Fault lines pose challenge to Austin’s mass transit, I-35 tunnel projects

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AUSTIN (KXAN) – With the approval of Project Connect and the eventual construction of the downtown transit tunnel, Austin seems poised to finally see the construction of an underground metro system. Elon Musk has also announced plans to hire staff in Central Texas for “The Boring Company,” his own underground tunneling company.

Now the Texas Department of Transportation is proposing the construction of a tunnel system beneath Interstate 35 to help reduce congestion on one of Central Texas’ busiest roads.

One of the greatest challenges these projects will face is one that’s laid dormant for millions of years, the Balcones Fault Zone.

“Any kind of tunneling operation through here is going to have to deal the with the fact that faults are present,” says Dr. Mark Cloos, a professor at the Jackson School of Geology at the University of Texas at Austin.

According to Dr. Cloos, the Balcones Fault Zone stretches north to south across Central Texas and is visible to the naked eye. Mt. Bonnell is a result of the fault zone and tiny faults can be found along creeks throughout the area.

Underground, 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.

Tunneling through Austin Chalk

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.

Digging for solutions

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

“There’s some ability to detect (any issues) by various types of studies that can be done,” Dr. Cloos says. 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,” Dr. Cloos says, “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.”

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