Sizing Up Courthouse Crack

by, CJ Siebeneck

DUE TO THE WAY THEY CONSTANTLY CHANGE, ARE A SOURCE OF USEFUL RESEARCH INTO LANDSLIDES AND HOW THEY HAPPEN.

When landslides and slope failures occur in our built and natural environments, damaging property and threatening life, there’s a scramble to secure reliable assessments to prevent further damage. But what if there were ways to measure the character and instability of rock and soil beforehand and to predict potential disasters?

Recently, PhD student Erin Jensen used seismic resonance measurements to characterize the Courthouse Crack, a potentially hazardous rock slope near Moab, Utah that is part of the Courthouse Mesa. “It’s important to be able to see a site like this in person,” Jensen says, “and really appreciate the size and scale. I get to experience firsthand all the different mechanisms and influences that are happening at a particular site.”

Seismic resonance is an emerging technique within the field of geohazards and has allowed Jensen to collect more data on the Courthouse Mesa instability than can be obtained with traditional approaches.

Perhaps surprising to the uninitiated, structures like buildings, bridges, as well as natural rock formations like arches have natural vibration modes and are constantly in motion at their resonance frequencies. The new technique can help detect and characterize rock slope instabilities. Using sensitive seismic instruments has changed how researchers detect changes in slope stability and what those changes look like.

“Traditional techniques are easy to implement, and fairly inexpensive,” Jensen says. “But the main limitation is that they’re really only measuring the surface of an instability. They aren’t providing much information about the internal structure, or what’s going on at depth.”

Seismic monitoring not only bridges the gap between surface and subsurface techniques but does so without being structurally invasive, though it can be costly. In the end, Jensen used a combination of new and traditional techniques to create a clearer picture of the instability of Courthouse Crack as a whole.

THE MOTHER OF INVENTION

At sites like Courthouse Mesa, traditional methods include expensive means of drilling and field mapping which means measuring the cracks you can see, plotting it out on a map, and viewing the geometry of instability. Alternatively, generating field data with seismic resonance and then coupling the data with numerical models result in an improved picture of crack conditions, which Jensen then uses to describe the instability geometry and how the Courthouse Crack’s stability might fail. “The combination of new and traditional techniques,” Jensen says, “generates an improved picture of landslide behavior and failure development.”

“We aren’t really concerned about imminent failure or any hazard to the public,” continues Jensen, specifically about Courthouse Mesa. “So it’s a really good spot to use as a field laboratory” and to use different seismic resonance techniques to understand work with rock slope instabilities and how they can be applied to different types of landslides, an obvious application for civil engineers, planners, and builders. Jensen’s work is a reminder that scientific inquiry is not just about discovering unknowns in the natural world but also about developing and refining new tools that have broader implications elsewhere. In this scenario, geological necessity has become the mother of invention.

“I came to the U because I was interested in working with Jeff,” she says of Associate Professor Jeff Moore who is her advisor and leads the geohazards research group. His work focuses on the mechanics of processes driving natural hazards and shaping the evolution of bedrock landscapes. Utah is in fact a prime location for research into geohazards and understanding the instability of rock formations because of the abundance of natural rock formations found in places such as Arches National Park.

Jensen received her undergraduate degree in physics and civil engineering. Before coming to the U, she worked on a variety of landslide projects during her master’s degree work in geological engineering and with the US Geological Survey. At the U, she had an opportunity to develop and apply techniques that the geohazards group had been using for a decade. Before this, Moore and his group had used seismic resonance techniques to study natural arches and towers but had not yet applied these methods to large rock slope failures like those at Courthouse Mesa.

Jensen and Moore build on past studies in order to refine and move instrumentation forward by answering basic questions such as how the techniques of seismic resonance measuring can be used at other sites. Seismic resonance methods enable geohazard practitioners to better characterize and monitor potentially hazardous unstable rock slopes, especially those where invasive equipment cannot be installed, and again providing a potential service for developers and engineers.

Another benefit of the instruments Jensen is using is that she can continuously track seismic data to monitor how the site’s instability responds to temperature and rainfall changes. Jensen can use this data to check if the changes are associated with progressive failure of the rock slope. For this project, she used a single seismometer installed on the rock surface for three years and tracked the resonance frequencies of the landslide over time. What she found was that the Courthouse instability is particularly affected by thermal stresses created by heating and cooling, which causes the crack to open and close both daily and on a seasonal cycle. “We see a pretty big seasonal change,” Jensen says. “The Courthouse Crack opens and closes about fifty millimeters annually. It’s very slowly increasing and opening by millimeters per year.”

In the future, characterization measurements repeated in another season at the same site could be useful to observe the changes based on larger swings in temperature and climate. These measurements could also detect a continuing extension and failure of the cracked mesa. Coming back to the site several years later would be useful to observe changes in the overall geometry of the Courthouse Mesa.

Creating another technique in the toolkit of geological engineering is important for Jensen and her group because it helps mitigate outside risks. Her work, which is being published soon, is instrumental in pushing the new technique for practical implementation and helps show how one can monitor landslide behavior. Conceptually, seismic resonance measuring can anticipate what kinds of other data and observations might be seen in other landslides.

Part of the project was stepping back from the site and doing conceptual and numerical modeling, such as testing out how frequency decreases with slope failure. This helps to predict how resonance frequencies will respond during progressive rock slope failures of different types. These models give new insights where field data does not exist, because instrumented rock slope failures are very rare.

Sometimes complex patterns of resonance frequency change before failure, and the models showed, for the first time, the expected form of resonance frequency change as ultimate slope collapse approaches. Field measurements like those at Courthouse Mesa are invaluable for establishing the new approach and understanding the limitations.

Erin Jensen’s work is taking her far afield from Utah. She is preparing for a postdoctoral fellowship with the US Geological Survey as part of the Mendenhall Research Fellowship Program. Her research will focus broadly on landslides in Alaska, as well as how landslides are affected by glacial retreat and climate change. <