PART 2: THE SOLUTION AND IMPLICATIONS
The Alaska Department of Transportation & Public Facilities (DOT&PF) selected a pipe pile-based column-supported embankment alternative as the most viable solution to mitigate long-term settlement issues at the west end of Runway (RW) 10-28 at Nome Airport (read more about the differential settlement issues on the runway and how this solution was selected in Part 1: Issue and Finding a Solution). The first structure of its kind installed beneath an airport runway in Alaska, this innovative design solution provides long-term stabilization for the runway surface.
The Nome Airport is the only year-round, reliable means of transportation for passengers, supplies, and emergency medevac to and from Nome. The Nome Airport serves as the transportation hub for many surrounding communities, making it a vital asset to this region of Alaska.
Innovative Pile-Supported Embankment
The selected pile-supported embankment solution ultimately consisted of driving more than 1,200 8’ nominal diameter extra strong steel pipe piles in a closely spaced triangular grid pattern, 8’ apart, covering an area the approximate size of a football field. Each pile was capped with a specially designed reinforced concrete pile cap. Pile spacing was optimized by the geotechnical engineer based on the geometry of the pile caps, thickness of the overriding embankment and design of the load transfer platform, following methods outlined by FHWA.
The west end of RW 10-28 was excavated down to approximately 10’ below the runway surface. Then, the pipe piles were driven from the excavation base down to depths of approximately 30’-50’ and into the underlying bedrock. After meeting the pile completion criteria (defined using pile dynamic analysis testing on select piles), the piles were cut off slightly above the excavation base grade and the pile caps were installed on each pile. A specialized load transfer platform (LTP) embankment section consisting of multiple layers of high friction aggregate and extra high-strength geogrid was then constructed surrounding and above the pile caps. Lastly, the excavation was backfilled with suitable material above the LTP to the base of the pavement structural section.
With this design, settlement will continue to occur below and between the pile caps, but the piles should experience minimal settlement and the LTP will help bridge the settlement between the pile caps. This configuration essentially distributes the stress above the pile caps analogous to a gothic vaulted ceiling so the entire load of the runway embankment is transferred to the piles and down to the underlying bedrock. This design should minimize future total and differential settlement reflected at the runway surface and provide an enduring smooth operating surface for aircraft operations.
The specially designed reinforced concrete pile caps were approximately 4’ in diameter and used a unique trapezoidal section and hexagonal plan shape. They were all premanufactured at a facility in Kenai for barge delivery to Nome.
“This unique trapezoidal/hexagonal shape should improve the friction and interaction between the soil and pile cap because the primary stress from the bridged embankment load is oriented more perpendicular to the pile cap surface,” said Brian Mullen, PE, Lead Geotechnical Engineer for this project. “This design also reduces material cost due more efficient use of reinforced concrete and allows greater spacing between the piles because of the larger plan surface we were able to develop for the pile caps using a tapered section.”
Reducing pile and concrete quantities and premanufacture of materials saved a significant amount of construction time and cost. Considering the project scale, small increases in pile spacing and small reductions in pile cap materials had a significant impact on construction cost, making this unusual pile cap not only an effective solution, but a cost-effective design, as well.
A Long-Term Solution for Decades-long Settlement Issues
This innovative deep ground improvement solution is likely the first application of a pile-supported embankment in Alaska, and of any type of column-supported embankment under an airport runway in the state. The pile-supported embankment at RW 10-28 in Nome was constructed by Knik Construction and pile installation subcontractor STG Incorporated in June and July 2022, on schedule and without any notable construction problems. Designed to span the deeper soil issues causing the decades-long operations and maintenance problems at select portions of Nome Airport’s main runway, this unique design solution has resiliency to withstand settlement for the overlying runway surface for decades to come.
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