With San Antonio’s Interstate 35 corridor already over capacity – and exponential growth expected throughout the area in the coming years – the Texas Department of Transportation (TxDOT) needed to find a way to reduce congestion and improve safety. However, because of commercial and residential development along the corridor, conventional widening wasn’t a viable solution.

Instead, the I-35 Northeast Expansion (NEX) project will add capacity via elevated, non-tolled lanes built on two new bridges between existing main lanes and frontage roads. Two general-purpose lanes and one high-occupancy vehicle (HOV) lane in each direction will expand I-35 from eight to 14 lanes while leaving the existing facility as is. Broken into three sections (Central, South, and North), the project covers a total of approximately 20 miles on I-35 from I-410 South to FM 1103 in Bexar, Comal, and Guadalupe counties.

For the first phase – the $1.5 billion I-35 NEX Central project – TxDOT awarded the design-build contract to Alamo NEX Construction, LLC (ANC), a joint venture led by Ferrovial Construction and Webber LLC. Design work started in October 2021, construction began in June 2022, and the team anticipates project completion in late 2027.

The I-35 NEX Central project will add approximately 9.5 miles of elevated lanes from I-410 North to FM 3009, with direct connections to I-410 North and Loop 1604 West. Covering 5.9 million square feet of bridge area, the new lanes will require construction of more than 800 bridge spans.

The project also incorporates elements to transition the elevated lanes and connectors with the existing highways. That includes constructing or reconfiguring 20 ramps – 12 on I-35, three on I-410 North, and five on Loop 1604 – as well as revisions to frontage roads, drainage, utilities, and other highway improvements.

Financed through federal funds, the Texas Clear Lanes initiative, and other state funds, the I-35 NEX Central project spans the cities of San Antonio, Live Oak, Selma, and Schertz, Texas, in Bexar and Guadalupe counties.

In planning for the three-phased I-35 NEX project, TxDOT determined that with the extensive development along frontage roads, the existing space between main lanes and frontage roads would allow for construction of the new bridges and minimize disruptions to local businesses and neighborhoods.

In addition, studies in the 1990s and early 2010s showed that a significant percentage of traffic passes all the way through the corridor or travels between major interchanges, so TxDOT decided to construct elevated lanes. Longer-distance, regional travelers – including heavy truck traffic – will be separated from local traffic that remains on existing I-35 main lanes, resulting in less congestion there.

As TxDOT proceeded with the I-35 NEX Central project, they chose design-build delivery.

“The project will benefit from the contractor’s innovation in accelerated construction methods such as prefabricated materials,” said Laura Lopez, TxDOT Public Information Officer. “In addition, the contractor is able to develop the sequence of work to minimize impacts to the traveling public. This method also provides opportunities for alternative technical concepts, allowing the contractor to value-engineer the schematic to find areas for innovation or efficiencies.”

Design-build delivery also facilitated starting construction sooner. ANC reached 60 percent design completion in 2022, with a goal to complete 100 percent of the design by summer 2023. The project’s official groundbreaking occurred in May 2022 and construction began a month later.

In 2022, crews set up the first maintenance-of-traffic phase for construction of the Loop 1604 direct connection to I-35 elevated lanes. They also placed all the temporary paving needed to start the first stage of the new bridge construction. In addition, crews started installing drill shafts along southbound I-35 and, at the end of the year, began substructure – underground – work.

In the first half of 2023, crews will continue building the substructure in all project segments along southbound I-35, including the two interchanges, with the goal to start superstructure – aboveground – work in the second half of the year. Along northbound I-35, foundation work will commence in the second quarter, followed by substructure work. Utilities relocation began early this year and will continue through the remainder of 2023.

Most construction work will occur on weeknights, with temporary ramps and temporary road widening to minimize traffic impacts.

Of course, I-35 NEX Central isn’t the only project underway in that fast-growing region of the state.

With so many projects under construction, “This creates a real challenge to ensure adequate resources to meet the project schedule,” said Jose Luis Beltran Simal, Vice President of Construction at Alamo NEX Construction. “In order to mitigate the high demand for resources, ANC has looked for different ways to diversify sources, including precast elements, diversification of resource suppliers, and seeking resources outside of the central Texas market.”

Other aspects of the work also require creative solutions. For instance, with the project’s right-of-way limitations, ANC chose monoshafts – large, single cylindrical foundations – rather than a combination of multiple, smaller cylindrical shafts with a large rectangular footing.

Additional complications come from the variety of large cap elements that will support the elevated lanes over the existing main lanes. In just the Central portion of the project, there are 900 bents and 530 symmetric hammerheads (of which 450 will be precast).

“Precasting them will allow ANC to tap into additional resources, improve construction timelines, reduce traffic impacts, and minimize safety risks associated with working at heights to construct the elevated lanes,” Beltran Simal said.

Similarly, for the structures work, “Our effort has been put into precast solutions to minimize health and safety risks, reduce closures, and expedite the construction schedule,” he said.

In addition, “Innovative solutions are in development for the precast slabs installation that again allow us to find a safer and more efficient way to complete that key item while working in very close proximity to high-volume traffic and utilities,” Beltran Simal said.

Around the I-410 North/I-35 interchange, tight spaces and complex terrain require extensive planning for delivery of large, precast elements.

“Both the length and weight of these elements will play a major role in how they’re delivered to the job site and installed,” Beltran Simal said. “In some instances, we modified the length and size of precast elements to allow for proper delivery and installation.”

Throughout the project, ANC continues to work on mitigating impacts. In addition to construction challenges, “The I-35 NEX Central project has various major interactions with stakeholders such as city governments, emergency services, business owners, community organizations, and homeowner associations,” Lopez said. “We’re working to ensure stakeholder needs are met and that the impacts to local businesses and government entities are minimal.”

As construction for this first phase ramps up, TxDOT is preparing to execute the second phase – the $852 million I-35 NEX South project – in June 2023. Stretching from I-410 South to I-410 North in Bexar County, this project will continue the six elevated lanes and add direct connectors at I-410 South.

The last phase, I-35 NEX North, will add elevated lanes from FM 3009 to FM 1103 in Comal County. That project is currently unfunded.

Today, more than 200,000 drivers travel within the I-35 NEX project limits every day. With that number expected to more than double by 2044, the three projects will significantly reduce travel time while improving safety and mobility.

This project is being completed through a partnership between TxDOT and Alamo NEX Construction (ANC), a joint venture between Ferrovial Construction and Webber.

ANC’s subcontractors and suppliers include:

• A.H. Beck Foundation Co., Inc., Converse, Texas
• Ancortex, Dallas
• Bexar Concrete Works, Ltd., San Antonio
• Heldenfels Enterprises, Inc., San Marcos, Texas
• INDUS Road and Bridge, Dallas
• Road Maker Contractors LLC, Fort Worth
• Texas Concrete Partners, Victoria, Texas
• Tricon Precast Ltd., Houston
• Urban Infraconstruction, Irving, Texas
• Vulcan Materials Company, Birmingham, Alabama

To build the elevated lanes for the I-35 NEX Central project, crews will install:

• 637,000 linear feet of concrete girders
• 20,500 linear feet of steel girders
• 844 bents
• 285,000 cubic yards of concrete (excluding beams)
• 79,000 linear feet of drill shafts

Photos courtesy of Alamo NEX Construction