DIRT ALERT

By Justin Velasco

Update: Caltrans officials said Monday that the transition
road from the westbound 10 freeway to the northbound 57 freeway
will remain closed indefinitely while a plan is formulated
stabilize the hillside.

Last week’s landslide may have caused headaches for
commuters, but it could provide educational opportunities for Cal
Poly geology students

Last Thursday’s landslide may have resulted in a traffic
nightmare for commute-weary locals, but for a Cal Poly geology
professor, it represents an educational gold mine.

“Certainly we’ll be talking about it in our classrooms,” said
Jon Nourse, chair of the Geology Department. “You’re talking about
a natural laboratory ” and it’s right here in our backyard at Cal
Poly.”

Caltrans worked earnestly over the weekend to clear the slide,
which sent approximately 2,500 cubic yards of earth tumbling onto a
transition road that leads from the westbound 10 freeway to the
north 57 freeway, even as weekend rains threatened to hamper
progress.

Originally, Caltrans officials said the cleanup might take as
long as a week, but later reports said the roadway could be open by
Monday.

Considerable progress had been made by Sunday afternoon, with
much of the dirt having been trucked away.

A large plastic sheet was placed over the steep cliff at the top
of the slide to protect it against rainfall.

Nourse said the cliff represented the most unstable part of the
hillside because it had not yet collapsed and it would have to be
graded to be rendered safe.

He expressed excitement over nearly every aspect of the slide,
from the fact that it took approximately 50 years for the hill to
collapse after it was cut for the freeway, to the speed at which
Caltrans worked to clean it up.

“I’m surprised they didn’t take dynamite to [the slide],” he
said. “It was amazing that they actually had their trucks up there
[cleaning up].”

To some, it seemed strange that a landslide would occur during
what had been unseasonably warm winter weather, but Nourse said
there is often a delay between a rainy weather and when a landslide
occurs.

“It’s not unusual for this to happen a week or two or even a
month after a rain,” Nourse said, adding that the delay is
representative of the time it takes for the rainwater to percolate
through the ground.

Nourse said the water works to facilitate a landslide in three
ways: by increasing the weight of the earth; by creating a pressure
that expands the space between soil particles; and by lubricating
the soil, especially when clay is present.

Those effects combined with a relatively steep slope and earth
that was labeled as unstable as early as the 1950s to create what
Nourse called a “textbook example” of a rotational slide.

“It’s dramatic how close [USGS diagrams look] to what you see
out there today,” he said.

Nourse, who was on the scene on the day of the slide, said he
will work to find an arrangement for taking his students out to the
site.

“I’d really like to be able to get out there and look closely,”
he said. “It would be especially good ” to be able to take the
students out there and show them ‘here’s a real event that happened
and the mechanisms that caused it.'”

Reach Justin Velasco at: news@thepolypost.com

A tumultuous year comes to an end

Justin Velasco/Poly Post

A tumultuous year comes to an end

DIRT ALERT

Illustration by Justin Velasco/Information courtesy of Jonathan Nourse

DIRT ALERT

DIRT ALERT

Illustration by Jonathan Nourse/Justin Velasco

DIRT ALERT

DIRT ALERT

Illustration by Jonathan Nourse, professor of geology, California State Polytechnic University

DIRT ALERT

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