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Dam plan meets nature: Best-laid Milltown design adjusts to reality in the field
By PERRY BACKUS of the Missoulian
Matt Fein, Envirocon's senior project director for the Milltown Superfund cleanup, explains the process for removing sediment behind Milltown Dam Thursday afternoon. Tree stumps were found beneath 20 feet of sediment, and are estimated to be 100 years old.
Photo by LINDA THOMPSON/Missoulian
BONNER - Oodles of computer-generated models, notebooks filled with engineering designs, the best guesses from a plethora of well-paid minds.
All of that's well and good - but sometimes the only way to truly know if something is going to work, you have to leave the office and step out into the muck.
For the last month, that's exactly what Missoula-based Envirocon has been doing at the Milltown Superfund cleanup site near Bonner. After months of review and design work, the company has put the pencils away and set its earthmoving machinery to work digging a portion of what will eventually become a bypass channel for the Clark Fork River.
“This is a very important juncture for us,” said Matt Fein, Envirocon's senior project director. “Our design was based on people's experience and knowledge, but on projects this large, there are always going to be challenges.
“You can't design something in a room by yourself and then just hand it to the folks in the field and expect that everything is going work just fine. There are always going to be variables that no one can predict.”
The company knew it faced some uncertainty when it began digging through a 20-foot-deep layer of river sediments just upstream from Milltown Dam.
In 2004, Envirocon dug a test pit not far from where its machinery is now working.
“Almost as soon as we started to dig, the water began pouring in,” Fein said. “It was really spooky. You can't dig effectively when the water comes pouring in on you like that. The hole just keeps getting wider as the sides cave in. These sediments have no structural strength in them when they're wet.”
So the company's engineers went to work to put together a plan to dewater the sediment.
Their challenge was twofold.
First, the 20-foot-thick layer of sediment was made of a variety of different deposits, including a few layers of clay that kept the water from dropping down into the aquifer. To address that problem, the company punched a number of small holes through the sediment and inserted some cloth-lined pipe. The wick drains allowed the water to move vertically.
The second challenge was to make sure the water in the sediment had a place to go.
The aquifer was under pressure just beneath the sediment layer. To alleviate that, the company drilled six wells - three on either side of their new test pit. The wells pulled about 350 gallons a minute from the aquifer.
“It turned out that there wasn't as much water as we thought,” Fein said. “That was good news. We were prepared to pump a lot more water, but it turned out that we had to reduce the size of pumps we installed initially. The first ones were too powerful. They'd suck the well dry and burn up.”
By last week, the company had dug about half of the 600,000 cubic yards of material it planned to move during the sediment dewatering test. The hope is the rest of the hole will be dug over the next couple of weeks.
When that work is done, Fein said Envirocon will incorporate everything it's learned from the test into the final design for the bypass channel, which is scheduled to be completed sometime next fall.
The 100-foot-wide channel will divert the Clark Fork River around the 2 million to 2.5 million cubic yards of contaminated sediment that will eventually be moved to the Opportunity Ponds near Anaconda.
The ground assessments, like this dewatering test, will likely play an important role in making sure the dam and sediment work are completed in a safe and efficient manner.
“We're literally designing things and then building them as we go,” Fein said.
As anticipated, there have been lessons learned as part of the dewatering test.
For instance, no one knew for sure what was underneath the 20-foot layer of sediment that lined the bottom of Milltown Reservoir.
Envirocon's crews have discovered the remnants of what apparently was a relatively thick forest that filled the valley floor at the confluence of the two rivers. The bottom of the hole is filled with stumps left behind by crosscut saw-packing sawyers over a century ago.
“We certainly didn't think we'd hit as many stumps as we have,” Fein said. “Their presence tells us exactly where the old forest floor was located. We know from that level down, the soil doesn't have significant levels of contamination.”
They've also learned that sediment removed from the area can be unruly.
“It starts to move like lava under its own weight when we try to stack it,” Fein said. “This sediment has no strength whatsoever when it's wet. We have to maintain a healthy respect for that fact.”
Plans call for stockpiling the estimated 600,000 cubic yards of sediment that will be removed during the bypass channel construction on the site.
“There will be a massive stockpile of sediment here,” he said. “We'll use up about every square foot of flat ground.”
The company has also faced a challenge in moving heavy equipment around the site. There is a layer of soft sand about 2 feet underneath the surface in some areas that can easily bog down machinery.
“We've needed to build more roads out of gravel than we anticipated,” Fein said.
By the first of February, the company hopes to begin constructing an earthen flood berm along the edge of the Clark Fork River that will begin near the remnants of Duck Bridge and run down to the confluence with the Blackfoot River.
“In my mind, that's when this project will really begin,” Fein said. “We're in the process of working with the agencies to get approval on the final design for the berm.”
Berms will be constructed along both the Clark Fork and Blackfoot rivers to protect the work site from flooding.
Envirocon is already stripping off the top 3 feet of topsoil at the upper end of the work site. That material will eventually be used to cap a lot of area disturbed during the construction phase.
“The soil has been thoroughly tested and it's so low in metal contamination that we're allowed to use it as topsoil,” Fein said. “There's a lot of material here. We estimate there's about 125,000 cubic yards. We'll use every bit of it in our reclamation efforts.”
Fein said the project appears to be adhering to the expected timeline, but people need to remember that this is a huge undertaking and there is plenty of opportunity for something to go astray.
The project's timetable is tied to the ebb and flow of the river system, he said.
Missing one deadline - like perhaps the upcoming bridge mitigation work - could push the whole project back a year.
“Mother Nature has a way of slipping in something unexpected just when you think you have it all figured out,” Fein said. “There are no guarantees.”
