TACOMA, Washington –– It’s safe to say 2020 was an odd year – a pandemic broke out – but even closer to home, sinkholes opened under the Joint Base Lewis-McChord Airfield. Yes, large caverns that would expand and cause the soil to weaken around the JBLM runway, a major element in getting mission-essential aircraft where they need to be.
To fully understand what happened, we need to go back to the early 1950’s when the JBLM runway was last renovated.
Underneath the runway, were two steel culverts each measuring 12 feet in diameter and less than 6 feet apart. Back then, the project cost about $4 million, covered the Clover Creek fish passageway and allowed the culverts to cross underneath the airfield.
According to the McChord Airfield Museum website, the construction that lengthened the runway to 8,100 feet was originally planned to accommodate new aircraft and equipment as well as new fighter operational facilities and air defense tracking system facilities.
Fast forward 70 years to 2020 and one could say the culverts were showing significant signs of wear and tear. The 1,800-foot-long steel culverts began to erode and thin out to a quarter of an inch thick, causing major structural issues and debris blockages to the runway and the stream that ran beneath.
John Norquist, resident engineer with Seattle district’s Lewis and Clark Project office, said that some of the sinkholes had caved in some places, almost to the point that there was blockage to Clover Creek, the stream that runs through the culverts, creating gaping holes in the culverts that were leaking water.
Norquist went on to say that when water leaks underground, it will wash out all the binds in the soil, so there was nothing keeping the soil together ― creating a cavern underground.
“So, it started creating sinkholes once the caverns opened up, and structurally the ground could not hold itself up any longer,” Norquist explained. “There were sinkholes that were opening up across the airfield. They were opening in the landscape area surrounding the runway. That was sort of the first indication that something was going on.”
The sinkholes started to show up at the end of January 2020, and by February there had been multiple sink holes and several rain events. Although there was no evidence of sinkholes opening beneath the runway, the airport runway operations were working on a short runway basis for approximately 11 months as a precaution.
“I think the most immediate thing that was done was testing the structural integrity of the runway to see if there was a risk in having any heavy aircraft traverse that area,” said Bruce Okumura, principal contracting officer with the district.
In fact, the runway supports not just any aircraft, but the weight of a C-17 Globemaster 3, the primary aircraft used in the U.S. Air Force to deliver troops and cargo to main operating bases or directly to bases overseas.
To begin construction on the culverts, the stream had to be diverted and the stream bed had to be dried out.
“We expected ground water as we were digging pretty deep,” Okumura said. “It just so happened that we had to pump a lot more water than we anticipated and that sort of threw us for a curve in the beginning.”
In total, 282,441 cubic yards of soil were excavated and 762 million gallons of ground water were pumped out of the stream bed and treated for chemicals such as Perfluorooctanoic Acid (PFOA) and Per-and Polyfluorinated Substances (PFAS). Once the water was treated, it was then released back into the creek once construction had allowed for the reintegration to begin.
When the culverts had first failed, a rough estimate had been made to replace the existing culverts. The initial estimate, made for two 12-foot-wide box culverts, came in at $20 million. That estimate did not allow for the volume of water that was expected nor was it big enough for the fish.
“It didn’t take into account revising the design to meet modern-day standards, so once we did that and we looked at the fact we were going to have to excavate a lot more and pump a lot more water, then our estimate jumped up to the tune of $75 to $80 million,” said Katherine LaPonte, project manager with the district.
The selected proposal plan replaced the two steel pipe culverts, that had been conveying Clover Creek, with one single span concrete arch. LaPonte said the decision to replace steel with concrete had more to do with concrete’s life span and ability to create a better overall stream bed system.
The new, $88.6 million construction project consists of a reinforced 1-foot-thick concrete structure, sitting on 3-by-4-by-9 feet footings; making it capable to withstand a 100-year stormflow compared to the previous design that handled a 50-year stormflow.
“What we did is we took a very restrictive culvert system, that changed significantly the natural way out of the stream, and we made it fish friendly,” Norquist said. “We put in its place something that mimics the stream bed, down stream and up stream. We’ve definitely improved that and solved a pretty significant stream issue.”
In addition to the concrete bridge, there were some additional features to help the fish passage that mimicked the rock type and shape, the ability of the stream bed to scour or not scour, and an LED lighting system that will help salmon, trout, and other living organisms pass through as natural and free flowing as possible.