Up to 50 trains a day cross the Cowlitz River on the BNSF Bridge 81.4 in the rural area of Vader, Washington. It may be remote, but its presence is an essential link in transporting goods. The bridge and the trains it carries have been important to the region for more than 110 years, especially as the single North-South rail route in the area.

In 2013, BNSF determined Bridge 81.4 had reached the end of its 100-year service life and needed to be replaced to meet current standards and to replace the non-redundant pin-connected truss main span.

“This bridge is an important link in our network, supporting both our business and the passenger operations of several agencies. Replacing the old structure was vital to maintaining the reliability of this corridor,” said Michael Schaefer, assistant director bridge construction, BNSF Railway.

BNSF brought in the HNTB team as designers on the Construction Manager/General Contractor (CMGC) delivered project. The CMGC project delivery model is a process where the contractor acts as a construction manager during preconstruction stages and as the general contractor during the actual construction period.

The project team evaluated a range of replacement options and ultimately selected an offline alignment due to the high amount of train traffic the bridge carries. It was an ideal solution, even though designing and building the new bridge presented a range of challenges, including its near inaccessibility, environmental concerns, geotechnical obstacles and the weather.

The old Bridge 81.4 was a 663-foot-long double main track over the Cowlitz River. HNTB replaced it with a new structure, 1,100-feet-long and with 12 spans, four of them long deck plate girder spans ranging in length from 143 feet 4 inches to 207 feet 6 inches long.

“We’ve used similar span lengths on other projects, but the permitting requirements limited the number of piers in the river, and that necessitated a longer span,” said Temple Overman, HNTB construction project manager. “However, we were able to ship the girders in different splice lengths, so when they arrived at the site they could be bolted together.  The deck plate girder spans consist of 4 girders per track which provides a more redundant structure than the old truss span.”

Keeping trains moving

Concerns from abutting landowners prevented the team from drilling borings at the proposed pier locations until full access was made possible by the construction of a temporary work bridge built adjacent to the new bridge and access roads pioneered on one end from an existing rock quarry. The work bridge was 44-foot-high and 42-foot-wide, 45 feet above the Ordinary High Water Mark (OHWM) and made of steel pipe piles and beams that could be reused for other temporary work bridges in the future. The work bridge facilitated precise placement of the foundations in the river and the ability to pick the steel girders and set on the new foundations.

About 720 feet across the Cowlitz River from bank-to-bank, a gravel road on the south side of the bridge and a 20-foot-tall geofabric wall off of the north bank were constructed to provide bank-to-bank access.

The work bridge and road were removed upon completion of the project, leaving no evidence that they had ever been there.

“It was difficult to fully understand the geology of the area during the design process,” said Overman. “We were able to drill two borings during the design phase, but they were further away from the bridge due to access issues and not at the proposed substructure locations. So, we had to make some assumptions in our design plans regarding the top of rock elevation.”

The risks to not having the borings during the design phase include the potential for not having adequate casing or rebar lengths and also schedule impacts. The team handled these risks through the CMGC delivery method.

When precisely placed borings were finally made possible, the team discovered a significant elevation change — the basalt rock layer was unexpectedly deep at one end of the bridge and almost nearing the surface at the other. Also, numerous boulders made driving H piles as planned difficult even with pile points installed. So, the team modified the plans to drill rock sockets for the H-piles.

This project also included temporary piles for the drilled shaft oscillator frame, girder erection falsework pile below OHWM, four, 9 feet and 10-inch diameter shafts approximately 180-feet deep, sheet pile shoring, permanent piling and cast-in-place concrete.