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Building Bridges Better

Taking a Deep Dive into I-35’s Southbound Mainlane Bridge Construction

Back to Volume 10, Number 4


Lately, curious commuters along I-35 have been treated to an out-of-the-ordinary construction scene: Weber LLC—the contractor hired by the Texas Department of Transportation (TxDOT)—hard at work demolishing and rebuilding the I-35 southbound mainlane bridge over the Brazos River. Overseeing the effort is construction and engineering firm BGE, Inc. To drivers, it might look like one more construction zone, but working on the river carries with it unique safety concerns and demands special construction practices.

Good planning is vital when replacing bridges. The Texas Department of Transportation (TxDOT) spearheads the planning process way ahead of the actual construction effort. TxDOT provides contractors with a detailed project plan, secures the proper building and environmental permits necessary for construction, and coordinates with regulatory agencies like, in this case, the Brazos River Authority and U.S. Army Corps of Engineers. This measure twice, cut once method maximizes the odds that construction will go as smoothly — and safely — as possible.

A crane on a barge lifting a steel girder

Crews from Webber LLC transport a center-span steel girder for installation on the I-35 southbound mainlane bridge over the Brazos River.


“I have to commend TxDOT for how well this project was planned,” BGE Project Manager Blake Axen says. “The department’s approach was efficient, thorough and accounted for just about every contingency, from the type of bidding process used (A+B or cost + number of days to complete) to accounting for how businesses would be affected.”

Part of that prep work is engaging local stakeholders in the Waco community. For example, Baylor University uses the Brazos River for its athletic program, so making sure construction didn’t interfere with university events was vital. TxDOT and BGE also talked with business interests and local officials in Waco, as well as private organizations that might be planning events on the river. All that coordination was done up front before the first piece of bridge deck was even removed.

Making sure construction doesn’t adversely impact the environment was also a principal concern during the planning stage. Swallows and other migratory birds often nest in and around a bridge’s superstructure, and Axen says a contractor must be proactive and mindful of migratory patterns when making the construction schedule. Likewise, during the demolition phase — when the old bridge structure is destroyed to make room for the new — making sure debris doesn’t affect the ecology (or other structures) downriver is mapped out in detail.

Keeping tabs on water levels is important, too, and not just to be environmentally responsible. High rainfall can create flooding, and that can present a safety hazard. Workers on the river move hundreds of tons of concrete rubble using a crane sitting on a barge. But the barges and cranes are so well engineered that day-to-day river activity like boating don’t present a significant danger.

A worker in a harnes inside a shaft.

Workers place underwater concrete for the 108-inch diameter drill shaft.


“Water levels affected by heavy rainfall are always a concern, though,” explains Chief Inspector Kyle Dillow with BGE. “But we’ve been really lucky on this project.” In fact, Webber was able to complete the foundational work for the southbound mainlane bridge and build all the necessary drill shafts for the new southbound bridge without missing critical time due to inclement weather.

Before demolition began, a traffic control management plan was created to monitor and control waterborne traffic. Off-duty police officers patrolled the river during demolition operations to ensure everyone’s safety.

Demolition of the original bridge columns required cutting the shafts near the mudline — i.e., one foot above the average bottom line of the river — and lifting the columns via those barge-based cranes mentioned earlier. The work can be tedious because the process is slow to ensure safety protocols are followed to the letter. Once the old columns are cut and removed, what’s left at the mudline are nubs of the old shafts. Underwater, the nubs resemble a former forest of concrete tree trunks.

The traditional approach to removing the old columns involves sending certified divers down to cut a ring around shafts before the cranes take over; that was the original plan on this project. But to enhance safety, the contractor employed a different strategy, using mechanical cutters instead of human divers working below the surface.

“We called it a mechanical cigar cutter, and the concept is pretty simple,” Dillow says. “The cutter slides over the shaft to be removed, then descends into the water until it reaches the river bottom. We then raise it a foot to the mudline, and the cutter does the work.”

Workers on a barge over the Brazos River

Webber crews are seen here lifting and removing old bridge columns from the Brazos River after the mechanical cutter operation.


Once the old shafts are removed, new shafts are seated into the riverbed to support the reconstructed bridge lanes. The new columns measure 108 inches and are sunk 67 feet down. In this case, that means 50 feet of concrete column is sunk below the mudline.

Begun in April 2019, I-35’s southbound mainlanes (including the bridge over the Brazos River) should be completed by summer 2021, when work on the northbound side will kick in to high gear. Lessons learned in that process will help make an already efficient, and safety-oriented construction process for the northbound mainlane bridge over the Brazos just a little bit more efficient, and a little bit safer. The northbound mainlanes and bridge over the Brazos is scheduled for completion by fall 2022.

A few of the significant, long-term benefits to Waco of the new bridges — and the My35 project in general — are already evident. Increasing each direction’s capacity from three to four lanes will improve the free flow of traffic through Waco, which will be particularly helpful during high-traffic events like Baylor football games. Modern design elements are now implemented (e.g., on-ramps) that emphasize safety and mobility over their predecessors. And a new traffic control management system that provides messages along the roadway are already alerting motorists north and south of the city of any traffic issues.

“We’re getting this project done in a time frame that is, I have to say, extraordinary,” Axen says. “TxDOT and local stakeholders have really worked together to help the Waco community continue the kind of success we’ve seen in recent years through initiatives like the city’s downtown revitalization project. Speaking on behalf of BGE, we’re proud to be part of the team that’s making that success happen.”

Back to Volume 10, Number 4

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Jake Smith
I-35 Public Information Officer
254-867-2705
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TxDOT Waco District
100 S. Loop Drive
Waco, TX 76704

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