Around March 1, 2021, the U.S. Air Force C-17 Globemaster 3 flew over the Lewis-McChord Joint Base in Tacoma, Washington. The construction of the single-span concrete arch that replaced two steel tube culverts continued. Built in 1950 The original culvert in the 1970s began to erode and thin, causing major structural problems and debris blockages in the runway and the Clover Creek flowing below it.

Tacoma, Washington — 2020 is a strange year to be sure — a pandemic broke out — but closer to home, a sinkhole appeared under the Lewis-McChord Joint Base Airport . Yes, large caves will expand and cause the soil around the JBLM runway to weaken, which is the main factor in sending mission-critical aircraft to where they are needed.

To fully understand what happened, we need to go back to when the JBLM runway was last renovated in the early 1950s.

Below the runway, there are two steel culverts, each 12 feet in diameter, less than 6 feet apart. At the time, the project cost approximately US$4 million to cover the Clover Creek fish passage and allow culverts to pass under the airport.

According to the McCord Airport Museum website, the building that extended the runway to 8,100 feet was originally planned to accommodate new aircraft and equipment, as well as new fighter operating facilities and air defense tracking system facilities.

Fast forward 70 years to 2020, and it can be said that there are obvious signs of wear and tear in the culvert. The 1,800-foot-long steel culvert began to erode and thinned to a quarter of an inch thick, causing major structural problems and debris blockages on the runway and the flowing stream below.

John Noquist, a resident engineer at the Lewis and Clark Project Office in the Seattle area, said that some sinkholes collapsed in some places, almost to the extent that the Shamrock Creek was blocked, the stream passed through the culvert and was in the leaky culvert.

Norquist went on to say that when the ground leaks, it will wash away all the restraints in the soil, so there is nothing to keep the soil together-forming a cave in the ground.

"So, once the cave is opened, it starts to form a sinkhole, structurally speaking, the ground can no longer support itself," Norquist explained. "There are sinkholes throughout the airport. They are open in the landscaped area around the runway. This is the first sign that something is happening."

Tiankengs began to appear at the end of January 2020, and there were multiple tiankengs and several rainfall events in February. Although there is no evidence that there is a sinkhole under the runway, as a precautionary measure, the airport runway was operated on a short runway for approximately 11 months.

Bruce Okumura, the chief contract officer for the area, said: "I think the most direct thing is to test the structural integrity of the runway to see if there is any risk of heavy aircraft crossing the area."

In fact, the runway can support not only any aircraft, but also the weight of the C-17 Globemaster 3, which is the main aircraft used by the US Air Force to transport troops and cargo to major operational bases or directly to overseas bases.

In order to start construction on the culvert, the stream must be diverted and the river bed must be dried.

"Because we dig deep, we expect there will be groundwater," Okumura said. "It happened that we had to pump much more water than expected, which got us into trouble at the beginning."

A total of 282,441 cubic yards of soil were excavated, 762 million gallons of groundwater were extracted from the riverbed, and chemicals such as perfluorooctanoic acid (PFOA) and perfluorinated and polyfluorinated substances (PFAS) were processed. Once the water has been treated, it will be released back to the creek as soon as construction allows the reintegration to begin.

When the culvert first failed, a rough estimate had been made to replace the existing culvert. The preliminary estimate for two 12-foot-wide box culverts is 20 million U.S. dollars. The estimate does not take into account the expected amount of water, nor does it have sufficiently large fish.

 "It did not consider modifying the design to meet modern standards, so once we did that, we saw the fact that we would have to dig more and pump more water, and then our estimate jumped by as much as $75 to $80 million, "Katherine LaPonte, the project manager for the area, said.

The selected proposal plans to replace the two steel tube culverts carrying the Shamrock Creek with a single-span concrete arch. LaPonte said that the decision to replace steel with concrete is more related to the service life of concrete and its ability to create a better overall river bed system.

This $88.6 million new construction project includes a 1-foot-thick reinforced concrete structure on a 3 x 4 x 9-foot foundation; it can withstand 100 years of storm flow compared to previous designs that dealt with 50 years of storm flow .

"What we did was we adopted a very strict culvert system, which significantly changed the natural outlet of the river, and we made it friendly to fish," Norquist said. "We placed something imitating the riverbed, downstream and upstream in its location. We did improve this and solve a very important flow problem."

In addition to concrete bridges, there are some additional features that can help imitate rock types and shapes of fish passages, the ability of riverbeds to scour or not scour, and an LED lighting system that will help salmon, trout and other creatures to be as natural and free as possible To pass.

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The official public website of the US Army Corps of Engineers in the Seattle area. For website corrections, please write to paoteam@nws02.usace.army.mil.