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Watching hydraulic jacks lower a million-pound box into the Willamette River is about as exciting as watching paint dry, but the work is difficult and about as risky as it gets, said Ted Aadland, area manager for Tempe, Ariz.-based Sundt Construction Co. It’s partnering with Stayton-based Slayden Construction Group Inc. as the construction manager-general contractor for the project to replace the Sellwood Bridge.

“We’re dealing with a million pounds 鈥� we want to take it easy,” Aadland said.

Footings constructed inside two perched box caissons will be used to support the steel arch spans that cross the channel above water, he said.

The first perched box caisson was lowered over the past week as crews set the foundation piece for the $300 million bridge, which is expected to open by fall 2015. The entire project is slated for completion by summer 2016.

Significant obstacles were encountered last summer, when an excavation company dug four shafts deep into the riverbed. During the process, crews struck boulders unexpectedly, and the project was delayed six to eight weeks, Aadland said.

The project is still on schedule and on budget; however, crews have to be prepared, he said.

“It’s the unknown that makes it so risky,” he said. “You may have boring logs down there. You never know for sure until you get into it.”

The team did not originally plan to build a perched box caisson, which involves construction of a concrete foundation about 28 feet below the surface of the water, Aadland said.

“The original design was a full coffer dam at the bottom of the river with 185-foot deep shafts,” he said. “The purged footings saved money and time because they would not disturb the river bottom during fish migration.”

Instead of the coffer dam, crews are building purged footings 鈥� essentially a suspended foundation.

After the perched box caisson is lowered into the river, water will be pumped out of the box and cement will be poured into the foundation. The purged footings will then be connected to the four deep shafts that were dug last summer. Anchor bolts will connect the footings with steel arches that will be built beneath the bridge deck, Aadland said.

“Each arch has two locking points at its base,” he said. “They will lock into the purged footing, which sits right at or above the surface of the water.”

Divers will perform much of the underwater work 鈥� such as guiding locking bolts into place, Aadland said. Dry-land crews will use the hydraulic jack and lower the box down, he said.

Lowering the box caisson is a significant milestone because it represents one of the last real project obstacles, said Mike Pullen, a spokesman for Multnomah County.

“We are fortunate that the surprises have already happened,” he said. “We’re not behind schedule. Now that we’re above ground, there will be fewer surprises.”

Use of the perched box has played a key role in how the project team achieves budget and deadline targets, Pullen said.

“It’s a cheaper way to build that part of the bridge,” he said. “It was decided it wasn’t disruptive to the river bottoms, so it gave us greater schedule flexibility,” he said.

Building the perched box caisson

Step 1: Build drilled shafts. Crews last year built four 10-foot-diameter drilled shafts into the bedrock beneath the river’s surface. A 50,000-pound chisel was used to break up rock until a depth sufficient to support the bridge’s weight was reached. The shafts were reinforced with steel bars and filled with concrete. No two shafts were the same length. Their depths ranged from 104 feet to 185 feet below water level.

Step 2: Assemble box caissons. This occurs above water. The box caissons, which measure about 100 feet long by 22 feet wide by 30 feet tall, are assembled around the drilled shafts at the two bents, or piers, in the water. They are supported on steel piles driven into the river bottom. Each caisson has a concrete floor built in place above the riverbed. The caisson walls are built from steel sheet piling.

Step 3: Sink caissons. This process started last week on one caisson and will continue next month on the second. Using hydraulic jacks, each caisson is lowered with final elevation of the concrete floor about four feet below sea level. As water enters the caisson, the floor will sink along the drilled shaft casings.

Step 4: Anchor caissons to drilled shafts. After sinking each caisson to its final elevation, divers working in the water will connect the floor to the drilled shaft casings using 鈥渢ie-down collars鈥� and steel bolts. Rubber gaskets, meanwhile, prevent the river water from leaking into the caisson after it is pumped dry. Once the caisson is secured to the shafts and piles, the jack system used for lowering the box is removed.

Step 5: Build bridge bent 鈥渋n the dry.鈥� Once the caisson’s inside is dry, the contractor will install steel reinforcing bars and fill the forms with concrete. After curing, form panels are removed and workers will apply a charcoal-colored stain to the concrete surfaces to give them an architecturally pleasing finish.

Steps 6 and 7: Flood the caisson and dismantle it. After the concrete bent is finished, the contractor will fill the caisson with water to equalize water pressure. They will then remove the steel sheet piles that make up the walls of the box. Other supports are removed as well, leaving the concrete floor permanently in place attached to the bent.

Step 8: Finish bridge construction. The bridge’s steel arch and roadway support structure will be constructed on the finished bents. The concrete roadway deck and sidewalks will be placed on the steel roadway supports.

Source: Multnomah County