Austin tunnel projects could face challenges from fault lines

November 17, 2020

The Texas Department of Transportation is progressing with plans for a transit tunnel system in downtown Austin as well a tunnel system beneath Interstate 35 to help ease traffic congestion, however, fault lines in the area could present challenges to those plans.

Mark Cloos, a professor at the Jackson School of Geology at the University of Texas at Austin told Austin’s KXAN network that the Balcones Fault Zone runs north and south across central Texas and is visible to the naked eye.

Cloos said that these faults create uneven rock layers that are challenging to bore through. If a bore unexpectedly hits a layer of hard rock, the bore could become damaged and get stuck. Likewise, if the bore hits soft soil, the tunnel could collapse around it.

The other problem with fault zones is caves form around them. A bore that suddenly hits a cave could find itself in empty space.

One of the advantages of tunneling in Central Texas is a geological formation called Austin Chalk, which has proven itself to be suitable for building an underground system in the past in places like Dallas. However, Dr. Cloos says that the layer of Austin Chalk in Central Texas isn’t consistent and has a “highly varied thickness.”

While some layers of Austin Chalk are three hundred feet thick, others are thin, with layers of hard rock and soft soil surrounding them. Tunneling through this material causes a similar problem to tunneling through uneven layers around the faults.

While uneven layers and the Balcones Fault Zone will create challenges for these projects, Cloos says they’re still very doable.

“There’s some ability to detect (any issues) by various types of studies that can be done,” Cloos said. CapMetro has several years of studies included in their construction timeline.

There are over 150 underground metro systems in the world in places as varied as Beijing, Kazakhstan and Washington D.C., proving they can be built just about anywhere.

“Engineers can do anything,” Cloos said, “I don’t know if you’ve ever been to Washington D.C. and seen the tunnels there for the subways. They basically put a subway system in a swamp. If you can deal with that, you can deal with the problems we got here.”

Recently the Boring Co. announced plans to hire tunnel engineers in Austin, a move that has caused speculation that the company will be involved a tunneling project.


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