Effective stress and FDL science

Retrogressive thaw slump / Image Laura Nielsen

“It’s a very dynamic slope,” Margaret Darrow said, standing in front of frozen debris lobe -A. FDL-A is a slow landslide; among the frozen debris lobes documented it’s the closest to the Dalton Highway and the Trans-Alaska Pipeline. Although the lobes likely began their life as debris left over when Pleistocene glaciers disappeared 10 to 14 thousand years ago, their speed has recently increased. Now when Darrow describes FDL-A she states truly: “It moves so fast that you can watch it move.” And the force coming downhill is substantial.

Exposed wall / Image Laura Nielsen

I followed the scientists in their hike up FDL-A, borrowing slippery moose tracks alongside water rushing in the opposite direction. Toppled trees and twisted cracked stumps lay interspersed with thickets of stubborn brush while moss and tundra plants felt springy underfoot… Until we reached the retrogressive thaw slump.

This mud pit was an open wound on the face of the lobe. The vegetative mat was falling apart and away like a collection of bad toupees, baring gray sandy silt to sunlight. Ice chunks embedded in the wall melted and rainfall joined in, trickling downhill. The water’s journey coaxed along clumps of silty goop which clumped and fell and flowed. As more ground succumbed to the pit, more roots were bared and more plants at the uphill precipice tumbled into the mud. Around us were toppled trees. In the distance, closer to the highway, I could hear the roar of waterfalls careening off the side of the lobe. Shortly their silted water would join the drainage ditch alongside the road not 142 feet away.

The bulk is moving downhill. Frozen debris lobe -A, massive geohazard, nears the highway.

Lobe motion

“FDL-A moves two ways: most of it is in the shear zone 66 to 74 feet below the surface and then there’s another minor portion which is internal flow so the whole thing is flowing as well.” ~ Margaret Darrow

Margaret Darrow is an engineer, an associate professor in the Mining and Geological Engineering Department at the University of Alaska Fairbanks. She studied FDL-A this summer (2014) with Ronald Daanen and Trent Hubbard, Engineering Geology section geologists with the Alaska Department of Natural Resources’ Division of Geological & Geophysical Surveys

Frozen debris lobes are mobile; even in the winter, FDL-A slides en-masse downhill. A juggernaut.

Vegetative mat collapsing / Image Laura Nielsen

Hubbard talks about his impression of the motion of the lobe as a whole:

“It’s not instantaneous, but it’s certainly fast enough that on a year-to-year basis or even every couple months you can see the dynamic changes. I think that that’s what impresses me the most about these features. As a geologist you’re used to changes – but sometimes over long periods of time. So here, we come back and, wow, there’s been a lot of changes.” ~ Trent Hubbard

Effective stress

The scientists measure the temperature of the lobe at -1.1 degrees Celsius [30 degrees Fahrenheit], 2 degrees warmer than the surrounding permafrost. Still, the lobe’s temperature sits below freezing.

“The temperatures that we have indicate that they are frozen yet movement occurs all winter long; it may be due to this water that is under high pressure and yet at sub-freezing temperatures.” ~ Margaret Darrow

Water which remains liquid at a negative temperature is called supercooled water. The pressure caused by the immense weight of the lobe is one reason the water at the shear zone is liquid (though the weight of the lobe is not sufficient by itself to explain the presence of liquid water at the measured temperature). Liquid water helps lubricate the way, encouraging the lobe’s bulk to slide downhill with gravity’s pull.

People who study slope processes and slope stability use the term effective stress to describe the pressure that the soil particles “feel” at depth. Lobe FDL-A is composed of frozen, silty sand with gravel. Effective stress helps to keep that soil together atop the bedrock 86 feet [26 meters] below. In 2012, the scientists and the Alaska Department of Transportation and Public Facilities were able to drill to 100 feet [30 meters] below the surface of FDL-A to take measurements, which is why we know the shear zone exists.

As water is injected by snowmelt or rainfall events the liquid water acts to destabilize the lobe, decreasing stability.

“Water in a landslide causes the grains of soil to force apart, it decreases something that we call effective stress and basically lowers the soil’s strength and it causes it to move faster. So the fact that there’s liquid water in these is a tremendous indicator of faster movement.” ~ Margaret Darrow

In a similar vein, projected warmer temperatures will likely increase the liquid water content and lower the lobes’ effective stress. For now freezing temperatures help to maintain effective stress and keep the lobe sluggish.

