Taking a landslide's temperature to avert catastrophe

Date:
June 15, 2020
Source:
Duke University
Summary:
Engineers have developed a comprehensive model of deep-seated
landslides and demonstrated that it can accurately recreate the
dynamics of historic and current landslides occurring under varying
conditions. The model points to the temperature of a thin layer
of clay at the base of the landslide as critical to the potential
for sudden cataclysmic failure.

The approach is currently monitoring a landslide in Andorra and
suggests methods for mitigating the risk of its escalation.



FULL STORY ========================================================================== Engineers from Duke University have developed a comprehensive new model of deep-seated landslides and demonstrated that it can accurately recreate
the dynamics of historic and current landslides that occur under various conditions.


========================================================================== Peering past the standard measurements of velocity and water levels,
the model points to the temperature of a relatively thin layer of clay
at the base of the landslide as critical to its potential for sudden cataclysmic failure. The approach is currently being used to monitor an evolving landslide in Andorra and suggests methods for mitigating the
risk of its escalation as well as any other future deep-seated landslides.

The results appear online on June 15 in the Journal of Geophysical
Research - - Earth Surface.

"I published a paper more than a decade ago that explained what happened
at the Vajont Dam, one of the biggest humanmade disasters of all-time,"
said Manolis Veveakis, assistant professor of civil and environmental engineering at Duke.

"But that model was extremely limited and constrained to that specific
event.

This model is more complete. It can be applied to other landslides,
providing stability criteria and guidance on when and how they can be
averted." The disaster Veveakis is referring to occurred at the Vajont
Dam, one of the tallest in the world at 860 feet, in northern Italy in
1963. After years of attempting to mitigate a slow, incremental landslide
of roughly an inch per day in the adjoining mountainside by lowering the
water level of the lake behind the dam, the landslide suddenly accelerated without warning. Nearly 10 billion cubic feet of rock plummeted down
the gorge and into the lake at almost 70 miles per hour. That created
a tsunami more than 800 feet tall that crashed over the dam, completely
wiping out several small towns below and killing nearly 2,000 people.

Before the catastrophe occurred, scientists did not believe any potential landslide would result in a tsunami more than 75 feet tall. They remain
puzzled at how this landslide had moved so violently and so suddenly.



==========================================================================
In 2007, Veveakis put the pieces together and developed a model that fit
the scientific observations of the disaster. It showed how water seeping
into rock above an unstable layer of clay caused a creeping landslide,
which in turn heated up and further destabilized the clay in a feedback
loop until it rapidly failed.

"Clay is a very thermally sensitive material and it can create a
shear band that is very susceptible to friction," said Carolina Segui,
a PhD candidate in Veveakis's laboratory and first author of the new
paper. "It's the worst material to have in such a critical place and
is a nightmare for civil engineers constructing anything anywhere."
This early model, however, used only the last month of data from the
Vajont Dam, when the water level was almost constant. It ignored any
sort of groundwater variation, essentially assuming that the external
loading remained constant. While that model worked to explain the
unexpected failure of the Vajont landslide, the model's assumptions made
it impossible to offer real-time assessments or use in other scenarios.

In the new study, Veveakis, Segui and Hadrien Rattez, a postdoctoral
researcher in Veveakis's laboratory, plug the old model's holes and
provide the ability to incorporate a combination of time-dependent
external loading and internal degradation. The resulting model is
able to recreate and predict observations taken from very different, deep-seated landslides.

"Traditional landslide models have a static internal material strength,
and if you exceed it the landslide fails," said Veveakis. "But in
examples such as these, the landslide is already moving because its
strength has already been exceeded, so those models don't work. Others
have tried to use machine learning to fit the data, which has worked
sometimes, but it doesn't explain the underlying physics. Our model incorporates the properties of soft materials, allowing it to be applied
to more landslides with different loading characteristics and provide an operational stability criterion by monitoring its basal temperature."
Besides using the model to recreate the movements of the Vajont slide
and explaining the mechanisms underpinning its motion for more than
two years, Veveakis and Segui show that their model can accurately
recreate and predict the movements from the Shuping landslide, another slow-moving landslide at the Three Gorges Dam in China, the largest dam
in the world. But while that landslide is also the result of a humanmade
lake beside a dam, that's where the similarities end.



========================================================================== Before the Vajont Dam failed, there was a fairly linear relationship
between the lake level and the velocity of the creeping landslide. The
lower the lake level, the slower the landslide. The Shuping landslide,
however, behaves in the opposite manner -- the lower the lake level,
the faster the landslide. And while the relationship between lake level
and velocity was roughly linear at the Vajont Dam, the velocity of the
Shuping landslide is non-linear, responding to additional sources of
water and loading, such as seasonal monsoons. It is also composed of
different materials.

Despite these differences, the researchers' new model is able to
accurately reproduce the Shuping landslide's movements over the past
decade.

In this case, the researchers do not have direct access to measurements
taken from the shear band, which is less than one meter of brown breccia
soil and silty clay. They have to make assumptions about the levels of
friction and the internal temperatures to make their model work.

In the mountains of Andorra, however, the slow-moving El Forn landslide threatens the safety of a nearby village called Canillo and is being
closely monitored by the government. Unlike China or Italy, there is no
dam or lake involved -- this landslide is being accelerated by melting
snow feeding the groundwater levels in the mountains above the city.

Even though the conditions are completely different from the previous two landslides, the researchers are confident their model is up to the task.

Thanks to numerous boreholes that have been taken to gain a better understanding of the El Forn landslide, Veveakis and Segui have been
able to insert thermometers directly into the shear band of a small lobe
that is sliding faster than the rest. With this level of data available,
the researchers expect to validate and refine their model even more,
and even provide advice as to how to avoid a potential catastrophe should
one begin to develop.

"One could imagine pumping water out of the ground, or circulating
another cold fluid through the shear layer to cool it down and slow
the landslide," said Segui. "Or at the very least, if we couldn't stop
it, to provide enough warning to evacuate. That is exactly why we are
there." This research was supported by the National Science Foundation (CMMI-2006150).


========================================================================== Story Source: Materials provided by Duke_University. Original written
by Ken Kingery. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. C. Segui', H. Rattez, M. Veveakis. On the stability of
deep‐seated
landslides. The cases of Vaiont (Italy) and Shuping (Three Gorges
Dam, China). Journal of Geophysical Research: Earth Surface, 2020;
DOI: 10.1029/2019JF005203 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200615155112.htm

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