Concrete crumbling beneath giant culverts at 19 Mile
June 16, 2022
Concrete foundations are crumbling beneath four giant culverts built at 19 Mile Haines Highway last year, apparently defying the State of Alaska's most recent effort to tame a mountainside that's been sliding into the Chilkat River for more than a century.
The 1,200-foot section of road that straddles the mountain has been described by state Department of Transportation officials as among the most expensive in Alaska to maintain due to the severity of mudslides that cross it during heavy rains, frequently making the road impassable.
For years, DOT saved time and money by leaving road-clearing equipment at the site.
But reconstruction of the highway brought a new plan that raises the roadway as much as 40 feet, into what today resembles a giant dike, with four culverts – virtual tunnels – designed to allow water, rocks and debris to pass under it.
At 60 feet long, about 20 feet high and roughly as wide, the culverts were built large enough to allow a front-end loader inside to clear them of debris.
The culverts have yet to be tested against the movement of the mountainside. In the year since the elevated culverts were built, debris flow hasn't reached sufficient volume to pass through them. (A smaller, fifth culvert on the west side of the structure passes routine volumes of water.)
But there are already signs that the concrete foundations supporting the corrugated steel passageways are failing.
Chunks of cement are falling off. Cracking in the concrete has continued since last summer, despite $1 million in reinforcement work done last summer after the cracks first appeared, DOT construction project manager John Kajdan said this week.
"The cracking started soon after they were built and the cracking is progressing," Kajdan said. "It's concerning that we're seeing damage in these structures that are supposed to be new."
This week the state was fabricating 20-foot long, steel struts to place between the cracking walls of the easternmost tunnel, which is exhibiting the most severe damage, including a dip in the roadway above it. A gauge has been placed along a crack in the concrete there to see if it widens.
"They'll reinforce (the culvert) while we're evaluating what can be done," Kajdan said of the struts. "We're not waiting and seeing. Our evaluation is not completed. When it is, we'll work on either repair or replacement of the structure."
Each of the four culverts shows concrete failure at its four corners as well as seepage through concrete along the face of their retaining walls. In the easternmost tunnel, the retaining wall is disintegrating along two, 10-foot sections where concrete has collapsed, exposing rebar framing.
During remedial efforts following a stop-work order last year, the state added "tie-backs," buried concrete anchors attached by steel cables to the concrete "head walls" and "wing walls" at the entrance of the culverts.
About 10 of the tie-backs have been found to be loose, which is also part of the failures at the site. Kajdan said the state doesn't know why the anchors have come loose. "I can't say. That's something we're looking into."
The anchors are intended to reduce soil pressure pushing from the road embankment down on the head wall and wing walls.
Kajdan said a certain amount of settling might be expected after construction but also said, "I don't think that's everything that's causing the damage. That's what our department is trying to figure out – what's causing the damage and whether it can be repaired."
DOT also has ruled out faulty materials as a cause. The compression strength for the concrete met the state's specifications, Kajdan said.
"If (the cause) is underlying conditions not foreseen by the project designer, that needs to be determined. Why the cracking is occurring, that needs to be determined. Then DOT will decide if (the culverts) can be repaired or the structures need to be replaced," Kajdan said.
Kajdan also responded to a rumor that the state planned to backfill the easternmost culvert. "If worst came to worst, to keep the road open, we could fill in the structure. We could do it, but that's not the plan."
Resident Roc Ahrens worked as an equipment operator for Haines DOT for 30 years, sometimes spending weeks at a time clearing debris from 19 Mile slides that were as wide as 200 feet and as deep as six feet.
A team of operators sometimes worked more than a month moving debris there, filling hundreds of dump truck loads per day, Ahrens said.
The work included taking machinery up the slide to create channels to ensure that debris flows came down at 19 Mile and didn't move elsewhere, potentially endangering nearby homes, he said.
Ahrens said the biggest slides there are triggered by extremely hot or extremely wet weather, moving what he called a giant "oreo cookie" of snow and mud that would accumulate in a bowl hundreds of feet up the mountain.
Operators would build embankment walls as high as 50 feet on the mountainside to contain slides, which sometimes filled the walls to the brim.
Ahrens said he's still puzzled by the state's new design of the area, particularly the dramatically raised roadbed.
"It doesn't make sense to me. It's looking to me they're building a dam there. I can't tell you what's going to happen the next time they get a big slide. I wonder what they gained by all that elevation. I know there's a method to what they're doing, but I don't understand it," Ahrens said.
Ahrens said the installing culverts big enough to accommodate heavy equipment was a necessity, as unplugging smaller culverts during his time at DOT was a laborious process requiring special equipment.
The 19 Mile section was estimated to cost $5 million. DOT spent an additional $1 million on last year's reinforcements, which included using crushed rock atop the culverts to reduce pressure from above.
The engineering firm DOWL designed the 19 Mile section. The project is funded by the state and federal governments. The cost of last summer's modifications were shared between DOWL and the state.
The Tlingit village of Kaatxawultu was destroyed by a landslide from the same mountain in 1895.