A Bridge to Greater Connectivity
While designing a much-needed interchange, Montana called in the cavalry -- experts in accelerated bridge construction -- to discuss state-of-the-art methods to avoid a long-term closure.
The tale of the new Custer Interchange in Helena, MT, starts like many other transportation stories: population growth, aging infrastructure, changing land uses, increasing traffic. The Custer Avenue Bridge crosses over I–15 inside the urban limits of Montana’s capital city. As a bridge along one of the few east-west routes that crosses the interstate, the structure could no longer meet the needs of its growing community. In fact, by 2003 the bridge was carrying more than 16,000 vehicles per day.
Another problem: The existing crossing offered no access between Custer Avenue and the interstate, a growing inconvenience to the increasing number of businesses setting up shop in the area. The solution? Replace the old bridge with a wider one, construct a new interchange to connect the two routes, and do it all while minimizing adverse impacts on nearby businesses, residents, and commuters.
"The greater Helena community really rallied around the need for increased accessibility -- access to and from the interstate and Custer Avenue, and access from the interstate to the growing number of businesses in the area,” says Ron Alles, Helena city manager. "Our challenge was meeting this need for a new interchange while replacing the old bridge and avoiding potentially costly traffic backups or lengthy detours.”
Using standard procedures and contract requirements, the project would have required closing Custer Avenue for approximately 4 months, according to engineering estimates from the Montana Department of Transportation (MDT). Given the limited number of interstate crossings in the area, MDT quickly ruled out closing the bridge for that long. What to do?
Call in the experts. MDT officials invited industry experts to a technology transfer workshop and later a conference on accelerated bridge construction to help them sift through the growing number of technologies, tools, and innovations available to help get the job done faster. MDT added flexibilities into the contract that enabled its contractor to complete the bridge and interchange well ahead of schedule and without major disruption to the community. In the end, the contractor employed mostly conventional construction techniques, but innovated and accelerated its work through ideas generated from the workshop and conference.
When Custer Avenue was constructed in 1955 as a two-lane road, the interstate did not yet exist. In fact, the area where the new interchange is now located was dotted with farmhouses, barns, and outbuildings -- a far cry from today’s growing commercial landscape.
Seven years later, in 1962, the State constructed the interstate and a bridge that carries Custer Avenue traffic over I–15. Although Custer Avenue provided a vital link between Helena and Helena Valley farms and ranches, the road carried very little traffic at the time the interstate was built, so ramps and an interchange to connect the two roadways were deemed unnecessary.
By 2003, the Custer Avenue Bridge had become functionally obsolete. In addition to lacking an interchange with the interstate, the bridge had no sidewalks or shoulders, presenting a barrier to mobility for bicyclists, pedestrians, and motorists alike.
In 2003 MDT completed an environmental impact statement for the I–15 corridor. A year later, the department signed a record of decision concluding that an interchange was needed at Custer Avenue. The interchange would provide access to I–15, improve east-west travel for all transportation users, create a safer roadway for bicyclists and pedestrians, and manage approaches to the ever-expanding commercial activity located along and near Custer Avenue by providing access to existing and projected land uses.
Soon after MDT decided to construct an interchange, commercial developers recognized the potential business opportunity and began preparing for the increased traffic exposure that would exit I–15 onto Custer Avenue once an interchange was built. However, preparations for commercial development began before MDT had final design plans for the roadway and interchange, which posed a potential risk for buildings being constructed in the right-of-way. To mitigate this risk, MDT and the city of Helena assisted the developers with determining where to position their facilities, including buildings and parking lots.
Technology Transfer Workshop
Design for the new Custer Interchange began in 2006 when MDT hired a consultant engineering firm. As the planning progressed, the need to minimize the impacts of construction on the Helena community became apparent. For example, in addition to minimizing traffic congestion and maintaining access to businesses, the project had to avoid interfering with public utility lines that run through the affected location. Due to the complexities of the project, MDT coordinated with the Federal Highway Administration (FHWA) to host an Accelerated Construction Technology Transfer workshop.
The workshop, held in January 2008 in Helena, occurred during the design stage. Transportation experts from the public and private sectors shared design ideas, insights into opportunities to reduce project impacts, and ways to mitigate potential problems. The workshop proceedings, documented in Improving Connectivity: The Custer Inter-change Project (FHWA-IF-08-012), are available at www.fhwa.dot.gov/construction/accelerated/if08012.
"The workshop brought together some of the best and brightest national experts who interacted with the preliminary engineering team and the MDT construction engineering staff,” says Mick Johnson, then MDT District Administrator. "It provided an array of options for consideration and inclusion into the design of this critical traffic element.”
The major concern addressed during the workshop was how to reduce the duration of conventional construction to minimize impacts on the community. Acting on one of the recommendations from the workshop, MDT officials modeled the traffic impacts of conventional techniques for phased bridge construction, which enabled traffic to use the bridge but with significant construction delays, compared to full closure of the Custer Avenue Bridge over I–15 and then analyzed the results.
Using CORridor SIMulation (CORSIM), a software program that performs traffic simulation, MDT found that a complete bridge closure would limit most impacts to just a couple of months versus conventional techniques, which would lead to greater congestion for a longer period of time. After sharing the simulation results with the local government and other stakeholders for feedback, MDT officials concluded that complete bridge closure offered the optimal solution.
