NIGLIQ BRIDGE WINS ANOTHER NATIONAL AWARD
NCSEA Excellence in Structural Engineering Outstanding Project
On April 8, 2015, the day the Nigliq Bridge project wrapped the Arctic Circle’s maiden major river crossing due north of Nuiqsut, North Slope Borough, Alaska, it was snowing, the temperature was a lonely single digit, and an ice fog hovered over the Colville River’s groundbreaking bridge.
“This bridge is an engineering feat,” said Michael Culmo, PE, CME Engineering, a judge in the National Steel Bridge Alliance’s prize-bridge competition that voted PND Engineers’ Nigliq Bridge the 2018 medium-span bridge national award winner in March.
Seven months later, PND Engineers' Nigliq Bridge won another national honor: the National Council of Structural Engineers Associations' 2018 Excellence in Structural Engineering Awards' Outstanding Project. The award was announced at the NCSEA Structural Engineering Summit on Oct. 26 at the Sheraton Grand Chicago.
“Launching a girder system is hard enough in ideal conditions but to do so under extreme conditions is an accomplishment to be lauded," Culmo said. "The use of steel bents in this harsh environment is a testament to the versatility of steel.”
Because the Colville River is frozen for more than half the year and floods each spring, there’s a short construction window in this remote arctic environment that regularly features blizzards and temperatures/wind chills plummeting to 50-below. Nuiqsut’s coldest month on average at -18°F is January, which is when the ice roads typically open to the surrounding oil development sites in the National Petroleum Reserve-Alaska.
“With only three-month windows in each of the two construction seasons due to working on top of frozen tundra, in order to get materials out there it was critical to minimize the amount of fieldwork,” said PND Engineers Principal-In-Charge Kenton Braun, who accepted the NCSEA honor on PND's behalf.
PND Engineers had to think efficiently so construction crews could work effectively.
The Nigliq Bridge, eight spans and 1,421 feet long, was engineered with twin fully enclosed steel box girders to support six pipelines along its length on the downstream side, as well as the primary service-vehicle traffic required for maintenance and upkeep of oilfield operations. Pipeline placement on the bridge was chosen to protect against 6-foot-thick ice floes that can exceed 200 feet in width. The bridge substructure was designed with sloping heavy wall pipe icebreakers that significantly reduce horizontal ice loads on the piers by bending the ice rather than crushing it.
The geotechnical profile of this bridge and its foundation is a case study in frozen-ground engineering. Nearly every major condition faced in designing structures on frozen ground is present. Accordingly, the bridge substructure utilized 48-inch-diameter steel pipe piles in three major configurations: adfreeze piles with and without passive refrigeration, driven piles with PND Engineers’ SPIN FIN™ tips for extra bearing capacity in deeply thawed zones, and tremie grout piles in soils with high salinity.
Superstructure elements were fabricated from steel plate, maximizing shop welding and minimizing fieldwork. Segments were incrementally launched on a custom hydraulic launch bed and lowered onto bridge bearings. Finally, 219 22.5-by-5-foot precast concrete deck panels were set and grouted on the girders to form the composite section necessary to support all design loads.
On April 8, 2015, the Nigliq Bridge became the first launched bridge built north of the Arctic Circle in North America; the incremental launch was completed in two months.