Outdated interchange geometry and merging difficulties were causing congestion on a 4.7-mile stretch of I-275 in Tampa, Fla., between State Road 60 and the Hillsborough River. Traveled by an average of 200,000 vehicles per day, the I-275 corridor plays a critical role in the Tampa community and economy. In May 2012, the Florida Department of Transportation (FDOT) initiated a reconstruction project aimed at enhancing safety and creating better traffic flow by awarding a design-build contract to the joint venture of Skanska and Ajax Paving Industries, with WSP | Parsons Brinckerhoff as design engineer.
One of the biggest improvements made possible by the project was construction of five braided interchanges that were designed and constructed to make it easier for vehicles to enter and exit the highway. At the old interchanges, cars were entering and leaving the highway in the same space. The new interchanges eliminated those conflicts. In addition, I-275 was expanded from three to four lanes in each direction, and the width between the lanes was increased to accommodate future lane expansion, if needed. A total of 21 bridges and more than 1,000 drainage structures were built as part of the project.
Keeping the traffic moving
The I-275 reconstruction project included a number of challenges, among them expanding the roadway and reconstructing five interchanges while maintaining movements on, off, and through the interstate. Because this segment of I-275 is highly urbanized, it traverses a network of city streets, most of which predate construction of the interstate in the 1960s.
By carefully analyzing current and projected volumes, the WSP | Parsons Brinckerhoff design team was able to sequence the closing and opening of ramp facilities between interchanges to allow access to commercial and residential areas. The sequencing maintained access while also maintaining levels of service and preventing weave/merge problems and traffic queuing on the interstate main line. The result was a maintenance of traffic scheme that provided for continued public use of the roadway while allowing the maximum possible work zone for the contractor. Maximizing access for the contractor was one of the biggest contributors to enabling construction on the project to be completed 139 days ahead of schedule.
Much of the initial work involved demolishing the existing infrastructure. However, this presented the challenge of ensuring that demolition would support the construction of new facilities. This was a factor that played heavily into development of the temporary traffic control plans. For example, many of the old bridges were preserved after traffic shifts to allow for haul routes for construction materials, which served several purposes. First, it was essential because construction staging took place in highly constrained right-of-way, so maintaining direct haul routes afforded maximum economy. Second, because of the project’s urban character, it minimized the amount of disruption in residential neighborhoods and commercial areas.
In addition to maintaining interchange functionality and creating smart, efficient work zones, a primary objective of temporary traffic control was maintaining capacity for the 200,000 vehicles traveling daily through the corridor as a new roadway was being constructed quite literally on top of the existing freeway. In fact, for much of the alignment, the proposed main line was in direct conflict with the existing facility. By tying new pavement to old, reversing flow on existing pavement, and partially demolishing and constructing bridges where footprints conflicted, existing capacity was maintained at all times.
Equally important was appropriate temporary signage and pavement marking, as good communication to the traveling public was essential to driver expectancy and safety. To this end, temporary signage was developed that was effective in terms of both cost and communication. Signage was relocated to existing overhead trusses and partially removed overhead structures. Pavement shields and other pavement markings were employed where maintaining static signs was impractical.
While maintaining main line capacity in the urban area was imperative, another important consideration was traffic management on the streets that feed into, from, and through the interstate system. Dale Mabry Highway, an urban principal arterial with ramp connections on I-275, is one of the busiest arterials in Tampa. The scope of improvements to Dale Mabry included widening to provide additional through capacity (one more lane in each direction) while creating sufficient space for the median piers for the I-275 bridges spanning overhead. Constructing these features while maintaining existing bridge openings (much narrower than proposed), and the existing four-span configurations, was key to delivering the project $35 million below the original estimate.
With an average annual rainfall of 46 inches in the Tampa region, managing stormwater is a major part of roadway safety. This task on I-275 was addressed with more than 1,000 drainage structures, more than 10 miles of pipe, and five stormwater ponds (in addition to four pre-existing ponds). While the original design specified lined ponds to increase volume, through prudent redesign and efficient use of available space all but one lined pond was eliminated in lieu of considerably less expensive unlined ponds. It was determined that the unlined ponds would provide sufficient attenuation volume for both the project and the ultimate lanes, even with the comparatively shallow groundwater elevations found in this part of Florida.
