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Project Links: Project Home Vicinity Map |
FACT SHEET Franklin Boulevard/Thornton Road Bridge Replacement Project
Project Completed and Opened in September 2005 The Franklin Boulevard/Thornton Road Bridge replacement project is competed and will be opened to the public on September 30, 2005. This project is a great example of providing habitat protection as part of a large ($14 million) bridge replacement effort. This important farm-to-market bridge and roadway has been closed since 1997 when a flood damaged parts of the trestle and approaches. Replacement of the bridge with a new concrete structure should have been routine. However, 40,000 Mexican free-tail bats living in the bridge trestle needed temporary and permanent housing. This project is a joint effort by Sacramento County and San Joaquin County and is funded by the Highway Bridge Rehabilitation and Replacement (HBRR) program with local matching funds.
Background A 1997 flood crippled Franklin Boulevard’s aging 1,700-foot timber trestle which is located near Highway 99 and the Sacramento County/San Joaquin Border. The structure, originally constructed in the mid 1930’s, spanned an overflow floodplain for the Cosumnes River, a riparian environment rich in plant and animal life. The trestle’s southern end connected to a 230-foot steel truss swing bridge, relocated in 1950, that crossed the Mokelumne River, a prime migration habitat for salmon. The bridge was designed as a swing span to accommodate barge traffic, but has not been opened since 1955, after channel dredging ceased. Sacramento County (owner of the trestle and half the river bridge) and San Joaquin County (co-owner of the river bridge) decided to replace both structures.
Bridge Replacement The new bridge over the Mokelumne River was designed as a spliced bulb-T girder structure with a composite-reinforced, cast‑in‑place concrete deck. Two 120-foot side spans flank a 140-foot center span. Each of the two support piers consists of a pair of cast-in-drilled-hole concrete pilings topped with a 4-foot diameter concrete cap. Raising the profile of the river bridge meant increasing the height of the approach ramps as well, which would, in turn, widen the side slopes. To avoid a private residence that abutted the right of way south of the river, the roadway alignment was shifted 5-foot to the west. The new alignment tapers over to the Bean Ranch Road intersection.
Bat Habitat Early in environmental planning phase of the project, it was discovered that the bridge provides habitat (February to October), for a maternal colony of approximately 40,000 Mexican free-tailed bats. Bridge demolition could have harmed the colony, which is a highly beneficial ecological and agricultural resource. To protect the colony, County staff developed an aggressive bat-protection program that was incorporated into old bridge demolition and new bridge construction. The first element of the project was to build temporary bat houses during old bridge demolition. The second part of the process was to incorporate new bat housing into design and construction of the new bridge. To ensure that new bat habitat equivalent to that provided by the original design could be incorporated into the cast-in-place slabs, DMJM+HARRIS designers worked with the Contractor and consulted with advisors such as Bat Conservation International (Austin, Texas). Together, they devised a system of narrow units running longitudinally along the 1,700-foot long trestle, spaced 4-foot apart across the 36.7-foot width of the structure. The exterior units would consist of three parallel 1 x 12-inch redwood planks, separated by 1-inch spacers, to form two 3/4 inch slots, considered the optimal opening width for the expected residents. After the slab was cast, the forms are stripped to expose the deep, vertical crevices for the bats' use. The cost of the permanent habitat, which was designed for at least 600,000 bats, was estimated at $120,000. This effort was successful and all of the returning bats call the new bridge home while in the Sacramento area.
Project Innovation Bridge construction was faster and less expensive then anticipated due to the use of cast-in-place concrete. The original design for the new trestle was for precast, prestressed voided slabs placed on 14-inch and 16-inch concrete piles. The designer expected the bats to roost in gaps left between the slabs. MCM construction submitted a cost reduction incentive proposal to change to a cast-in-place prestressed slab and T-beam system. This included a new concept for the bat habitat that included a “box” section made from two redwood boards, each 1-inch thick by 12-inches wide by 6-foot long, separated by small square blocks to provide a 1inch opening for the bats. The assembled boxes were inserted in the underside of the concrete bridge. This alternative made for better bat habitat then would have been possible with pre-cast bridge sections. This suggestion saved the County $400,000 and also provided a better habitat for the bats.
