In-water work window closing for light-rail bridge
POSTED: Thursday, October 27, 2011 at 03:05 PM PT
BY: Angela Webber
Daily Journal of Commerce
Since July 1, crews have worked to set the stage for construction of the Portland-Milwaukie Light Rail Bridge. Now the curtain will rise, so to speak, for the main attraction.
Monday marks the end of the window that allows crews to work in the Willamette River. The deadline also marks the beginning of the project’s next phase, when the bridge itself will begin to take shape.
For the past few months, work has focused on preparing a staging area. Crews recently completed cofferdams – giant tubs in the water to hold workers so that they don’t disturb the rest of the river. Installation of the cofferdams involved drilling and installing piles that extended into layers of sand and gravel underwater. The steel piles support the “walls” of the cofferdams, which can be drained so work can take place inside more easily.
“When we’re working in the cofferdam we’re not interfering with marine life around the bridge, so we can work year-round,” said Ralph Salamie, project sponsor with the bridge’s design-build contractor, Kiewit Infrastructure West. It has a $118.9 million fixed-price contract for the bridge construction.
Crews also took advantage of the summer in-water work window to install pillars to support temporary work bridges that provide access to the cofferdam sites. It was critical for crews to meet the deadline, said Robert Barnard, a project director for TriMet.
“You don’t want to miss the in-water work window. You want to be done on time,” he said. “You make it, or you lose a year.”
To ensure that didn’t happen, crews planned ahead. They even prepared for the unexpected.
“The key to any operation is detailed planning,” Salamie said. “The other key is to plan for the what-ifs.”
Kiewit and TriMet planned for only one shift, Mondays through Fridays. They left weekends open for catch-up, Barnard said.
“We used most Saturdays and a few Sundays, but we were able to work a single shift and not have to work through the night,” Salamie said.
Occasionally, crews encountered some surprises, but they are to be expected, Barnard said, when drilling takes place underwater. When installing piles, crews found that they needed to hammer through surfaces harder than expected, or dig holes deeper than anticipated. Some piles planned to go only 100 feet deep ended up being up to 140 feet deep, Barnard said.
“It’s a challenge to estimate what the conditions are below the water,” Salamie said. “We have to react quickly (and) get some more material, and we’re back on schedule.”
Crews have only one more task to complete before the in-water work window closes. They are installing “scour protection,” which means dumping heavy rock on the bottom of the river around the cofferdams in order to reduce the amount of sand and sediment at the bottom of the river that will be lifted up by the current.
Because the cofferdams present an obstacle to the flow of the river, water can kick up whatever is lying on the floor. Scour protection provides a heavy protective shield to mitigate the cofferdams’ disturbance, Barnard said.
Salamie said he is looking forward to Monday.
“But it’s just the beginning,” he said. “It’s just the end of the beginning.”
With staging areas essentially in place, crews will dig deeper drill shafts to install the piles that will support the bridge. These piles go deeper than the ones supporting the cofferdams, down to the Troutdale Formation. It’s a layer of compressed cobblestone and rock that Barnard calls “God’s own concrete” because of its natural hardness and stability.
Once those piles are installed, crews will pour a concrete “pile cap” on each tower.
Then the bridge will start to appear, Barnard said.
“Now we’re going to start building a bridge,” he said.
http://djcoregon.com/news/2011/10/27...t-rail-bridge/
Prev
Linear Mode
