for building a single span bridge were extremely expensive. The County needed a solution to quickly and cost-effectively build the new bridge.
To meet the limited budget for building the abutments, the Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) was chosen. This system provides an economical solution to accelerate bridge construction and is derived from examples of ancient structures that utilize closely spaced organic materials to reinforce soil. The principles are the same for this modern design adaptation by using angular, crushed gravel with a high-strength geotextile or geogrid in frequent layers. The interaction of these components stabilizes the structure internally. MIRAGRID® 7XT and MIRAFI ® HP570 were chosen for this project. The construction technique does not require deep foundation design and thus can be completed in less time, usually weeks. It also utilizes basic earthwork methods and practice with minimal environmental impact.
MIRAFI and MIRAGRID provided optimal solutions for two components of this bridge system. The design included gravel compacted and encapsulated in the MIRAFI HP570 geotextile fabric to function as an integral base for the abutment. Next, the abutment was built by placing alternating layers of gravel and geosynthetic reinforcement in four-inch to eight-inch lifts. The project engineer chose MIRAGRID 7XT for the reinforcement between the compacted fill material for its strength and constructability properties. The MIRAGRID 7XT was placed horizontally at eight inches between concrete masonry facing units and at four inches near the top to reinforce the bearing bed, which supports the precast concrete bridge beams. The ease of construction using the flexible materials along with the simple installation techniques increased worker safety and decreased the time to complete the project.
Although there are now over 100 GRS-IBS bridges in service throughout the United States, the Rock County GRS-IBS bridge #67564 is the first in the state of Minnesota. It is also one of the tallest, standing at 24 feet high.
The bridge abutments were constructed using alternating layers of compacted gravel and MIRAGRID 7XT geosynthetic reinforcement, ensuring stability and strength while reducing construction time
MIRAFI HP570 geotextile encapsulated the compacted gravel base, providing structural integrity and forming a solid foundation for the abutments without requiring deep foundations
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