Sunday, November 5, 2000

Cracking up: Sea ice under stress

Published in the Antarctic Sun

It’s strong enough to land planes on, too thick for a small drill to get through and cracks under pressure.

Sea ice is vital to the early-season research based at McMurdo Station.

Scientists base themselves on the frozen ocean to study the marine world. And when it breaks up and blows north, it leaves a spectacular expanse of open water.

David Cole and John Dempsey have forged a partnership out of the study of fracture of sea ice. Their work has involved lab work and field research in Alaska and now Antarctica.

Cole, from the U.S. Army Corps of Engineers Cold Regions Research and Engineering Lab, and Dempsey, from Clarkson University, are studying how ice behaves when under stress, in the breaking process.

Their project has faced some difficulty this year. The sea ice is thicker than usual, which is hard on their equipment.

They were expecting to find some ice as thin as 36 inches, and have equipment that can cut ice up to 84 inches, though very slowly. The thinnest they’ve found is 45 inches, with most of the ice 55 to 60 inches thick.

The amount of time required to cut through this thickness of ice is more than the team has.

“Because of ice thickness we can’t do the research we proposed,” Dempsey said. “We probably need a Ditch Witch,” a trenching machine for cutting through the ice faster.

Right now it takes too long to cut blocks the size they need. The biggest piece they’ve been able to study was three meters square. They would like to be studying deformation and fracture of blocks of sea ice up to 30 meters on a side, 100 times larger than they can get.

“The underlying theme of our research is to look at scale,” Dempsey said. Without large blocks of ice to study, they can’t get the data they would like.

Cole’s part of the study happens first. He wants to know how ice deforms when under stress. His work stops when the ice actually cracks, but the information he gathers helps Dempsey watch the right area of a floe when they do crack it.

“It starts with the microstructure,” Cole said.

The way ice crystals form and align themselves as the ocean freezes makes a difference in how the ice will crack, even months later. When there is a small current, ice crystals line up in one general direction.

That, in turn, makes the ice relatively weak in one direction, so it tends to crack for long distances in straight lines, Cole said.

“The properties are different depending on the direction,” Cole said. “It’s not just a homogeneous material.”

Some things are very different in the field from in the lab. For example, brine drains out of the ice when it’s brought into the lab, which changes the characteristics of the ice.

They have a camp about three miles (five km) from the ice edge, on fast ice. Their cutting area is a short distance away, but on much thinner, floating ice.

Cole and Dempsey and their two students mark out an area in which they want to work. They cut a block free of the ice sheet and then cut a starter crack, into which they insert a balloon-like loading device.

This “flat jack” has a computer-controlled inflation valve, which lets the team vary the pressure in the crack. The computer is set to stress the block of ice until they are ready to break it.

“We don’t want to accidentally break it,” Cole said.

As the ice deforms, they monitor it for stresses and tensions, as well as how it deforms in response to the pressure on the crack. Some of these processes, Cole said, vary with the size of the piece of ice, while others do not.

Eventually, though, they are ready to break the floe.

Ice breaking

“Ice fracture is a very complicated process,” Dempsey said.

They have learned that at the tip of a crack that is about to break further, a series of micro-cracks form. They have equipment listening for the noise of those tiny cracks, to warn them before the block actually breaks.

Cole always looks carefully at the structure of the ice as well as these micro-cracks, to estimate where the block will break.

“It’s nice to have nature verify your direction,” Cole said. But he’s never sure if he’ll be right until the chunk of ice opens up entirely,

“Until you come down and try to do some tests you don’t know,” Dempsey said. “There are so many different types of ice.”

The ice thickness affects the breakup, but the more significant factor is the nature of the ice itself, which depends on how old the ice is, how it formed, local landforms and other environmental factors.

“We’re getting different ice wherever we move,” Dempsey said.

The models Dempsey and Cole have made about the behavior of ice under tension are based on smaller, more homogeneous sections of ice. They are checking to see how well those models predict the behavior of the ice they find in McMurdo Sound, and in larger sections.

They’ve wrapped up for this season, but are ready to come back and keep working, perhaps with better equipment and ice conditions.

“We have two field seasons,” Cole said.