Mark Twain’s fictional Mississippi River rafters, Huckleberry Finn and Jim, would see quite a collection of bridges if they navigated that fabled waterway today. The river awaits landmark new crossings but construction now taking place covers a variety of methods and materials.

Many of the river’s 100-plus crossings are reaching the end of their design life and planners along the length of the river are investigating rehabilitation or replacements. Traffic, economic development and seismic retrofits are spurring the recent spate of work.

"This is a period when bridge construction is flourishing up and down the river," says Steve Hague, bridge engineer with HNTB, Kansas City, and a veteran of some 40 Mississippi River projects. "A lot of the bridges were built in the 1920s and ’30s, so it’s time to look at replacements."

State transportation departments are doing just that. Many future crossings are depending on the transportation bill now hung up in Congress. Still, there are several projects under way.

Building within and over the river banks presents challenges, particularly in shifting soils. "You have an old historic river meandering around; it has deposited materials over thousands of years," says Alec Smith, vice president with Haley & Aldrich, Boston, the geotechnical engineer for a proposed Great River Bridge that would stretch five miles between Cleveland, Miss., and McGehee, Ark.

U.S. Coast Guard engineer David Stupp notes that locks and dams populate the river north of St. Louis, mitigating scouring conditions. By the time the river reaches Memphis, "it’s nothing but mud" and fast currents, he notes. As river barges increase in size, so does the need for new bridges with at least 1,000-ft horizontal and 60-ft vertical clearances.

CONCRETE. Caissons rest on articulated concrete mats value-engineered for the Greenville bridge. (Photo Courtesy of HNTB)

In Greenville, Miss., near a bend in the river, the existing U.S. Route 82 truss is hit about twice a year by barges, says Hague. HNTB designed a $200-million crossing 1,500 ft downstream that will not only reduce collisions but stand as North America’s third-longest cable-stayed span when completed in 2006.

HNTB designed the original truss, and Massman Construction Co., Kansas City, built it. Now, Massman, in a joint venture with Traylor Brothers Inc., Evansville, Ind., is building the new bridge, with its 1,378-ft-long main span. The team value-engineered a proposal to use articulated concrete mats rather than willow mats during caisson installation to cushion the riverbed from scouring. Massman got the idea from the Army Corps of Engineers, which uses the concrete mats for bank protection. "From a time standpoint it was faster because the Corps already has a stockpile," says Henry Massman, president of the firm.

Mitch Carr, Mississippi Dept. of Transportation bridge engineer, says the method saved $500,000 and two months. A special Corps machine rolled mats in 124-ft widths and sunk them into the river, overlapping three sections to create a 314 x 403-ft footprint. The larger of two caissons is 121 x 70 x 98 ft; both reach about 180 ft below the riverbed to support approximately 200-ft-high towers.

Crews are just beginning the roadway portion. MDOT plans to let contracts for the approaches this fall for the Mississippi side and next spring for the Arkansas side. Structures include welded plate girder and precast concrete spans on drilled shafts.

The superstructure will include seismic shock absorbers consisting of 3-ft-dia cylinders containing flowing silica putty to allow the bridge to move laterally in a quake, says Hague. The putty was previously used in California’s Carquinez Bridge and upriver, at Cape Girardeau.

The new $100-million cable-stayed Cape Girardeau bridge between Missouri and Illinois, including a 1,150-ft main span, opened last December. The $2-million demolition of the old truss bridge began in August. The third phase dropped not only the intended 671-ft span but caused an unintended chain reaction that dropped the other 671-ft main section and part of a pier. MoDOT must decide whether demolition is still needed for the remaining 300 ft. click here to view map

Further south, engineers are seismically strengthening the crucial Interstate 40 link to downtown Memphis. The 35-year-old crossing, situated with the active New Madrid seismic zone, consists of two 900-ft tied steel arch spans connected to a 1,860-ft-long steel box girder. Its approaches are built of mostly prestressed concrete spans with some steel plate, says Dennis Cook, Tennessee DOT assistant chief of planning. The $171-million job is Tennessee’s largest seismic project.

Imbsen & Associates, Sacramento, began retrofit design 10 years ago. So far $74 million in work has been completed, with bids for rebuilding precast concrete girders as steel beams to come next year, says Fred Stephenson, project manager. Massman built 18 friction pendulum bearings, which resemble giant woks and allow the piers to move up to 22 in. in any direction, and bolted decks to floorbeams to make them composite. New cast-in-place drilled piles, shear blocks, stiffener plates and other methods should help the bridge withstand a catastrophic event and enable the crucial truck route to reopen quickly.

Tennessee DOT hopes to begin a feasibility study soon to identify locations for a new crossing to provide an alternative to I-40. Estimated at $600 million, the new bridge would carry highway traffic and freight trains, says Cook.

Seismic design for a new Mississippi River bridge in potentially liquefacting soil is a familiar and evolving challenge. The proposed Great River Bridge, now finishing design, would be whopping 25,000 ft long with a main channel span of nearly 1,500 ft. Due to the river’s chameleon nature, an old channel is now filled with soft clays, which will require drilled shaft foundations as deep as 130 ft, notes Haley & Aldrich’s Smith. The firm has recommended a $1-million separate contract for preconstruction load tests that may eliminate contingencies and unknowns in the main bid. Arkansas DOT is considering the idea while waiting on funding.

In Louisiana, state engineers are addressing funding issues with new design-build legislation. A $200-million bridge near St. Francisville, expected to be complete by 2011, will follow a $4-million design-build job over the Tensas River. The state...