If a gas pipeline were to be buried in permafrost-laden soils in the Arctic, how would those soils react? How would climate change affect the ground around the pipe? How will it affect the ground beneath vital roads and infrastructure throughout cold regions of the world?
Margaret Darrow, researcher and assistant professor in the Department of Mining and Geological Engineering, has a plan to answer these and similar questions. Thanks to a major award from the National Science Foundation’s Faculty Early Career Development Program (CAREER), Darrow is set to embark on a five-year study of unfrozen water in frozen ground. Her goal is to improve understanding of frozen ground behavior as it interacts with thermal changes in the environment. Her results will lead to (1) better predictions of permafrost response to climate change and (2) development of improved building designs for cold climate regions.
What would permafrost do?
Darrow said there is one main key to understanding the behavior of frozen ground: Water. “Water is the key thing from an engineering standpoint,” she explained. It is the water within the soil, particularly unfrozen water (water that remains liquid at sub-freezing temperatures), that responds to temperature changes. When the ground freezes, the water moves toward the freezing front near the surface.
Christine McCabe
Margaret Darrow, researcher and assistant professor in the Department of Mining and Geological Engineering, collects soil samples during drilling.
“It is the attraction of water to the freezing front that causes frost heave problems,” she said.
During a thaw, the ice melts and oversaturates the soil so it has no strength. It essentially becomes a mush of mud. The collapse of that saturated ground is the final result of frost heave. Ground heaving due to the freezing phase is the other half of the problem.
“This phenomenon is critical to the frost heave process. It is the key factor in determining the strength of the ground and how frozen ground responds to temperature (changes),” Darrow said.
While frost heave has been studied extensively in the past, Darrow is taking an innovative step by addressing three factors together, hoping to gain a more accurate understanding of the process. First, she’ll measure the mass and molecular mobility of the unfrozen water using pulsed nuclear magnetic resonance (NMR) methods. Then she’ll see how those results relate to measurements using other methods such as CT scanning and X-ray diffraction. Finally she’ll correlate those measurements with measurements related to the charges of the soil particle surfaces.
Photo courtesy Margaret Darrow
A researcher in the field holds up samples of ice-rich soil collected during some of Margaret Darrow’s research. Her new project, funded by NSF, will use innovative methods to gain a better understanding of unfrozen water in soil.
“We will quantify the unfrozen water content using the NMR method; we will observe the soil in the frozen state at a microscopic level and measure what is going on at the surface of all of these microscopic particles,” Darrow explained.
The results of all these measurements together, Darrow said, will help develop a model that is based on the relationships between unfrozen water thickness, viscosity and connectivity. She emphasized that her ideas are not exactly new—“I’m walking on the shoulders of giants,” she said—but her approach of “combining all these components together” is unique. She expects it will provide “a more comprehensive picture” of permafrost and its response to changes in climate.
“That’s where a new understanding of how these processes work together will form,” she said.
Previous models of unfrozen water used what Darrow called a “simple empirical relationship,” which was really an approximation of what the unfrozen water content actually looks like. Her approach will go beyond that simple model and provide a more complete and complicated model that better represents natural soils found in the field.
“In many equations and theories used now there is always unfrozen water there but it is not a precise measurement,” she said. “If you change that just a little bit it will have a profound effect on the model results.”
Photo courtesy Margaret Darrow
Close-up image of ice-rich soil. A new project headed Margaret Darrow, funded by NSF, will use innovative methods to gain a better understanding of unfrozen water in soil.
Why does that matter? Think gas pipeline. If you buried a chilled gas pipeline, the ground would freeze around the pipe. Engineers would need to know how that soil would respond not only to the pipe but to any potential temperature changes, either natural or related to construction and use of the pipe. Since we know the soil’s response will depend on what the unfrozen water inside it will do, we need an accurate model of the unfrozen water.
“This has so many applications (in regard to planning for cold climate construction),” Darrow said.
Not only will the knowledge offer cost savings in the form of better planning (i.e. less structural damage and safety risks) for infrastructure such as gas pipelines and roads, it will also help with things like improved planning for the relocation of Arctic communities due to permafrost instability.
The project will begin Sept. 1, 2012 and continue for five years. Darrow called the opportunity a “huge” boost for her career because of the many doors it could open for her as a researcher.
“If we can understand this, there are so many directions I could take it,” she said, listing things like landslides in clay-rich soils, construction of pipelines and bridges and other infrastructure, and planning for roads as a few areas that could benefit from the knowledge she expects to gain.
“It plays into all different elements of frozen ground engineering. It is like the nexus,” she said.
