FLASH BRIEFING

Holes delay MoPac project

Filling 'karst features' in highway's path will slow construction timeline

Phil Jankowski pjankowski@statesman.com
Construction workers marked an area where so-called karst features were found near MoPac Boulevard, north of LaCrosse Avenue. Addressing the fissures will delay construction along the highway, transportation officials have said. [RICARDO B. BRAZZIELL/AMERICAN-STATESMAN]

Construction along a stretch of MoPac Boulevard is facing delays after crews discovered dozens of fissures and holes in the ground along the freeway that help recharge the Edwards Aquifer and Barton Springs.

The discovery of 72 so-called karst features has forced the Texas Department of Transportation to redesign the La Crosse Avenue bridge over MoPac (Loop 1) and push its opening to late this year at the earliest. TxDOT has not given a firm timeline on how much of a delay the finding will have on its overall plans to upgrade the highway's intersections with Slaughter Lane and La Crosse Avenue.

TxDOT is building underpasses at the two intersections. Already in place at Slaughter Lane is the novel "diverging diamond intersection," which forces eastbound and westbound motorists to, in effect, switch sides just before getting to MoPac.

La Crosse Avenue will have a conventional intersection.

The $53.5 million project was slated for completion in early 2021, according to TxDOT.

"Construction has experienced a delay, due to the discoveries, but we continue to work diligently to get the project completed as quickly as possible," TxDOT spokeswoman Alex Navarro said in an email.

Karst refers to the limestone that prevails in the ground in the Austin area, especially in Southwest Austin. Karst features are fissures, holes or caves in the rock that can serve as intakes for rainwater, which eventually makes its way to the Edwards Aquifer and Barton Creek.

TxDOT will have to spend roughly $600,000 to mitigate the holes. The Texas Commission on Environmental Quality has oversight of proposed mitigation plans, which could include filling the holes in with a gravel mixture.

The impact of filling in any of the karsts to continue construction was not immediately clear. However, any mitigation efforts must first be approved by TCEQ. The Barton Springs Edwards Aquifer Conservation District is also involved in discussions.

The holes need to be filled to ensure the structural integrity of the highway.

Many of the features lie in the direct path of MoPac's freeway extension. The agency will not reroute the highway project but has to halt when a new fissure is found.

"Each time a unique feature is discovered, they pause construction in that area and barrier it off with hay bales and caution tape," said Vanessa Escobar, senior regulatory compliance coordinator with the Barton Springs Edwards Aquifer Conservation District. "They just kind of isolate the area, and everyone can kind of discuss it together" with TCEQ.

Similar limestone fissures forced the Central Texas Regional Mobility Authority to reroute part of the Texas 45 Southwest tollway flyover to MoPac, which opened last month.

Escobar and the district had an oversight role on that project as part of a legal settlement. Their participation in the MoPac project is less formal, but staffers at the district have been involved in mitigation discussions.

"It is very common to see these kind of features at any point in the Edwards recharge zone," Escobar said. "The overall big picture is, (the karsts) take in the water, rainwater, and that ultimately makes it to the aquifer and Barton Springs. You have municipalities and rural folks that depend on groundwater."

Environmental concerns have been at the fore of the MoPac intersection project since at least 2016, when the Save Our Springs Alliance sued TxDOT to force the agency to submit to a federal examination of the project's effects on endangered species, such as the Barton Springs salamander and the golden-cheeked warbler.