Dam plan meets nature: Best-laid Milltown design adjusts to reality in the field
By PERRY BACKUS of the Missoulian
Matt Fein, Envirocon's senior project director for the Milltown Superfund cleanup, explains the process for removing sediment behind Milltown Dam Thursday afternoon. Tree stumps were found beneath 20 feet of sediment, and are estimated to be 100 years old.
Photo by LINDA THOMPSON/Missoulian
BONNER - Oodles of computer-generated models, notebooks filled with engineering designs, the best guesses from a plethora of well-paid minds.
All of that's well and good - but sometimes the only way to truly know if something is going to work, you have to leave the office and step out into the muck.
For the last month, that's exactly what Missoula-based Envirocon has been doing at the Milltown Superfund cleanup site near Bonner. After months of review and design work, the company has put the pencils away and set its earthmoving machinery to work digging a portion of what will eventually become a bypass channel for the Clark Fork River.
“This is a very important juncture for us,” said Matt Fein, Envirocon's senior project director. “Our design was based on people's experience and knowledge, but on projects this large, there are always going to be challenges.
“You can't design something in a room by yourself and then just hand it to the folks in the field and expect that everything is going work just fine. There are always going to be variables that no one can predict.”
The company knew it faced some uncertainty when it began digging through a 20-foot-deep layer of river sediments just upstream from Milltown Dam.
In 2004, Envirocon dug a test pit not far from where its machinery is now working.
“Almost as soon as we started to dig, the water began pouring in,” Fein said. “It was really spooky. You can't dig effectively when the water comes pouring in on you like that. The hole just keeps getting wider as the sides cave in. These sediments have no structural strength in them when they're wet.”
So the company's engineers went to work to put together a plan to dewater the sediment.
Their challenge was twofold.
First, the 20-foot-thick layer of sediment was made of a variety of different deposits, including a few layers of clay that kept the water from dropping down into the aquifer. To address that problem, the company punched a number of small holes through the sediment and inserted some cloth-lined pipe. The wick drains allowed the water to move vertically.
The second challenge was to make sure the water in the sediment had a place to go.
The aquifer was under pressure just beneath the sediment layer. To alleviate that, the company drilled six wells - three on either side of their new test pit. The wells pulled about 350 gallons a minute from the aquifer.
“It turned out that there wasn't as much water as we thought,” Fein said. “That was good news. We were prepared to pump a lot more water, but it turned out that we had to reduce the size of pumps we installed initially. The first ones were too powerful. They'd suck the well dry and burn up.”
By last week, the company had dug about half of the 600,000 cubic yards of material it planned to move during the sediment dewatering test. The hope is the rest of the hole will be dug over the next couple of weeks.
When that work is done, Fein said Envirocon will incorporate everything it's learned from the test into the final design for the bypass channel, which is scheduled to be completed sometime next fall.
The 100-foot-wide channel will divert the Clark Fork River around the 2 million to 2.5 million cubic yards of contaminated sediment that will eventually be moved to the Opportunity Ponds near Anaconda.
The ground assessments, like this dewatering test, will likely play an important role in making sure the dam and sediment work are completed in a safe and efficient manner.
“We're literally designing things and then building them as we go,” Fein said.
As anticipated, there have been lessons learned as part of the dewatering test.
For instance, no one knew for sure what was underneath the 20-foot layer of sediment that lined the bottom of Milltown Reservoir.
Envirocon's crews have discovered the remnants of what apparently was a relatively thick forest that filled the valley floor at the confluence of the two rivers. The bottom of the hole is filled with stumps left behind by crosscut saw-packing sawyers over a century ago.
“We certainly didn't think we'd hit as many stumps as we have,” Fein said. “Their presence tells us exactly where the old forest floor was located. We know from that level down, the soil doesn't have significant levels of contamination.”
They've also learned that sediment removed from the area can be unruly.
“It starts to move like lava under its own weight when we try to stack it,” Fein said. “This sediment has no strength whatsoever when it's wet. We have to maintain a healthy respect for that fact.”
Plans call for stockpiling the estimated 600,000 cubic yards of sediment that will be removed during the bypass channel construction on the site.
“There will be a massive stockpile of sediment here,” he said. “We'll use up about every square foot of flat ground.”
The company has also faced a challenge in moving heavy equipment around the site. There is a layer of soft sand about 2 feet underneath the surface in some areas that can easily bog down machinery.
“We've needed to build more roads out of gravel than we anticipated,” Fein said.
By the first of February, the company hopes to begin constructing an earthen flood berm along the edge of the Clark Fork River that will begin near the remnants of Duck Bridge and run down to the confluence with the Blackfoot River.
“In my mind, that's when this project will really begin,” Fein said. “We're in the process of working with the agencies to get approval on the final design for the berm.”
Berms will be constructed along both the Clark Fork and Blackfoot rivers to protect the work site from flooding.
Envirocon is already stripping off the top 3 feet of topsoil at the upper end of the work site. That material will eventually be used to cap a lot of area disturbed during the construction phase.
“The soil has been thoroughly tested and it's so low in metal contamination that we're allowed to use it as topsoil,” Fein said. “There's a lot of material here. We estimate there's about 125,000 cubic yards. We'll use every bit of it in our reclamation efforts.”
Fein said the project appears to be adhering to the expected timeline, but people need to remember that this is a huge undertaking and there is plenty of opportunity for something to go astray.
The project's timetable is tied to the ebb and flow of the river system, he said.
Missing one deadline - like perhaps the upcoming bridge mitigation work - could push the whole project back a year.
“Mother Nature has a way of slipping in something unexpected just when you think you have it all figured out,” Fein said. “There are no guarantees.”