Flood Plain Part of the complexity of project design was that it was in a floodplain area and the old structure provided a certain amount of flood resistance. The design had to be analyzed very closely to make sure that the new structure would be flood-neutral, neither allowing a greater capacity of flow underneath the trestle, nor restricting it. The challenge for the design team was to make the conveyance capacity of the replacement structure as close to that of the original as possible in order to achieve the desired flood-neutral condition. The design of the overall replacement structure had to improve freeboard at the river crossing, reduce the number and size of substructure elements in the overflow area and keep the resulting water surface the same. The original trestle’s 20-foot spans were replaced with 55-foot spans, reducing the number of bents from 85 to 35. The substructure for the new bridge over the Mokelumne River also presents less of an obstruction to flows. Two bents consisting of 5-foot diameter concrete columns piles replaced the swing span’s 30-foot diameter pivot pier.
Access Road Another component to the overall hydraulic character of the site was the road that came in from the west and joined the trestle in a T-intersection just north of the river crossing. In addition to providing access to a television transmission tower and two gas wells, Bean Ranch Road was the only route for transporting crops from about 2,000 acres of prime farmland, land locked between the Mokelumne river to the south and the sloughs to the north. The embankment supporting this road gave designers a way to compensate for the reduction in support piers. Computer modeling was used to calculate the dimensions of a 200-foot long addition that would be built up along the north side of the existing embankment. The designers also had to consider the impacts the enlarged embankment would have on sensitive fish habitats.
Old Growth Redwood One of the primary environmental concerns with demolition of the timber trestle was conservation. The structure was constructed of clear, prime-grade old-growth redwood. According to mill stamps found on some of the piles, the wood had been milled in the 1920s, and it had been air-dried. More than 260,000 board feet of this valuable material was salvaged from the piles, supports, and deck of the trestle. The decking planks and some of the braces measured 4 x 8 inches, and other braces were 6 x 10 inches. The 20-foot long support timbers were 6 x 16 inches. The contractor collected the wood, stripped it of its lead-based paint, and arranged for storage. Sacramento County advertised the availability of this resource, and found at three markets:
In all, the County the recovered redwood generated nearly $500,000 in cash or offsetting costs. That comes very close to balancing the total environmental mitigation expenses of the project.
Community Asset A project should also be judged by the value it provides to the community. This project succeeds on several counts. Interstate 5 runs roughly parallel to San Joaquin/Sacramento County Route J8, which is known as Franklin Boulevard north of the Mokelumne River and as Thornton Road south of the river. The county route carries primarily local traffic, as well as recreational users such as bicyclists, and visitors to the Cosumnes River Preserve. However, in this particular area, I-5 is built low and it occasionally floods. The bridge replacement was designed to be as functional as possible, in case it has to be used as a relief route during a shutdown of I-5. In addition an effort was made to make the bridge more aesthetically pleasing. The Sacramento County Board of Supervisors felt strongly that motorists on the trestle should have a good view of the landscape. As a result, the structure was equipped with an unobtrusive barrier wall with gaps to increase visibility. To enhance safety without sacrificing the view, a bicycle railing was installed on top of the barrier.
What Makes This Project Special Although replacement projects for roads and bridges are common for Sacramento County, the development of a project that included so many environmental and community benefits is not. The special aspects of this project are summarized below:
Date of Completion September 2005.
Construction Cost $14,000,000
Project Design Firm Tom Barnard, Project Manager, DMJM +Harris
Construction Management Kim Grigsby, Sacramento County
Construction Contractor Jim Carter, MCM Construction Inc.
Sacramento County Contacts
Press Releases/Franklin Backgrounder |
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