No one can predict exactly when a coal mine will collapse, but a $1.1 million grant from the National Institute for Occupational Safety and Health aims to change that, according to a Penn State mineral engineer.
Shimin Liu, assistant professor of energy and mineral engineering, and Joseph Kreutzberger Early Career Professor, both in Penn State's College of Earth and Mineral Sciences will begin a comprehensive five-year laboratory and field study aimed at understanding the degradation of shale rock roofs found above coal seams in mines. Although rare, these failures account for about half of all injuries sustained in coal mines.
These roofs are initially very strong, Liu said, but over time become compromised by the elements. During the summer, warm, humid air enters the cooler mine, creating condensation. In the winter, the reverse occurs. This humidity cycling degrades the shale, which is especially prone to moisture damage. How quickly and by how much is what Liu hopes to find.
"The critical question we ask is, what does this water really mean to shale strength over time?" said Liu. "It's a four-dimensional problem -- the three-dimensional shape of the mine factored against time. How does time shape the strength of the rock? When does the shale roof degrade to a critical low strength to where the ground control plan needs to be reinvestigated?"
Last month, Liu began traveling to three partnering mines to gather rock samples. He will use those samples to create replica shale roofs. In the lab, he will perform a series of tests to determine how the shale responds to moisture-induced swelling. He will also test enhanced bolting methods, an industry standard where companies brace the mine's roof for support. The existing practice is to use vertical bracing, but Liu thinks cross-bracing or other patterns could offer more stability.
"A major goal is to understand the science that's taking place in these mines, and then we will see how to best use these results," said Liu. "We are focusing on the scientific behavior of the rock under the real conditions. This stage is really important. Once we analyze the data, we can offer the best practices to the industry."
Modeling software to assess roofs exists, said Liu, but it accounts for so few real-life variables. He hopes his research will fill in those gaps and add to the model, leading to safer mines and miners.
One piece of equipment, obtained though this grant can test rock strength without destroying it. That's key for Liu's research but it also will benefit other engineering- and energy-related research.
Nondestructive testing allows for prolonged testing and is an ideal way to account for variables over time because the composition and strength of the rock is really varied throughout the testable area, said Liu, who said the research could pave the way for technologies to control the moisture cycle or reinforce the mine. That innovation will come later. The first step, he said, is understanding the problem.