Sandpoint, Idaho, is the home of a significant amount of transcontinental freight rail traffic. BNSF’s mainline track in Sandpoint merges with the former Montana Rail Link before spanning the lake as a single track, which often creates bottlenecks for the approximately 60 freight trains that cross the lake each day.
To reduce the congestion and promote efficient movement of train traffic, BNSF built a second track through the bottleneck area.
Hanson served as the design engineer and provided on-site construction observation services for the project, which includes new bridges over Lake Pend Oreille, Bridge Street and Sand Creek. The project involved developing final bridge and track designs, as well as permitting and construction documents. Project design included administrative tasks, preliminary studies and investigations, engineering and design, construction plans, specifications, estimates, as well as attending public hearings and providing bidding support services. Construction observation includes project setup, administrative tasks, on-site construction coordination and closeout.
Overcoming soft soil and other challenges
One of the biggest challenges the Hanson team encountered while undertaking the design was confirming the required length of the nearly 300 foundation piles needed to accommodate the unique soft clay soil over two-thirds of the nearly mile-long bridge. An initial review of the soil conditions found that the upper portions, extending some 75 feet below the mudline, provided only minimal capacity. The soil was so soft and sensitive that installing a pile caused excess pore water pressure to develop, making the soil softer and weaker. The project team brought the piles to capacity by extending the pile length to nearly 240 feet in many locations and allowing them to set up for about 100 days.
For the southern third of the bridge, the Hanson team conducted supplemental H-pile probing to determine that the piles for that section of the bridge could be driven to capacity in bearing. The team used a high-tech monitoring system, with tie-mounted sensors at 12-foot spacing, to check for track movement during pile driving.
Hanson led an experienced team of geotechnical and environmental consultants, considering input from BNSF and a railroad bridge contractor, to help resolve various design challenges related to the two other, less visible, bridges’ proximity to the existing track and local roads. The project team lengthened the Sand Creek bridge, in lieu of constructing the track embankment in stages, to mitigate the settlement risk of the active track and the adjacent highway bridge. The clearance between the new Bridge Street bridge and the existing bridge is just 1-2 feet, and an adjacent state roadway is only 50 feet away.
To economize the construction approach, the project team used similar pieces for the 4,875 ft bridge over Lake Pend Oreille. For instance, the supporting piles have the same 3-foot-diameter section, the caps have the same design, and the superstructure uses multiple spans of identical prestressed concrete girders with the same concrete deck section. This approach saved time and money throughout construction.
Because of its environmental significance and recreational impact, the project required extensive permitting. To maintain boaters’ access to the water, the construction team employed barges to transport equipment and materials instead of building a long, temporary work trestle across the lake. To further mitigate the environmental risks related to construction, Hanson also limited the amount of cast-in-place concrete to avoid spills in the lake and lowered the number of strikes per pile to reduce vibration and sound to protect fish.
Hanson’s goal was to set a high bar from design through construction and, by extension, meet BNSF’s goal of setting the gold standard for permitting. The double track was operational in 2023, on BNSF’s schedule.