Saturday, February 02, 2013

Making tunnelling under downtown safe

Dig a tunnel for Highway 99 under dowtown beneath many buildings? Under another tunnel? Yes. That is what they are doing. What could possibly go wrong? Of course they have done the civil engineering to make sure it is deep enough and that the soil will hold above it. But what if?

A team of engineers and technicians are putting sophisticated sensors in place to get early data on any potential problem. They are using at least seven types of sensors to watch 200 buildings.

For example: Extensometers measure the solid density. If it drops the boring has probably encountered a void in the soil. But the primary method is at the boring machine: checking the soil being removed and watching for changes in the density of the soil at the cutting heads.

History: What has gone wrong? I don't think I heard about the following recent history, which is in the Seattle Times story about the sensors:

Seattle Times
A few years ago, soil-measuring errors caused a void that nearly swallowed a house above Sound Transit’s Beacon Hill Tunnel. Contractor Obayashi Corp had to reimburse taxpayers $4 million to locate and fill gaps.
The photo: Sensor on the exterior of a Pioneer Square building. Seattle Times

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