PART 1: ISSUE AND FINDING A SOLUTION
How did steel piles make their way into an airport runway design?
Typically used as a deep foundation for bridges, docks and other facilities, steel pipe piles were selected to form a column-supported embankment underlying portions of the main runway serving Nome. Deep ground improvement was needed to mitigate severe differential settlement occurring at the west end of Nome Airport’s Runway (RW) 10-28. Rapid thawing of variable deep ice-rich permafrost and resulting unconsolidated soils contributed to ongoing severe distortion of the runway surface. This, in turn, created hazardous conditions for aircraft operations. A network of closely spaced steel pipe piles was installed under the runway to span the problematic soils and provide full support for the overlying embankment. 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.
Long-term Settlement Issues Plague Nome Airport Runway Operations
The Nome Airport has a history of maintenance and operations problems related to poor geotechnical conditions underlying its runways. Originally built in the 1940s, the airport embankments were largely constructed over refrozen dredge tailings (reworked fine- and coarse-grained soils with intermixed organics resulting from mining operations) and undisturbed soils containing variable ice-rich permafrost. For airport embankments that have been in place for up to 80 years, you would typically expect underlying soils to have stabilized with time – but that is not the case with thawing permafrost. Geotechnical explorations within the most problematic areas indicated relatively rapid thawing of ice-rich permafrost over recent decades. As the ice-rich soils underlying the runway thaw, unconsolidated soils containing significant excess moisture are generated in place of the previously frozen soil mass. Fine-grained soils impede water flow from the freshy thawed soils, resulting in unstable soil conditions at depth that may take decades to fully consolidate. Localized variabilities in water (ice) content, rate of permafrost thaw and permeability in the soil mass led to variable consolidation rates in localized areas. The ultimate result is severe and ongoing differential settlement for overlying pavements at the runway surface.
This differential settlement has progressively gotten worse over the decades. In particular, the west end of primary RW 10-28 experienced extensive differential settlement and needed a long-term solution to reduce the frequent maintenance burden and improve safety for aircraft operations.
The Alaska Department of Transportation & Public Facilities (DOT&PF), who operates the Nome Airport, routinely regraded and repaved this problematic portion of RW 10-28 to smooth out the runway surface. Although a relatively inexpensive solution, it doesn’t last and each repair required long-term closure of the main runway.
Seeking a permanent repair strategy, DOT&PF contracted R&M to develop a long-lasting repair solution as part of the wider Nome Airport Rehabilitation project.
Finding a Solution
Deep ground improvement methods previously implemented by DOT&PF included deep dig-outs and deep dynamic compaction, but these solutions proved unsuccessful. Implementation of deep dynamic compaction and deep dig-outs were complicated by frozen soils and shallow groundwater and much of the problematic soil was deeper than the 20’-30’ depths these techniques are capable of influencing.
With those techniques exhausted, R&M looked for other options that would meet DOT&PF’s design criteria of eliminating future settlement in the specified problem areas of the runway.
“We were looking for a way to bridge the problematic soils and provide a solid foundation that will support the runway embankment,” said Carla Baxley, PE, Project Manager for the Nome Airport Rehabilitation project and Group Manager of Airport Engineering at R&M. “We were looking for a 20+ year solution.”
To develop the long-term solution, R&M, with the support of a deep ground improvement expert subconsultant, first looked at the various traditional deep ground improvement techniques. These included stone columns, vibro-compaction, vibro-replacement, deep soil mixing, permeation grouting and compaction grouting. Stone columns, vibro-compaction, vibro-replacement, deep soil mixing and permeation grouting were deemed incompatible or too risky with the thick deposits of very soft fine-grained soils at the project site. Compaction grouting was considered potentially viable, but implementation on such a wide scale was considered problematic for airport operations and cost prohibitive.
The two primary alternatives remaining included:
- An aggregate/cement treated soil column (ATSC)-supported embankment, where existing soils are mixed in-place with aggregate and cement to form relatively solid columns spanning the problematic soils.
- A pile-based alternative using conventional steel pipe piles in place of the ATSC columns.
Both alternatives involve constructing a load transfer platform above the columns to help stiffen the base of the embankment and distribute loads from the overlying embankment onto the columns/piles.
The Chosen Alternative
After further analysis, R&M concluded the ATSC method was more expensive, or at least associated with higher cost risk compared to pipe piles, due to the high cost for cement in Nome and the large, specialized equipment and personnel required for mobilization from outside Alaska. There is also greater uncertainty with cost, complexity and installation problems for non-traditional construction methods in Alaska.
Conversely, pipe piles are commonly installed in Alaska. There are several Alaskan contractors specializing in pile driving, including some based in Nome, which lowers associated risk and costs. For these reasons, DOT&PF selected the pipe pile-based column-supported embankment alternative as the most viable solution to mitigate the settlement issues on RW 10-28 at Nome Airport.
Read Part 2: The Solution and Implications to learn more details about the selected alternative and the long-term implications for the Airport.
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