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Deep thawing soil will keep roads rough

Before being patched with thick black asphalt during recent weeks, the roads across Goldstream Creek’s swampy valley bucked my car like a whitewater raft in the Nenana River canyon.

This is not news. I’ve bobbed over these road waves for 60 years. I first felt them as a kid riding the original gravel roads in my parents’ dust-trap of a Land Rover. At some point, pavement arrived and the dust left. But the bumps have continued.

I understand how roads thaw the underlying permafrost, roiling the routes across Goldstream’s bottomland and the slopes that define the valley. But it seems like the thawing should have an end date, a point when the road settles into a smooth plane. Sixty years isn’t enough?

Not by a country mile, according to Billy Connor. A professional engineer, Connor has studied Alaska roads for almost 50 years. He spent 30 years with the Alaska Department of Transportation and Public Facilities before coming to the University of Alaska Fairbanks, where he most recently directed the Arctic Infrastructure Development Center.

Actually, “a country mile” exaggerates the estimate Connor gave me. What he said was “100 feet or more.” That’s how deep the problematic permafrost soils can be in valleys like Goldstream and another famously unstable spot on the Richardson Highway southeast of Fairbanks, the Tenderfoot Creek flats.

The extreme depth of the frozen soil, which accumulated when woolly mammoths walked this land, means it will continue thawing far into the future.

“Back in the days of glaciation, the winds off the glaciers at the base of the Alaska Range would pick up the silt and loft it and deposit it on this side of the valley,” Connor said.

Spring snowmelt and summer rains percolated into the silt deposits, then froze each winter. The subterranean ice would remain through subsequent summers.

“As it did that, it created an ice-rich permafrost. So that built up over tens of thousands, 40,000, 50,000 years,” Connor said.

Later, boggy vegetation, resistant to decay, built thick peat deposits over the silt in many places. The peat insulated the frozen silt and itself could freeze permanently a few feet below the surface.

Today, Connor said, the permafrost soil in some locations is 40% ice, by volume. Large lenses of pure ice often lurk within it. The Tenderfoot flats is such a place.

“Once we built the road, we disturbed all of that, and we're thawing it,” Connor said. “And then add to that some of our climate warming. Now, to be clear, the road is actually thawing the permafrost faster than the change in climate. But you add the two together and you get a cumulative effect.”

When soil that’s 40% ice thaws, much of it oozes away underground. The road above drops.

While roads in Goldstream, Tenderfoot and other similar valleys will continue to warp for decades, some ice-rich permafrost under other roads has already finished thawing. You can see this where permafrost layers are thin, such as near the Tanana River, the big glacier-fed waterway that drains much of Interior Alaska.

The Tanana’s meanderings have laid down a base of stable gravel, the silt and peat layers are shallow, and the water table is near the surface. 

“Water carries a lot of heat,” Connor noted. “I often say, ‘If you want to thaw permafrost, add water.’”

Sometimes, road builders have accelerated the thawing intentionally. For example, the state highway department cleared vegetation from the Mitchell Expressway route, which skirts the south side of Fairbanks, and let it sit for a few years to “prethaw” the permafrost before building the road. It remains blissfully level decades later.

In other spots, the agency has excavated the ice-rich permafrost to reach more stable sand or gravel, but “anything more than about 10 feet (deep) becomes prohibitively expensive,” Connor said.

Another expensive fix uses thermosiphons — sealed pipes buried in the roadbed that contain a circulating gas that carries the ground’s heat out and winter’s cold in. The idea is to keep the permafrost frozen.

Foam insulation boards, another way to protect the permafrost, can slow the thaw a bit. But installing the boards and excavating down to frozen ground, where they must be placed, are also costly tasks.

Money, in fact, determines much of what happens on our wacky roads.

“DOT looks at it and makes a determination,” Connor said. “Is it just cheaper to fix it, or is it cheaper to try to just live with the maintenance?”

When the problem is 100 feet thick, living with the maintenance wins. And it will for many decades to come. 

Since the late 1970s, the 51��������' Geophysical Institute has provided the Alaska Science Forum column free in cooperation with the 51�������� research community. Sam Bishop is an editor and writer with the 51�������� Advancement office.