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Montana DOT rebuilds an old bridge with a new system

Case Studies | April 1, 2014 | By:

Using geosynthetic reinforced soil, this two-lane project was completed faster with lower costs


The structure built at the South Fork Dry Fork Marias River crossing was selected for a new technology tested in other states and was proven to work. The new bridge—also designed as a grizzly bear undercrossing—was built with geosynthetic reinforced soil (GRS)–integrated bridge system (IBS) technology. GRS–IBS is a system that uses a series of alternating layers of granular fill material and geotextile sheets to create a composite reinforcement that provides support for the bridge slab.

The combination of the compressive strength of the granular soil and the tensile strength of the geotextile results in a strong internally supported structure able to handle a substantial load. Furthermore, this design provides a smooth transition from the roadway to the bridge since the construction is jointless and has no approach slab.

Another potential benefit to choosing this type of bridge design was that construction time of the structure could be reduced. Less concrete is used compared to conventional abutment designs, which can take up a sizeable amount of project time. On past projects, significant cost savings were realized through the combination of reduced labor costs from shorter construction time due to simpler construction techniques.

Evaluation procedures

Research will document the installation for best practice and any construction concerns germane to the performance of the GRS–IBS structure. Special consideration will entail documenting the placement of the alternating layers of compacted granular fill supported with the selected geosynthetic reinforcement, facing (block) wall unit, wing wall, precast beam placement, pavement approach, and other pertinent events.

Semiannual inspections (late fall/early spring) will report on GRS–IBS integrity and any other visually measurable outcomes. Additional site inspections may supplement the semiannual visits, based on need. Monitoring and reporting on long-term performance and condition will be performed for a minimum of 60 months (5 years).

Construction documentation will include information specific to the installation events. Post-documentation will entail semiannual site inspections for inclusion into the current construction report located here.

Initial remarks

The information presented in this report encompassed a period from June to November 2013, involving 10 site inspections.

District staff members did an excellent job informing MDT Research Programs when events of installation had progressed to the point when documentation needed to take place.

This report attempts to capture the pertinent elements of construction to describe the installation events on how this type of structure was assembled. This report also establishes a baseline of documentation for use in determining future performance of the GRS–IBS structure. As noted, semiannual site evaluations will be added annually to the construction report.

One issue with the project was poor logistics by the contractor in getting the appropriate and approved project materials on-site, specifically the concrete masonry unit (CMU) solid core and split-face hollow core block used for the structure’s face of the GRS abutment wall.

It was also reported that the GRS backfill material was difficult to locate due to the gradations requirements. Also, the contractor did not adequately prepare or train the project managers and workers in the basic elements of assembling this type of bridge support. These and other minor issues, including weather delays, led to an extended construction schedule of the GRS-IBS installation.

Other than the issues stated above, once construction began, the contractor’s team exhibited good attention to detail in the assembly stages of the GRS abutments.
Because of the team’s inexperience, the learning curve necessary to understand what needed to be done extended the installation time substantially to completion. The north GRS abutment #2 took nearly a month to complete, with the south abutment #1 section in about half that time. MDT staff members were an integral component in the team’s training and in oversight to get the project completed.

In what was observed and documented on-site, and with consensus with MDT district staff members associated with the project, there were no detrimental construction practices with the assembly of abutments 1 and 2, as well as the slab installation that may affect short- or long-term performance to date. As noted in the report, the project paving phase will not be completed until spring of 2014.

MDT Research would like to thank Mike Klette, Great Falls project manager, and Kevin Thielmann, civil engineer technician, for the help in the coordination of project activities. And a special thanks to Jay Manuel, civil engineering specialist, whose expertise and knowledge of the GRS–IBS installation on-site added value to this report.

Craig Abernathy is an experimental project manager with the Montana Department of Transportation (MDT).

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