CBC Visits Big Sky Country to Aid Montana DOT with Buried Flexible Bridge / Culvert Program

As part of Montana Department of Transportation’s (MDOT’s) comprehensive Culvert / Bridge Inspection Program, CBC Engineers & Associates, Ltd. was asked to to inspect, evaluate and provide remedial action in required for some 30 Buried Flexible Culvert / Bridge locations that have been on the MDOT’s watch list for several years. CBC was originally engaged by MDOT in 2005 when measurements were sent to CBC for analysis. The DOT recognized CBC’s ability with the recommendations and evaluation of the data given and decided to include CBC in a comprehensive Buried Flexible Culvert / Bridge Inspection Program starting in 2006. To date, CBC has observed, evaluated and in some cases designed remedial solutions at some 30 locations for MDOT and today only a handful remain on the DOT watch list.

Project summary

Montana Department of Transportation (MDOT), had been monitoring some 30 locations of Buried Flexible Culvert / Bridges across their state for a period of years. These structures became known as the watch list. It was obvious to MDOT that in many locations changes were occurring at many locations on this watch list, however, what to do about the changes or even if the changes constituted replacement was a hard call for the DOT. These changes were such things as corrosion, shape or deflection changes, and settlement of the roadway embankments containing these structures. As the DOT kept a watchful eye on these structures, they also knew eventually they were going to have to do something about them.

So the first call to CBC came in 2005 from MDOT inquiring on how CBC might be able to help them. We gave the DOT the evaluation process that is used to determine if a structure’s geometry is of worry, or if the pipe wall corrosion was a problem, and how we take these measurements in the field to run the necessary shape and structural checks to determine if a structure could be rehabilitated or needed replacement. Theses structures consisted of 6″ x 2″ corrugated steel structural plate in the shapes of an Underpass, Round and Pipe-arch shapes. These crossings were used for both animal crossings under major interstates and state routes and in some case they acted as hydraulic structures as well. The majority of these¬† locations were installed in the 1960’s and 1070’s making them 35 to 45 years old.

Of the 30 locations that have been measured and evaluated in some cases several times, 13 structures were deemed in need of some remediation to extend their useful service life. CBC used their field measurements of the locations in questions and compared different visits over time to see if any of the structures were moving and as a result structure geometry changes were obvious. The actual shape measurements were then used to draw a cross sectional shape in AutoCAD at each station in each structure. The Chords and Mid-ordinates (shown in picture below) comprising the structure geometry were then determined and then inserted into a computer program called MULTSPAN which gives the maximum deflection of the structure and information of what remedial action might be required until a permanent fix was decided upon.¬†Personnel within CBC helped write this software program under a grand from Ohio Department of Transportation (ODOT)and the Federal Highway Administration (FHWA) in the late 1980’s.

Through this process MDOT was able to let a project in 2008 that had CBC’s designed permanent solutions to 9 of the 13 troubled sites. These locations again were comprised of all 3 shapes, the Underpass, Round and Pipe-Arch shapes. The solutions for long term stability and the extension or each of these 9 structures service lives varied from a simple concrete paved invert field poured in the bottom for abrasion protection, to more structural solution requiring the insertion of “Ring Beam Stiffeners” and concrete paved inverts to raise the inverts or flowlines up and over the newly installed stiffeners.¬†

Round Shape Solution

Underpass Shape Solution

Pipe-Arch Shape Solution

Rehabilitation Versus Replacement

Montana DOT through this program was about to rehabilitate 9 of 13 structures without digging up I-94 in at least 4 placed to replace these structures which were all 100’s of feet long. They were able to avoid costly construction and headaches to their traveling public by using these solutions in place of replacement. This also saved the State of Montana Taxpayers millions of dollars in the process as the rehabilitation of each of the 9 locations was about 20% of the replacement cost and the rehabilitated structures are good for another 30 to 50 years. This was a win-win for MDOT and the taxpayer traveling public. The remaining four structures are being watched closely by the DOT as they decide when to rehabilitate. CBC stands ready to assist them in their efforts.