The lined pond that remained was critical to providing the necessary volume for stormwater conveyed to it by an 84-inch pipe within a very small parcel of available right-of-way. This required a design that provided sufficient mass for the pond liner to be held below the groundwater elevation in an empty condition. Achieving these elevations involved excavation through porous lime rock. During construction it was determined that the lime rock layer was much shallower in some areas than the geotechnical borings originally indicated. Through a combination of rapid adjustment of the pond contours and proactive coordination with the Southwest Florida Water Management District, a redesign was performed and the stormwater permit modified with virtually no work stoppage in the field.
As noted, the project involved some of the largest stormwater facilities found on interstates in Florida, including more than 5,800 linear feet of 84-inch pipe discharging into one of the ponds. One of the biggest design considerations for this pipe was ensuring it was aligned in a location where it could be constructed within the right-of-way. More specifically, the space between the existing main line interstate traffic at the time and the right-of-way line abutting a row of residential properties was so tight that other trunk lines, sign structure shaft foundations, mechanically stabilized earth walls, and the 84-inch pipe itself had to be carefully aligned and orchestrated such that the pipe could be constructed using double-stacked trench boxes.
With the myriad drainage structures, pipe, drilled shaft foundations, and bridge piles on this project there was an enormous utility coordination effort. Throughout the corridor, the project interfaced with 12 utility owners and varying utility types ranging from buried fiber and overhead electric to a buried jet fuel line traversing through the FDOT right-of-way. This made effective coordination essential. Wherever possible, structures and drainage alignment were redesigned to avoid utilities. Where conflict was unavoidable, keeping the cost of relocation or removal to an absolute minimum was a primary goal. What made this especially challenging was that fewer than half of the existing utility locations were known to the contractor at the time of bid, and were also unknown to the utilities themselves in many cases. Discovery and coordination continued well into construction.
Keeping stakeholders involved
Another vitally important coordination effort was with the other road operators and public agencies affected by the project. Most of the interchanges connect to streets owned and maintained by the City of Tampa. The project crosses the Hillsborough River over the proposed Tampa Riverwalk, which will be a signature feature of downtown Tampa. Tampa is also home to the Tampa Bay Buccaneers, and Raymond James Stadium is a huge traffic generator within the project limits.
Ensuring the city was engaged and informed about proposed design changes and upcoming traffic shifts (particularly how, when, and why) was important. Other stakeholders included Hillsborough County, the Florida Highway Patrol, the U.S. Coast Guard, Southwest Florida Waste Management District, and the Tampa Port Authority. Lastly, because of the project’s proximity to the Tampa International Airport, coordination with the Federal Aviation Administration was needed to secure 115 individual permits for features encroaching into Tampa International Airport’s airspace, including light poles, overhead trusses, and ITS features.
An aesthetic appeal
A highly visible aspect of the project turned out to be the architectural features included at the interchanges. Decorative architectural features and extensive landscaping were intended to aesthetically integrate the interstate into the neighborhoods through which it passes. This was accomplished by referencing the character of the varied neighborhoods, from historic West Tampa with 100-year-old brick cigar factories, to the commercial centers of Dale Mabry Highway and the Westshore Business district. The design of these features involved engaging the City of Tampa, the public, and several civic organizations, such as the Design Review Committee and the Westshore Alliance, to solicit input and build consensus.
Since work concluded, reaction from both FDOT and the public has been highly favorable. The I-275 improvements — better traffic flow, easier vehicle access, and safer sight distances — have also contributed to its being recognized with three awards for design and construction. Honors won by the project include the Florida Transportation Builders Association’s Best in Construction award in the urban category and the 2016 Project of the Year in the transportation-roadways category by the Design-Build Institute of America, Florida region.
John Dewey is a project manager with WSP | Parsons Brinckerhoff in Tampa, Fla.