Understanding What’s Happening Under Boise’s Sliding Foothills Neighborhood

May 25, 2016

The road is still closed to a Boise foothills subdivision where the land is slowly sliding beneath some high-end houses. We wanted to get a better understanding of what's happening underground. So, we spoke with a long-time Boise geologist. Not many people know as much as Spencer Wood about what’s happening under the grass of the foothills. The now emeritus Boise State University geosciences professor has been writing about the land here for decades.

What are the foothills?

The Boise foothills are soft. They’re almost entirely sand and silt.

“…that were deposits of the large lake that occupied this area, oh about four million years ago,” Wood says.

Later tectonic shifts he says, dropped part of the valley floor, leaving the hills to the north.

Wood says that soft sand and silt is not what makes the foothills potentially problematic for building. Here in northeast Boise where Wood is standing, the sand is mixed in with a few kinds of rock and, this is the important part when it comes to building, clay.

“The clay is weak,” Wood says. “It’s slippery. It slides, if it’s wet. Now when it’s dry of course it’s hard. But soon as it gets soft it just has no strength at all. As the water levels rise they tend to … I don’t quite want to say float. But they create an upward pressure. And that causes it to slide.”

Not just a theory

Looking to right or left Wood can see Table Rock or Castle Rock. And straight up are some expensive houses Wood says are sitting solidly on sandstone. A few blocks behind those is the Terra Nativa subdivision where the sliding ground has made one house uninhabitable and threatens others.

Wood takes out and reads from a paper he wrote in 1983 about this area.

“The basalt tuff unit crops out mostly in the eastern foothills,” he reads. “Because of the abundant clay in and derived from this unit, planning and construction near areas of the outcrop of the basaltic tuff should be accompanied by a thorough geo-technical investigation.”

Basically, Wood is saying that if you’re going to build in the eastern end, you’ve got to study the land more intently than you would even in the rest of the foothills. The problem here?

“Well I don’t know if they were that thorough,” he says.

Not shy about assigning blame

Wood says you can build safely in the Boise foothills. He notes he's lived in them for more than 40 years. He says you can even build safely in areas with this slippery clay if you avoid steep slopes and you’re very careful about water drainage.

But Wood thinks Terra Nativa is an example of not using enough caution.

“I am disappointed in the professional community that did not recognize it,” he says. “It was a very recognizable situation.”

Now he takes out two glossy eight by tens. They’re aerial photos. One is a recent shot of Terra Nativa and the other is the same spot before the houses were built.

“This is back in 1998 and I don’t know if you can see it but that’s an old landslide scarp right there,” he says pointing to a landscape feature. “And that’s really clear to me.”

With a laugh Wood says these pictures will definitely end up in court as exhibits in the inevitable lawsuits.

Not just Terra Nativa

Wood says the engineers should have known that the Terra Nativa site was prone to landslides and not given developers the OK. But beyond that he says there are a lot of houses in the foothills that are built on slopes that are too steep. He thinks the entire Boise geo-technical engineering community needs to be more cautious.

“They’re the key,” he says. “They’re the ones who basically put a warranty on an area and say this is safe for development. And I believe they’ve been a little cavalier in allowing some of these areas to be developed.” 

This is the part of the story when we'd typically bring in the voice of a geo-technical engineer to defend the industry. But we can't because they’re not talking. Strata, the company that did the original study of the Terra Nativa site, did not answer requests for an interview. Engineers at several other companies that have done work in the foothills either declined to talk or did not return calls.

Geologist Spencer Wood says he hopes that the people who may lose their homes in Terra Nativa get compensated. He says they took the word of experts that their homes were built on solid ground.

Find Adam Cotterell on Twitter @cotterelladam

Copyright 2016 Boise State Public Radio