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Precast Bridge Elements Help Zachry Construction Maintain Aggressive Schedule on Second Phase of San Antonio’s US 281 Upgrade

by: Julie Devine
Zachry Construction uses precast elements to accelerate construction on U.S. 281 near San Antonio.
Zachry Construction uses precast elements to accelerate construction on U.S. 281 near San Antonio.
As Zachry Construction Corporation of San Antonio, Texas, works to complete the $178.3 million second phase of Bexar County’s U.S. 281 improvement project, they’ve juggled challenges and silver linings to meet the accelerated construction schedule.

Surprisingly, one consequence of the pandemic actually helped them expedite work, as did unconventional precast elements as part of their Accelerated Bridge Construction (ABC). On the other hand, hidden karst features amid hard limestone excavations occasionally slow their efforts.

“It’s mostly about planning and sequencing the work, and being really efficient to get to all the work possible as early as possible, working multiple spots at the same time,” said Kevin Allen, Zachry’s Project Manager.

The 4.8-mile, Phase 2 upgrade on the north side of San Antonio, Texas, remains on schedule to finish in early 2023. Webber Construction of Houston, Texas, will finish the $192 million, 3.8-mile first phase of U.S. 281 improvements by this fall.

Together, the two phases will upgrade an 8-mile section of U.S. 281 from a four-lane, divided highway with numerous traffic lights into a full expressway designed to enhance traffic flow and safety for more than 91,000 vehicles that drive the route each day, as well as projected traffic growth over the next 30 years.

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The improved roadway includes a total of 10 lanes – two non-tolled, general purpose lanes; a High-Occupancy Vehicle (HOV) lane for those traveling with two or more people (including VIA Metropolitan Transit buses); and frontage roads in each direction. The work also adds new bicycle and pedestrian facilities.

Funded by the Texas Department of Transportation (TxDOT), the City of San Antonio, Bexar County, VIA Metropolitan Transit and the Advanced Transportation District, and federal dollars, Phase 2 of the U.S. 281 work broke ground in March 2019 after TxDOT awarded Zachry the low-bid contract.

Pandemic’s Silver Lining
As with many jobs, “The biggest construction challenge has been dealing with COVID-19 – how to manage around people being in and out and vendors and suppliers dealing with their own issues,” Allen said.

However, the pandemic also provided a silver lining to expedite work. “Once schools shut down in the spring and traffic volumes dropped quite a bit, we asked TxDOT if we could get some additional lane closures that weren’t originally allowed in our contract,” Allen said.

Typically, crews need to leave two lanes in each direction of U.S. 281 during peak hours. During the stay-at-home orders, “TxDOT allowed us to take a lane during peak hours, as opposed to only at night,” Allen said.

The lower traffic volumes especially helped construction on one of the project’s local roads.

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“The project was able to accelerate the work on Borgfeld Road with a complete closure,” explained Danny Gallegos, P.E., TxDOT’s Project Manager and Assistant Area Engineer. “Instead of switching traffic to construct the road in halves, Borgfeld Road was completely closed and substantially complete in less than half the time it would’ve taken to construct in phases.”

In total, “That road closure saved about a month of time,” Allen added.

Precast for Speed
With 12 different bridges included in their contract, Zachry’s ABC efforts also condense construction time while minimizing lane closures.

Before work began, “We approached TxDOT with some alternative ideas – some non-conventional precast elements to speed up construction on several of the bridges,” Allen said.

That included precast overhangs on some of the outside bridge girders, as well as precast caps.

For the caps, “Our fabricator builds them at their plant, then we truck them in two pieces,” Allen said. “Instead of a normal bridge cap that takes a couple weeks to form it, tie all the steel, and pour it, the precast elements allow us to set a whole bridge worth of caps in one day.”

Digging Through Limestone
The project’s excavation, on the other hand, proceeds a bit more slowly.
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“All our excavation is through rock,” Allen explained. “It takes more planning for the cuts because it’s not as forgiving as normal soil or dirt.”

The conditions also require some special equipment, including ground cutter attachments on excavators and rock saws for trenching. For major excavations, Zachry uses huge Wirtgen 2500 Surface Miners, bringing as many as three onsite during the peak of work.

“Because this area of San Antonio and Bexar County is predominately hard limestone, it necessitates this complicated machinery that’s only utilized in areas where you’re excavating through competent rock,” Gallegos said.

Resequencing for Karst
At times, karst features make excavation – and even some drilling – more complicated.

“TxDOT identified a few small karst features before we started,” Allen said. “For those, we took care of some predetermined measures before we started anything else. It changed our usual sequencing a little bit, but we took that into account in our initial plans.”

Since construction began, crews uncovered more than 15 additional, previously unknown karst features that required modifications to existing plans.

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“When we find a new one, we have to stop work within 50 feet of the karst feature, then let TxDOT know,” Allen said. “We keep out of that area while they do their biological investigation, and that definitely causes challenges for the team. We’ve had a couple that affected vertical work, and we’ve had a couple where we needed to change the traffic control to get around them.”

Because of the required adjustments, “The level of coordination and communication with all parties involved has been challenging, yet we’ve gotten through it by following a detailed plan when karst features are discovered,” Gallegos said.

Throughout the work, “Karst features, utility relocations, and corridor traffic have been the biggest overall challenges,” he added. “All three may be summarized as overcome by careful, continuous, and detailed coordination.”

By the Numbers
Phase 2 of the U.S. 281 project includes:
  • More than 380,000 cubic yards of concrete
  • Over 149,000 tons of asphalt
  • 91 retaining walls
  • 12 bridges
  • Over 18,000 linear feet of drilled shafts, with diameters ranging from 26 inches to 60 inches
  • Approximately 1.8 million cubic yards of excavation and embankment
Making It Work
Bexar County’s U.S. 281 upgrade was originally envisioned with toll lanes, but the passage of Proposition 1 and Proposition 7 provided funding that allowed the project to move forward without tolls.

Phase 1 of the work starts at Loop 1604 and ends north of Stone Oak Parkway in San Antonio, Texas. Phase 2 continues the corridor improvements north to the Bexar/Comal County line.

As crews work on the two phases, “Traffic patterns have been coordinated thoroughly between both projects since there is a tremendous amount of work to be performed in the overlapping section,” Gallegos said. “In addition, minimizing impact to traffic between both projects has been coordinated thoroughly so as to not affect each project’s anticipated schedule.”

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