How The City Determines If Boise Foothills Land Is Safe To Build On

May 9, 2016

The ground under the Boise foothills neighborhood called Terra Nativa has been sliding for weeks, possibly months. One house has been deemed unsafe to live in. At least one other has damage. And late last week the highway district closed two roads in the subdivision due to buckling streets and sidewalks and fear of landslides. 

We wanted to know how the city determines if a site in the foothills is safe to build on. Here’s what we learned.

If you want to build in the foothills you have to hire a particular kind of expert called a geotechnical engineer. He or she does tests at the site like drilling holes and sending what’s dug up to a lab for analysis. The engineer writes a report, which includes recommendations on what needs to be done to build safely on that spot. If you’re building a subdivision you have to do this once for the whole area and then a separate one for each house.

The developer gives the report to the city. Since Boise doesn’t have anyone on staff to analyze the reports, they’re sent to another outside engineer like John Barker from CH2M Hill. Barker reads the report to make sure everything was done properly. He says this relies a lot on trust. If the first engineer is certified by the state board of engineers, the assumption is everything is going to be correct.

But Barker says if something was missed on accident or intentionally fudged, it will stand out.

“There’s very much a linear process from start to finish of how you collected the data you did, how you analyzed and evaluated that,” Barker says. “Theoretically the unscrupulous engineer that would, you know, just make stuff up and hope not to get caught, I think it would be pretty evident.”

The city has said there were no red flags in the original geotechnical report for the Terra Nativa subdivision. A city spokesman says until it's clear what exactly is happening there, they won’t know if they need to reconsider the process for determining if foothills land is OK to build on. Right now the developer has geotechnical engineers trying to figure that out. The city has already hired CH2M Hill to review those findings when they come in. 

Find Adam Cotterell on Twitter @cotterelladam

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