Recommendations from the workshop also addressed the importance of building community support for the project and keeping the public informed. MDT created a Web site at www.mdt.mt.gov/custer to provide details, videos, and updates on the project. The department also worked extensively with the city of Helena, Lewis and Clark County, Helena Regional Airport Authority, and local business owners. For example, throughout the design phase, MDT held multiple public meetings for stakeholders to provide information and build community support for the project. Also, during the construction phase, MDT officials held weekly meetings with the public, which were well attended by nearby business owners and local media. The meetings provided opportunities for stakeholders to ask questions directly and resolve business concerns, while giving MDT engineers a chance to explain the work that would occur next.
"The weekly public meetings during the construction phase of the project were a key component of the overall public involvement process,” says MDT District Construction Engineer Doug Wilmot. "MDT’s project staff and the prime contractor were able to address all of the concerns brought forward by the public and resolve conflicts at the project level.”
Conference on Accelerated Bridge Construction
With the benefit of reduced traffic impacts apparent, MDT explored other ways to expedite the bridge construction. To further develop ideas generated during the technology transfer workshop, the department hosted a conference on accelerated bridge construction in July 2009 with experts from FHWA and the bridge industry in attendance. The conference focused on proven techniques as well as newly introduced construction strategies.
Among the key concepts ultimately used in the project were the following:
- Use of incentives and disincentives in the construction contract based on costs to road users. Road user costs are calculated using factors such as number of cars and trucks, speeds, and detours.
- Construction of interstate ramps and placement of fill to widen and raise Custer Avenue while keeping the road open, thus reducing the time the bridge was closed.
- Designing flexibility into the contract to enable the contractor to choose between precast (pour concrete elements offsite and then move and set in place), cast-in-place (pour concrete in place), slide-in-place (build bridge superstructure next to existing bridge and then slide into place), or move-in-place (build offsite and transport to location) construction methods, if necessary and cost effective.
"The key to accelerating the bridge construction was providing flexibility to the contractor,” says Kent Barnes, MDT bridge engineer. "To do that, we chose to design for acceleration and enable the contractor to simplify to conventional construction when necessary. For example, we provided design details for many precast elements and for bridge sliding. The contractor could choose to use these accelerated features or simplify to build conventionally to best fit the schedule demands of the overall project.”
During the design phase, MDT also conducted a value engineering review in August 2009. The multidiscipline value engineering team analyzed ways to improve the value and quality of the project along with reducing the time needed to complete the project. The review team provided useful information regarding the sequencing of constructing the approach embankment and the bridge. The analysis showed that bridge construction could be accelerated so much that placement of the embankment fill would have to occur after the bridge was completed, which was not what MDT officials desired. The review made the project team aware of the need to balance the timing of the bridge and the embankment.
Final Design and Construction
MDT and its design consultant incorporated select concepts from the workshop, conference, and value engineering review into the final design plans. Following a recommendation offered during the conference, MDT officials advertised the project contract for 6 weeks in summer 2011, increasing Montana’s standard response time of 4 weeks. The additional time enabled contractors to consider the complexities of the project and the various options for bridge construction before proposing their recommended solutions. The agency awarded the $23 million contract in August 2011.
MDT included three construction phases in the contract, with the first two phases offering incentives for finishing early and disincentives for exceeding deadlines. The first phase improved the detour route before bridge closure, the second included the bridge closure and replacement, and the third involved all remaining contract work such as opening the interchange ramps. MDT also decided upon a 75-day period for the bridge closure phase, with up to 35 days eligible for incentives of $14,500 per day.
Because the new bridge is wider, the contractor could build the new sections on both sides of the old structure and then close the bridge only to tear down and rebuild the center section. Therefore, before closing the old bridge, the contractor installed the outside bridge piers, columns, and deck beams for the new bridge just outside the foot-print of the existing structure. The contractor also built up the embankment fill before the bridge closure.
Using mostly conventional techniques and taking advantage of the flexibility afforded in the contract, the contractor was able to complete much of the remaining work prior to closing the road and removing the old bridge. Ultimately, the contractor completed the bridge replacement 35 days ahead of the allotted 75 days and received the maximum incentive amount.
The bridge and interchange opened to traffic on May 23, 2012. In June 2012, U.S. Senator Max Baucus and then Governor of Montana Brian Schweitzer attended a formal opening ceremony.
"The speed [at which] this project was constructed is a testament to all [who] were involved and can prove that working together, we can streamline the process, while ensuring that quality is maintained,” FHWA Division Administrator Kevin McLaury said during the ceremony.
By hosting an Accelerated Construction Technology Transfer workshop, MDT was able to hone in on the best options for completing the interchange project in a much shorter timeframe. The Custer Avenue Interchange was not a "business as usual” project and it required flexibility within all phases. By showcasing the possibilities of accelerated construction, MDT and FHWA created momentum to move all levels of leadership to commit to completing additional design work that helped reduce time and impacts to the community during construction. In the end, combining a thorough design phase, including traffic modeling and stakeholder involvement, with a contract that offered flexible options for the contractor equaled a winning combination for MDT’s Custer Avenue Interchange.
Mark Studt, P.E., is a consultant project engineer for MDT. Studt was the State’s consultant design project manager for the Custer Interchange. He has a B.S. and M.S. in civil engineering from Montana State University.
Alan Woodmansey, P.E., is an operations engineer in the FHWA Montana Division. He is the Federal oversight engineer for the district where the Custer Interchange is located. He has a B.S. in engineering from the United States Military Academy and an M.S. in engineering management from the Missouri University of Science and Technology.