Bridging the Yazoo

Contractor drills 84-in shafts for foundation

By Dana Crisson

Even though Mississippi has been experiencing a severe drought throughout the spring and summer, water levels on the Yazoo River and the adjacent Mississippi River have remained high, causing flooding problems that threatened to delay the construction of the new U.S. Highway 61 bridge.

“Many people have difficulty understanding that the Mississippi River has remained in a flood condition in spite of the drought,” says David Trevathan, president of Key Constructors Inc. of Madison, Miss.

“Our situation is unique because the Yazoo is so close to the Mississippi, which is only 1.5 mi away. Backwater from the Mississippi often causes flooding on the Yazoo. However, any time you are building a bridge, you have to be willing to be flexible.”

Trevathan is familiar with this type of situation because Key Constructors has been building bridges, overpasses, dams, culverts and levies since 1975.

The new, 4,000-ft bridge is part of a $32 million project that includes tearing down the existing two-lane bridge, which had structural problems and needed to be replaced. The design for the new bridge, which was drawn up by HNTB Consultants under a contract with the Mississippi Department of Transportation, is designed to allow for expansion at a later date as traffic increases. 

“This project was contracted in August 2006, and work began immediately and has been progressing well, in spite of flooding.”

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“Crossing the river is always the major milestone. We intended to work from the river crossing out to the bridge abutment, but we simply couldn’t afford to have 35 to 40 people sitting around waiting for the water levels to drop, so we had to change our schedule to reverse the process. We went to work on other phases of the project while we were waiting on the water levels to drop.”

The bridge foundation utilizes drilled concrete shafts, driven concrete piles and concrete footings, column and caps. The shafts are being installed by Russo Corp. of Birmingham, Ala.

“We determined that drilled shafts, which are somewhat unusual in Mississippi, were the most economical choice for this project.  We typically use driven pilings here, since the soil is softer. There aren’t any companies in Mississippi that install drilled shafts, so we contracted with Russo for the shafts since they have experience drilling Alabama’s harder soil and rock.”

Russo has drill rigs ranging from small, highly mobile pressure diggers to crane attachments capable of drilling 12-ft-diameter shafts over 200 ft deep.

Trevathan says the bridge requires drilled holes that are 84 in. in diameter, which are difficult to drill without collapsing. A special digger is used to drill the trenches, which are then filled with slurry, a mixture of bentonite and water, to keep them from collapsing. Then, reinforcing steel is lowered into the trenches, and concrete is pumped in from the bottom up, which displaces the slurry.

“A 12- to 14-hour retarding agent is added to the concrete to keep it from setting too soon,” Trevathan adds.  

To cross the main river pan, which is 400 ft long, the superstructure of the bridge will be made of 130-ft precast, prestressed concrete girders and a three-span structural plate girder system.

“This is an unusually large span, so we contracted for steel girders made in North Carolina by Carolina Steel,” Trevathan says.

The deck, or riding surface, is poured-in-place concrete.

Normal weather has a great impact on any construction schedule, which has to be changed depending on the time of year.

“The Department of Transportation allows us a certain number of time units for this project. They take many conditions into consideration when they assign those units, including the weather, so the system is fair. Currently, the project is 50% complete with 30% of the time utilized.”

The new bridge is scheduled for completion by fall 2009, and demolition of the existing bridge will begin as soon as traffic is diverted to the new bridge.

“We are not expecting traffic to be a problem, even though U.S. 61 is one of the major highways in the area, traveling from Baton Rouge to Memphis and beyond,” Trevathan says.

“We should be able to salvage a large amount of materials from the existing bridge. Depending on the market, we will try to sell the majority of the structural steel, and the concrete will be crushed and sold to use in various aggregates.”

The entire project should be completed by spring 2010.