Bridge Description

GRS IBS [Geosynthetic Reinforced Soil Integrated Bridge System]

0.1 Miles South of JCT. C.S.A.H. 4 on C.R. 55 over Minnesota southern railway in Magnola Township

Tier 5 (within 3 months)

• ABC: 60 days
• Conventional: same time

• reduced onsite construction time
• minimized environmental impacts

82.5-ft-long and 33-ft-wide one-span prestressed concrete adjacent box beam bridge, 0º skew

135

No bridge previously existed at this RR crossing.

The new bridge consists of 82-ft-long single-span prestressed concrete side-by-side boxes utilizing Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) technology. The bridge has a roadway width of 30 ft with two 11-ft wide traffic lanes and two 4-ft-wide shoulders. The cross-section consists of eight 2.75-ft-deep adjacent box beams with a 2- to 5.75-inch-thick bituminous overlay. The adjacent box beams are post-tensioned transversely through the deck.

The GRS abutments were founded on glacial till. The total height of the abutments from top of reinforced soil foundation to bridge low member is 22 ft. The front facing of the GRS abutment is gravity stacked 8-inch concrete blocks. The lower layers, where contact with water is anticipated, are constructed with solid concrete blocks to prevent water from accumulating and freezing.

The GRS technology used in this project consists of three main components: the reinforced soil foundation, the abutment, and the integrated approach. Alternating layers of compacted granular fill and geosynthetic reinforcement provide support for the bridge. The closely spaced reinforcement and granular soil create a composite material that is capable of carrying bridge loads with predictable performance.

The bridge abutment went up quickly. Three to five layers of the abutment were completed each day, about 220 to 360 square feet. Wing walls meet the abutment at an angle, so 45-degree CMU (concrete masonry units) were specified to help speed construction.

We would not envision using GRS abutments again for such a tall abutment (≈ 20-ft exposed height) which generates an excessively large foot print of geosynthetic reinforcement, and reduces the cost saving feature of GRS. The hollow core CMU facing is vulnerable to construction damage, stress concentrations, etc. In the future, we would opt to use wet cast solid concrete blocks for both better freeze-thaw protection and to better distribute possible stress concentrations, etc.