Yida Zhang is deeply passionate about assessing the stability of dams, but his approach is unconventional. Rather than studying the large aging structures, his research zeroes in on the tiny sand grains beneath and surrounding them. He investigates how the grain size distribution evolves under stress and its potential to compromise the stability of dams, especially in cases of extreme weather events.
“In geotech, we design based on the properties of soils at the moment of construction,” says Zhang, an assistant professor in CU Boulder’s Department of Civil, Environmental and Architectural Engineering. “That’s the end of the story. We don’t ask how the material evolves and degrades over time.”
Zhang was recently granted a prestigious CAREER award, a $600,000, five-year grant from the National Science Foundation, to support his research, “Decoding the Grain Size Evolution of Granular Soils under High Stresses.” The CAREER program stands as NSF's highest honor in support of early-career faculty with the potential to serve as academic role models in research and education.
The funding enabled Zhang to establish his research group, the Multiphysical Geomechanics Lab, to pursue research on granular soils under extreme stresses and environmental loadings over extended time spans.
Rising dams, extreme weather
Amid the construction of increasingly taller dams and the escalating threat of extreme weather events, Zhang underscores the critical importance of his research. During construction, carefully controlled grain size distributions and construction protocols are utilized to achieve desirable geotechnical properties of the soil and to ensure mechanical and hydrological stability. However, the long-term effects of high stress, which can crush, degrade and erode the grains, are often overlooked, he says.
For example, one of the most common methods for safe, long-term storage of tailings, a mining waste consisting of crushed rocks and residual chemicals, is to construct tailings dams. Tailings first go through a hydrocyclone process to separate the coarse grains for dam construction. Then, the finer fraction, mixed with water, is stored upstream of the dam to prevent it from transforming into a dense, toxic mudflow. Crushed sand grains would substantially hinder dam drainage, which is crucial to swiftly evacuate water from behind the dam; draining the water reduces the risk of liquefaction in the event of an earthquake.
Zhang’s goal is to develop a numerical approach that leverages grain-size distribution data to inform the engineering designs of large critical geostructures. Engineers would be able to use this tool when designing tall dams, enabling them to anticipate how grain size will evolve under stress and how the factor of safety will change over time. Unlike current technology, this grain-sized informed approach will empower engineers to design dams with a safety margin exceeding the minimum requirements over the long run, effectively preventing failures arising from grain size shifts or soil breakage, he says.
The project also includes educational tools designed to illustrate to high school students the ways in which civil engineers address climate change and to make civil engineering topics more accessible. This initiative aims to counter the national decline in the recruitment of civil engineers and secure a steady supply of skilled geotechnical engineers into the workforce.
“We need to create a pipeline for the next generation of geotechnical engineers,” Zhang says. “Without a continuous influx of talent into this field, we risk a shortage of skilled professionals as we confront new challenges related to climate change.”
Furthermore, he plans to equip graduate students with toolkits for adapting geo-structures to climate change.
“No one wants the failure of critical dams or levees, recalling memories of Katrina and, more recently, the Brumadinho dam failure in Brazil,” Zhang said. “We want to design our dams and levees with better knowledge about the material response to extreme loading after it ages. We don’t want the dams to fail.”
Photo above: Hoover Dam. Photo credit: Susan Glairon