Tunnel and earthquakes?

Hello everyone, as I mentioned in my last post I am going to use my blog here to answer questions I got during my tunnel lecture I gave last fall.

• What are types of tunnel earthquake reinforcement?

• To reinforce an inner tunnel lining the "normal" structural reinforcement bars are used. I put "normal" in quotation marks because the tunnel reinforcement has specific connection types, main bars in cross section are curved, and often tunnel linings are build segments of about 10 m or 30 ft in length so each edge has terminal bars.
• To get to the question against earthquakes: it depends. Building a tunnel in the ground goes along with a positive behavior against earthquakes - the ground shakes and it is assumed that the tunnel goes with the same movement.
• Challenges are at points of transitioning from a softer soil to, let's say, a hard rock. These points should receive the attention. A similar but more severe subject is the crossing of a fault line, e.g. the water mains in southern California cross the San Andreas Fault.
• Well, how do you build a tunnel at these difficult points? That is a difficult question to answer in just one blog article. It is less about reinforcement and more about the whole design of the tunnel. In March last year (2014), I attended parts of the Tunneling Class with Prof. Sitar at UC Berkeley as a guest listener and a representative from Jacobs Associates presented on the Claremont Tunnel with a diameter of 2.7 m or 9 ft crossing the Hayward Fault line near San Francisco. They constructed a pipe inside a tunnel that can withstand a lateral offset of 2.6 m or 8.5 ft. The whole project of seismic improvement costed $38 Million! Not every water department has such an amount for a seismic upgrade, but a loss of the main freshwater line would cost more after an event. For a more detailed description, the following two pictures have links below them.

Claremont Tunnel Design - Fault Crossing

Claremont Tunnel - Jacobs Associates

The subject of building tunnels across faults is very present and each solution is unique and innovative. And at some places, like L.A. and the San Andreas Fault with an estimated maximal offset of 7 m or 20 ft, solutions do not exist yet.

Thanks for "tuning in". If you have questions or comments, please contact me.

Stephie