Rapid replacement greases the skids

Just east of the Elk Creek Tunnel on Oregon 38 in the Coast Range, the bridge program had to replace a bridge where there was no room for a detour structure to carry traffic during construction.

To avoid subjecting local residents, coast-to-valley freight haulers and tourists to long detours or prolonged delays and lane closures, ODOT opted for rapid replacement. With this technique, crews build a new bridge beside the old one and then, during a short closure, slide the old bridge out of the way and the new one into place.

The logistics were tricky, so ODOT partnered with specialty contractor Mammoet, which pioneered the development and use of hydraulic skidding systems. Skids can move very heavy loads over short distances, in this case, a new bridge that was 222 feet long by just over 38 feet wide and weighed more than 2 million pounds.

When it came time to install the replacement, ODOT promised to limit the road closure to a single weekend, so at 9 p.m. on Friday, crews began cutting away the guardrails and concrete that joined the existing bridge to its substructure. In just 12 hours, the replacement Elk Creek Bridge began a two-hour, 15-foot journey to its new home. Crews spent the next 22 hours double- and triple-checking that the structure fit together properly and was ready for traffic. The process went so smoothly that the bridge reopened nine hours earlier than expected, in plenty of time for Monday morning commuters.

For the new Elk Creek Bridge on the west side of the tunnel, ODOT had the option of building a detour structure. However, because all the equipment was in place, it was more cost-effective to use the same technique again.

Rapid replacement was a more expensive solution, but ODOT applied for and received a $1 million Federal Highway Administration grant to help cover the additional costs.

Crane sets beams

For most of the water-spanning bridges we replaced, the terrain allowed us to build both a detour bridge and a work bridge, so we could reroute traffic during construction and avoid interfering with fish migration and breeding.

The wide and shallow basin surrounding the Sandy River bridges, however, is vulnerable to flooding from winter storms that send water rushing down from Mount Hood upstream; in the spring, it is susceptible to surges from the Columbia River downstream.

Under low-flow conditions, temporary work and detour bridges would have a minor effect on the Sandy River's water level. But during a major storm, their steel piles could increase backwater upstream of the bridge and act as a sieve, trapping large amounts of debris against the bridge rather than allowing it to travel down the river freely.

Residents and businesses nearby face the threat of floods every winter, and ODOT didn't want to increase the risk by leaving 168 debris-catching work bridge pilings in place to add to the mix of hazards.

In addition to other measures, the project team devised a unique solution for a unique situation: setting the beams from a crane that operated within the linear footprint of the bridge being built. Specialized twin gantry cranes, constructed on-site for the project, hooked, lifted and placed steel beams measuring up to 167 feet long and weighing up to 192,000 pounds, almost as much as the cranes' combined weight of 250,000 pounds. The gantry cranes were supported by only 12 temporary pilings, so they allowed us to keep debris-attracting obstacles out of the river.

Not so tiny bubbles

Fish don't have ears in the conventional sense: They perceive sound as movement and vibrations in the water via finely tuned nerve endings in their bodies.

The noise generated by heavy machinery — for example, driving piles into the riverbed to provide support for new bridges — would disturb them and other river creatures, and could negatively affect communication and migratory patterns. In consultation with biologists from the Oregon Department of Fish and Wildlife, contractors on the bridge program toned down their construction noise with an unlikely solution: bubbles.

On the McKenzie River Interstate 5 Bridge, Hamilton Construction used a bubble curtain to dampen noise; a ring on the bottom of the river discharged air, causing bubbles to rise continually and isolate vibrations. Later, on the larger I-5 Willamette River Bridge project, the company revamped and upgraded its noise attenuator and christened it a "bubbleator." The bubbleator is a custom-made circular device constructed of sheet metal and lined with high-density polystyrene foam. Aluminum pipes frame the piles to produce a thick wall of frothy bubbles, thus dampening sound from pile strikes around each two-pile template. Due to its size, the frame of these devices also served as a safe, sturdy work platform for crews during pile driving.

Hydro-acoustic monitoring on the project showed that the bubbleator maintained noise levels below thresholds required by ODFW.