“The [cold] temperature lowers that pressure and so the colder the feature is, the less pressure there is. If the pressure goes up the whole lobe is lifted and starts floating.” ~ Ronald Daanen

Fallen trees / Image Laura Nielsen

A working  theory

The lobe is deceptive, camouflaged even, because it’s mostly covered by vegetation. Alaska plants grow furious and swift in the short summers; life goes on even when unsteady ground half-topples trees. That vegetative mat makes FDL-A’s mass appear less imposing. But sometimes, the facial mask of plants breaks. Thaw slumps reveal underlying silt (see related project: Thermokarst). Additionally, large cracks have appeared on FDL-A’s back: crevices across the lobe which collect water.

Similar crevices were recorded on FDL-D before that lobe picked up speed aggressively.

“These cracks went all the way across the catchment, they were full of water, they were a place where a lot of water could infiltrate into the debris lobe and perhaps get down and cause it to move faster, lowering the effective stress. Here it is [photographed] last year. So the mass is gone, it went downhill. Here are some remnants of the cracks but: same location, no debris lobe. It’s already moved downhill. It moved fast in just a few years.” ~ Margaret Darrow

This has led to what the scientists call a working theory. Darrow, Daanen and Hubbard say the crevices may collect water and ice which melts, injecting liquid into the lobe, sparking more aggressive motion. And that’s worrisome since FDL-A sits so close to the the highway and Trans Alaska Pipeline System (TAPS). Darrow describes seeing massive ice (big chunks or wedges of ice) in the wall of the mud pit that we explored as it shifted lobe material downhill. The location of the largest ice wedge aligned with a crack in the vegetative mat above.

“We had hypothesized that the cracks on the surface might be one avenue for water infiltrating down into the lobe – a faster avenue for water migration than through the soil down to the shear zone. And today we found some cracks. We found massive ice: there was a break in the organic material and there was a crack in the surface that corresponded with the massive ice … that ice was present right below where a crack was on the surface and so that helps to support that theory. That was pretty exciting.” ~ Margaret Darrow

“The more ice that is in that wall the faster it will go and it doesn’t stop, meaning it is now at a size where it could take a whole section of the lobe away and that would have consequences for the downslope part as well because the permafrost gets warmer and therefore more water gets under in the shear zone and that feeds the motion of the whole feature.” ~ Ronald Daanen

“FDL-A as of June of 2014 was 142 feet off the highway so if it picks up and moves like FDL-D did it’s on the highway in the year.” ~ Margaret Darrow

Crack with water / Image Laura Nielsen

Water, pressure and temperature

The massive ice witnessed in the muddy walls of the retrogressive thaw slump lent to the scientists’ working theory. So does the data they gained by drilling.

“There are complicated relationships between water pressure, temperature, and ice pressure and how that may affect the shear zone in these features. … Supercooled water behaves according to the Clausius–Clapeyron equation, which is an equation that combines water pressure, temperature, and ice pressure in a rigid relationship. You can approach that equation various different ways by assuming ice pressure is constant, assuming water pressure is constant, or assuming temperature is constant.” ~ Ronald Daanen

Daanen describes the ice pressure as constant because the weight of the lobe rests upon the frozen ground beneath. He knows the temperature is -1.1 degrees Celsius. And, because the scientists were able to drill and take measurements, he knows water pressure at a depth of 85.5 feet measured 150 feet.

“The pressure at that depth should have been 615 feet; way way way higher. But you take the pressure needed to keep that water liquid and you correct for it with the negative temperature. … After I did the calculations I looked at the map and I said: wow, it’s right there where the shear zone actually comes in daylight, where we see some cracks in the feature. And so we are thinking, wow, if that’s the case then ok water goes in the crack and pressurizes that shear zone.” ~ Ronald Daanen

The crevices let water infiltrate the shear zone. That feeds the motion of the whole feature.

“Pressure is related to temperature and so the temperature causes a pressure reduction, and so if temperature was to rise we hypothesize that the pressure would be much greater than what we currently measure. And if you take away that negative temperature effect the pressure actually correlates to the cracks that are up in the catchment, 615 feet higher in elevation. ~ Margaret Darrow

They’ve learned much, but the scientists need more data to build an effective model, or simulation, of the frozen debris lobes which could test what mitigation options would be effective. They need funding.

“We really don’t know what could stop it. And that’s what everybody wants to know, you know, how can we prevent these things from going on the road? So. That’s what the research is for, right: to understand why they move and extrapolating that knowledge into, ok, if we understand why it moves, how it moves, what can we do against it?” ~ Ronald Daanen

Laura Nielsen

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

Frozen Debris Lobes project

  • Interviews with the scientists during their field work season, 2014